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tl/h/12593 lm2598 simple switcher power converter 150 khz 1a step-down voltage regulator, with features may 1996 lm2598 simple switcher power converter 150 khz 1a step-down voltage regulator, with features general description the lm2598 series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving a 1a load with excellent line and load regulation. these devices are avail- able in fixed output voltages of 3.3v, 5v, 12v, and an adjust- able output version. this series of switching regulators is similar to the lm2595 series, with additional supervisory and performance features added. requiring a minimum number of external components, these regulators are simple to use and include internal fre- quency compensation 2 , improved line and load specifica- tions, fixed-frequency oscillator, shutdown /soft-start, error flag delay and error flag output. the lm2598 series operates at a switching frequency of 150 khz thus allowing smaller sized filter components than what would be needed with lower frequency switching regu- lators. available in a standard 7-lead to-220 package with several different lead bend options, and a 7-lead to-263 surface mount package. typically, for output voltages less than 12v, and ambient temperatures less than 50 c, no heat sink is required. a standard series of inductors (both through hole and sur- face mount types) are available from several different manu- facturers optimized for use with the lm2598 series. this feature greatly simplifies the design of switch-mode power supplies. other features include a guaranteed g 4% tolerance on out- put voltage under all conditions of input voltage and output load conditions, and g 15% on the oscillator frequency. ex- ternal shutdown is included, featuring typically 85 m a stand- by current. self protection features include a two stage cur- rent limit for the output switch and an over temperature shutdown for complete protection under fault conditions. features y 3.3v, 5v, 12v, and adjustable output versions y adjustable version output voltage range, 1.2v to 37v g 4% max over line and load conditions y guaranteed 1a output current y available in 7-pin to-220 and to-263 (surface mount) package y input voltage range up to 40v y excellent line and load regulation specifications y 150 khz fixed frequency internal oscillator y shutdown /soft-start y out of regulation error flag y error output delay y low power standby mode, i q typically 85 m a y high efficiency y uses readily available standard inductors y thermal shutdown and current limit protection applications y simple high-efficiency step-down (buck) regulator y efficient pre-regulator for linear regulators y on-card switching regulators y positive to negative converter typical application (fixed output voltage versions) tl/h/12593 1 2 patent number 5,382,918. simple switcher and switchers made simple are registered trademarks of national semiconductor corporation. c 1996 national semiconductor corporation rrd-b30m66/printed in u. s. a.
absolute maximum ratings (note 1) if military/aerospace specified devices are required, please contact the national semiconductor sales office/distributors for availability and specifications. maximum supply voltage (v in ) 45v sd /ss pin input voltage (note 2) 6v delay pin voltage (note 2) 1.5v flag pin voltage b 0.3 s v s 45v feedback pin voltage b 0.3 s v s a 25v output voltage to ground (steady state) b 1v power dissipation internally limited storage temperature range b 65 cto a 150 c esd susceptibility human body model (note 3) 2 kv lead temperature s package vapor phase (60 sec.) a 215 c infrared (10 sec.) a 245 c t package (soldering, 10 sec.) a 260 c maximum junction temperature a 150 c operating conditions temperature range b 25 c s t j a 125 c supply voltage 4.5v to 40v lm2598-3.3 electrical characteristics specifications with standard type face are for t j e 25 c, and those with boldface type apply over full operating temperature range. symbol parameter conditions lm2598-3.3 (limits) units typ limit (note 4) (note 5) system parameters (note 6) test circuit figure 1 v out output voltage 4.75v s v in s 40v, 0.1a s i load s 1a 3.3 v 3.168/ 3.135 v(min) 3.432/ 3.465 v(max) h efficiency v in e 12v, i load e 1a 78 % lm2598-5.0 electrical characteristics specifications with standard type face are for t j e 25 c, and those with boldface type apply over full operating temperature range. symbol parameter conditions lm2598-5.0 (limits) units typ limit (note 4) (note 5) system parameters (note 6) test circuit figure 1 v out output voltage 7v s v in s 40v, 0.1a s i load s 1a 5 v 4.800/ 4.750 v(min) 5.200/ 5.250 v(max) h efficiency v in e 12v, i load e 1a 82 % LM2598-12 electrical characteristics specifications with standard type face are for t j e 25 c, and those with boldface type apply over full operating temperature range. symbol parameter conditions LM2598-12 (limits) units typ limit (note 4) (note 5) system parameters (note 6) test circuit figure 1 v out output voltage 15v s v in s 40v, 0.1a s i load s 1a 12 v 11.52/ 11.40 v(min) 12.48/ 12.60 v(max) h efficiency v in e 25v, i load e 1a 90 % http://www.national.com 2
lm2598-adj electrical characteristics specifications with standard type face are for t j e 25 c, and those with boldface type apply over full operating temperature range. symbol parameter conditions lm2598-adj (limits) units typ limit (note 4) (note 5) system parameters (note 6) test circuit figure 1 v fb feedback voltage 4.5v s v in s 40v, 0.1a s i load s 1a 1.230 v v out programmed for 3v. circuit of figure 12 . 1.193/ 1.180 v(min) 1.267/ 1.280 v(max) h efficiency v in e 12v, v out e 3v, i load e 1a 78 % all output voltage versions electrical characteristics specifications with standard type face are for t j e 25 c, and those with boldface type apply over full operating temperature range . unless otherwise specified, v in e 12v for the 3.3v, 5v, and adjustable version and v in e 24v for the 12v version. i load e 200 ma symbol parameter conditions lm2598-xx (limits) units typ limit (note 4) (note 5) device parameters i b feedback bias current adjustable version only, v fb e 1.3v 10 na 50/ 100 na(max) f o oscillator frequency (note 7) 150 khz 127/ 110 khz(min) 173/ 173 khz(max) v sat saturation voltage i out e 1a (notes 8 and 9) 1 v 1.2/ 1.3 v(max) dc max duty cycle (on) (note 9) 100 % min duty cycle (off) (note 10) 0 i cl current limit peak current, (notes 8 and 9) 1.5 a 1.2/ 1.15 a(min) 2.4/ 2.6 a(max) i l output leakage current (notes 8, 10 and 11) output e 0v 50 m a(max) output eb 1v 2 ma 15 ma(max) i q operating quiescent sd /ss pin open, (note 10) 5 ma current 10 ma(max) i stby standby quiescent sd /ss pin e 0v, (note 11) 85 m a current 200/ 250 m a(max) i jc thermal resistance to220 or to263 package, junction to case 2 c/w i ja to220 package, junction to ambient (note 12) 50 c/w i ja to263 package, junction to ambient (note 13) 50 c/w i ja to263 package, junction to ambient (note 14) 30 c/w i ja to263 package, junction to ambient (note 15) 20 c/w http://www.national.com 3
all output voltage versions (continued) electrical characteristics specifications with standard type face are for t j e 25 c, and those with boldface type apply over full operating temperature range . unless otherwise specified, v in e 12v for the 3.3v, 5v, and adjustable version and v in e 24v for the 12v version. i load e 200 ma symbol parameter conditions lm2598-xx (limits) units typ limit (note 4) (note 5) shutdown/soft-start control test circuit of figure 1 v sd shutdown threshold 1.3 v voltage low, (shutdown mode) 0.6 v(max) high, (soft-start mode) 2 v(min) v ss soft-start voltage v out e 20% of nominal output voltage 2 v v out e 100% of nominal output voltage 3 i sd shutdown current v shutdown e 0.5v 5 m a 10 m a(max) i ss soft-start current v soft-start e 2.5v 1.6 m a 5 m a(max) flag/delay control test circuit of figure 1 regulator dropout detector low (flag on) 96 % threshold voltage 92 %(min) 98 %(max) vf sat flag output saturation i sink e 3 ma 0.3 v voltage v delay e 0.5v 0.7/ 1.0 v(max) if l flag output leakage current v flag e 40v 0.3 m a delay pin threshold 1.25 v voltage low (flag on) 1.21 v(min) high (flag off) and v out regulated 1.29 v(max) delay pin source current v delay e 0.5v 3 m a 6 m a(max) delay pin saturation low (flag on) 55 mv 350/ 400 mv(max) http://www.national.com 4
electrical characteristics (continued) note 1: absolute maximum ratings indicate limits beyond which damage to the device may occur. operating ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. for guaranteed specifications and test conditions, see the electrical characteristics. note 2: voltage internally clamped. if clamp voltage is exceeded, limit current to a maximum of 1 ma. note 3: the human body model is a 100 pf capacitor discharged through a 1.5k resistor into each pin. note 4: typical numbers are at 25 c and represent the most likely norm. note 5: all limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face) . all room temperature limits are 100% production tested. all limits at temperature extremes are guaranteed via correlation using standard statistical quality control (sqc) methods. all limits are used to calculate average outgoing quality level (aoql). note 6: external components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. when the lm2598 is used as shown in the figure 1 test circuit, system performance will be as shown in system parameters section of electrical characteristics. note 7: the switching frequency is reduced when the second stage current limit is activated. the amount of reduction is determined by the severity of current overload. note 8: no diode, inductor or capacitor connected to output pin. note 9: feedback pin removed from output and connected to 0v to force the output transistor switch on. note 10: feedback pin removed from output and connected to 12v for the 3.3v, 5v, and the adj. version, and 15v for the 12v version, to force the output transistor switch off. note 11: v in e 40v. note 12: junction to ambient thermal resistance (no external heat sink) for the to-220 package mounted vertically, with the leads soldered to a printed circuit board with (1 oz.) copper area of approximately 1 in 2 . note 13: junction to ambient thermal resistance with the to-263 package tab soldered to a single sided printed circuit board with 0.5 in 2 of (1 oz.) copper area. note 14: junction to ambient thermal resistance with the to-263 package tab soldered to a single sided printed circuit board with 2.5 in 2 of (1 oz.) copper area. note 15: junction to ambient thermal resistance with the to-263 package tab soldered to a double sided printed circuit board with 3 in 2 of (1 oz.) copper area on the lm2598s side of the board, and approximately 16 in 2 of copper on the other side of the p-c board. see application hints in this data sheet and the thermal model in switchers made simple version 4.2 software. typical performance characteristics (circuit of figure 1 ) normalized output voltage tl/h/12593 2 line regulation tl/h/12593 3 efficiency tl/h/12593 14 switch saturation voltage tl/h/12593 15 switch current limit tl/h/12593 16 dropout voltage tl/h/12593 17 http://www.national.com 5
typical performance characteristics (circuit of figure 1 ) (continued) operating quiescent current tl/h/12593 4 shutdown quiescent current tl/h/12593 5 minimum operating supply voltage tl/h/12593 6 feedback pin bias current tl/h/12593 49 flag saturation voltage tl/h/12593 7 switching frequency tl/h/12593 8 soft-start tl/h/12593 9 shutdown /soft-start current tl/h/12593 10 delay pin current tl/h/12593 11 soft-start response tl/h/12593 12 shutdown /soft-start threshold voltage tl/h/12593 13 http://www.national.com 6
typical performance characteristics (circuit of figure 1 ) continuous mode switching waveforms v in e 20v, v out e 5v, i load e 1a l e 68 m h, c out e 120 m f, c out esr e 100 m x tl/h/12593 18 a: output pin voltage, 10v/div. b: inductor current 0.5a/div. c: output ripple voltage, 50 mv/div. horizontal time base: 2 m s/div. discontinuous mode switching waveforms v in e 20v, v out e 5v, i load e 600 ma l e 22 m h, c out e 220 m f, c out esr e 50 m x tl/h/12593 19 a: output pin voltage, 10v/div. b: inductor current 0.5a/div. c: output ripple voltage, 50 mv/div. horizontal time base: 2 m s/div. load transient response for continuous mode v in e 20v, v out e 5v, i load e 250 ma to 750 ma l e 68 m h, c out e 120 m f, c out esr e 100 m x tl/h/12593 20 a: output voltage, 100 mv/div. (ac) b: 250 ma to 750 ma load pulse horizontal time base: 100 m s/div. load transient response for discontinuous mode v in e 20v, v out e 5v, i load e 250 ma to 750 ma l e 22 m h, c out e 220 m f, c out esr e 50 m x tl/h/12593 21 a: output voltage, 100 mv/div. (ac) b: 250 ma to 750 ma load pulse horizontal time base: 200 m s/div. connection diagrams and order information bent and staggered leads, through hole package 7-lead to-220 (t) tl/h/12593 50 order number lm2598t-3.3, lm2598t-5.0, lm2598t-12 or lm2598t-adj see ns package number ta07b surface mount package 7-lead to-263 (s) tl/h/12593 22 order number lm2598s-3.3, lm2598s-5.0, lm2598s-12 or lm2598s-adj see ns package number ts7b http://www.national.com 7
test circuit and layout guidelines fixed output voltage versions tl/h/12593 23 component values shown are for v in e 15v, v out e 5v, i load e 1a. c in e 120 m f, 50v, aluminum electrolytic nichicon ``pl series'' c out e 120 m f, 35v aluminum electrolytic, nichicon ``pl series'' d1 e 3a, 40v schottky rectifier, 1n5822 l1 e 68 m h, l30 typical values c ss e 0.1 m f c delay e 0.1 m f r pull up e 4.7k adjustable output voltage versions tl/h/12593 24 v out e v ref # 1 a r 2 r 1 j where v ref e 1.23v r 2 e r 1 # v out v ref b 1 j select r 1 to be approximately 1 k x , use a 1% resistor for best stability. component values shown are for v in e 20v, v out e 10v, i load e 1a. c in e 120 m f, 35v, aluminum electrolytic nichicon ``pl series'' c out e 120 m f, 35v aluminum electrolytic, nichicon ``pl series'' d1 e 3a, 40v schottky rectifier, 1n5822 l1 e 100 m h, l29 r 1 e1k x ,1% r 2 e 7.15k, 1% c ff e 3.3 nf, see application information sec- tion r ff e3k x , see application information sec- tion typical values c ss e0.1 m f c delay e0.1 m f r pull up e4.7k figure 1. standard test circuits and layout guides as in any switching regulator, layout is very important. rap- idly switching currents associated with wiring inductance can generate voltage transients which can cause problems. for minimal inductance and ground loops, the wires indicat- ed by heavy lines should be wide printed circuit traces and should be kept as short as possible. for best results, external components should be located as close to the switcher lc as possible using ground plane construction or single point grounding. if open core inductors are used, special care must be taken as to the location and positioning of this type of induc- tor. allowing the inductor flux to intersect sensitive feed- back, lc groundpath and c out wiring can cause problems. when using the adjustable version, special care must be taken as to the location of the feedback resistors and the associated wiring. physically locate both resistors near the ic, and route the wiring away from the inductor, especially an open core type of inductor. (see application section for more information.) http://www.national.com 8
lm2598 series buck regulator design procedure (fixed output) procedure (fixed output voltage version) example (fixed output voltage version) given: given: v out e regulated output voltage (3.3v, 5v or 12v) v out e 5v v in (max) e maximum dc input voltage v in (max) e 12v i load (max) e maximum load current i load (max) e 1a 1. inductor selection (l1) 1. inductor selection (l1) a. select the correct inductor value selection guide from a. use the inductor selection guide for the 5v version figures 4, 5, or 6. (output voltages of 3.3v, 5v, or 12v shown in figure 5. respectively.) for all other voltages, see the design pro- b. from the inductor value selection guide shown in fig- cedure for the adjustable version. ure 5, the inductance region intersected by the 12v hori- b. from the inductor value selection guide, identify the zontal line and the 1a vertical line is 68 m h, and the inductance region intersected by the maximum input inductor code is l30. voltage line and the maximum load current line. each c. the inductance value required is 68 m h. from the region is identified by an inductance value and an induc- table in figure 8, go to the l30 line and choose an induc- tor code (lxx). tor part number from any of the four manufacturers c. select an appropriate inductor from the four manufac- shown. (in most instance, both through hole and surface turer's part numbers listed in figure 8. mount inductors are available.) 2. output capacitor selection (c out ) 2. output capacitor selection (c out ) a. in the majority of applications, low esr (equivalent a. see section on output capacitors in application series resistance) electrolytic capacitors between information section. 47 m f and 330 m f and low esr solid tantalum capaci- b. from the quick design component selection table tors between 56 m f and 270 m f provide the best results. shown in figure 2, locate the 5v output voltage section. this capacitor should be located close to the ic using in the load current column, choose the load current line short capacitor leads and short copper traces. do not that is closest to the current needed in your application, use capacitors larger than 330 m f. for this example, use the 1a line. in the maximum input for additional information, see section on output ca- voltage column, select the line that covers the input volt- pacitors in application information section. age needed in your application, in this example, use the 15v line. continuing on this line are recommended in- b. to simplify the capacitor selection procedure, refer to ductors and capacitors that will provide the best overall the quick design component selection table shown in performance. figure 2. this table contains different input voltages, out- put voltages, and load currents, and lists various induc- the capacitor list contains both through hole electrolytic tors and output capacitors that will provide the best de- and surface mount tantalum capacitors from four differ- sign solutions. ent capacitor manufacturers. it is recommended that both the manufacturers and the manufacturer's series c. the capacitor voltage rating for electrolytic capacitors that are listed in the table be used. should be at least 1.5 times greater than the output volt- age, and often much higher voltage ratings are needed in this example aluminum electrolytic capacitors from to satisfy the low esr requirements for low output ripple several different manufacturers are available with the voltage. range of esr numbers needed. d. for computer aided design software, see switchers 220 m f 25v panasonic hfq series made simple (version 4.2 or later). 220 m f 25v nichicon pl series 3. catch diode selection (d1) c. for a 5v output, a capacitor voltage rating at least a. the catch diode current rating must be at least 1.3 7.5v or more is needed. but, in this example, even a low times greater than the maximum load current. also, if the esr, switching grade, 220 m f 10v aluminum electrolytic power supply design must withstand a continuous output capacitor would exhibit approximately 225 m x of esr short, the diode should have a current rating equal to the (see the curve in figure 16 for the esr vs voltage rat- maximum current limit of the lm2598. the most stressful ing). this amount of esr would result in relatively high condition for this diode is an overload or shorted output output ripple voltage. to reduce the ripple to 1% of the condition. output voltage, or less, a capacitor with a higher voltage rating (lower esr) should be selected. a 16v or 25v b. the reverse voltage rating of the diode should be at capacitor will reduce the ripple voltage by approximately least 1.25 times the maximum input voltage. half. c. this diode must be fast (short reverse recovery time) 3. catch diode selection (d1) and must be located close to the lm2598 using short leads and short printed circuit traces. because of their a. refer to the table shown in figure 11. in this example, fast switching speed and low forward voltage drop, a 3a, 20v, 1n5820 schottky diode will provide the best schottky diodes provide the best performance and effi- performance, and will not be overstressed even for a ciency, and should be the first choice, especially in low shorted output. output voltage applications. ultra-fast recovery, or high- procedure continued on next page. example continued on next page. http://www.national.com 9
lm2598 series buck regulator design procedure (fixed output) (continued) procedure (fixed output voltage version) example (fixed output voltage version) efficiency rectifiers also provide good results. ultra-fast 4. input capacitor (c in ) recovery diodes typically have reverse recovery times of the important parameters for the input capacitor are the 50 ns or less. rectifiers such as the 1n5400 series are input voltage rating and the rms current rating. with a much too slow and should not be used. nominal input voltage of 12v, an aluminum electrolytic 4. input capacitor (c in ) capacitor with a voltage rating greater than 18v (1.5 c v in ) would be needed. the next higher capacitor voltage a low esr aluminum or tantalum bypass capacitor is rating is 25v. needed between the input pin and ground to prevent large voltage transients from appearing at the input. in the rms current rating requirement for the input capaci- addition, the rms current rating of the input capacitor tor in a buck regulator is approximately (/2 the dc load should be selected to be at least (/2 the dc load current. current. in this example, with a 1a load, a capacitor with the capacitor manufacturers data sheet must be a rms current rating of at least 500 ma is needed. the checked to assure that this current rating is not exceed- curves shown in figure 15 can be used to select an ap- ed. the curve shown in figure 15 shows typical rms propriate input capacitor. from the curves, locate the current ratings for several different aluminum electrolytic 25v line and note which capacitor values have rms cur- capacitor values. rent ratings greater than 500 ma. either a 180 m for 220 m f, 25v capacitor could be used. this capacitor should be located close to the ic using short leads and the voltage rating should be approxi- for a through hole design, a 220 m f/25v electrolytic mately 1.5 times the maximum input voltage. capacitor (panasonic hfq series or nichicon pl series or equivalent) would be adequate. other types or other if solid tantalum input capacitors are used, it is recom- manufacturers capacitors can be used provided the ended that they be surge current tested by the manufac- rms ripple current ratings are adequate. turer. for surface mount designs, solid tantalum capacitors are use caution when using ceramic capacitors for input by- recommended. the tps series available from avx, and passing, because it may cause severe ringing at the v in the 593d series from sprague are both surge current pin. tested. for additional information, see section on input ca- pacitors in application information section. conditions inductor output capacitor through hole electrolytic surface mount tantalum output load max input inductance inductor panasonic nichicon avx tps sprague voltage current voltage ( m h) ( y ) hfq series pl series series 595d series (v) (a) (v) ( m f/v) ( m f/v) ( m f/v) ( m f/v) 3.3 1 5 22 l24 330/16 330/16 220/10 330/10 7 33 l23 270/25 270/25 220/10 270/10 10 47 l31 220/25 220/35 220/10 220/10 40 68 l30 180/35 220/35 220/10 180/10 6 47 l13 220/25 220/16 220/16 220/10 0.5 10 68 l21 150/35 150/25 100/16 150/16 40 100 l20 150/35 82/35 100/16 100/20 5 1 8 33 l28 330/16 330/16 220/10 270/10 10 47 l31 220/25 220/25 220/10 220/10 15 68 l30 180/35 180/35 220/10 150/16 40 100 l29 180/35 120/35 100/16 120/16 9 68 l21 180/16 180/16 220/10 150/16 0.5 20 150 l19 120/25 120/25 100/16 100/20 40 150 l19 100/25 100/25 68/20 68/25 12 1 15 47 l31 220/25 220/25 68/20 120/20 18 68 l30 180/35 120/25 68/20 120/20 30 150 l36 82/25 82/25 68/20 100/20 40 220 l35 82/25 82/25 68/20 68/25 15 68 l21 180/25 180/25 68/20 120/20 0.5 20 150 l19 82/25 82/25 68/20 100/20 40 330 l26 56/25 56/25 68/20 68/25 figure 2. lm2598 fixed voltage quick design component selection table http://www.national.com 10
lm2598 series buck regulator design procedure (adjustable output) procedure (adjustable output voltage version) example (adjustable output voltage version) given: given: v out e regulated output voltage v out e 20v v in (max) e maximum input voltage v in (max) e 28v i load (max) e maximum load current i load (max) e 1a f e switching frequency (fixed at a nominal 150 khz). f e switching frequency (fixed at a nominal 150 khz). 1. programming output voltage (selecting r 1 and r 2 ,as 1. programming output voltage (selecting r 1 and r 2 ,as shown in figure 1 ) shown in figure 1 ) use the following formula to select the appropriate resis- select r 1 to be 1 k x , 1%. solve for r 2 . tor values. r 2 e r 1 # v out v ref b 1 j e 1k # 20v 1.23v b 1 j v out e v ref # 1 a r 2 r 1 j where v ref e 1.23v r 2 e 1k (16.26 b 1) e 15.26k, closest 1% value is select a value for r 1 between 240 x and 1.5 k x . the 15.4 k x . lower resistor values minimize noise pickup in the sensi- r 2 e 15.4 k x . tive feedback pin. (for the lowest temperature coeffi- cient and the best stability with time, use 1% metal film resistors.) r 2 e r 1 # v out v ref b 1 j 2. inductor selection (l1) 2. inductor selection (l1) a. calculate the inductor volt # microsecond constant a. calculate the inductor volt # microsecond constant e # t(v # m s), from the following formula: (e # t), e # t e (v in b v out b v sat ) # v out a v d v in b v sat a v d # 1000 150 khz (v # m s) e # t e (28 b 20 b 1) # 20 a 0.5 28 b 1 a 0.5 # 1000 150 (v # m s) where v sat e internal switch saturation voltage e 1v e # t e (7) # 20.5 27.6 # 6.67 (v # m s) e 34.8 (v # m s) and v d e diode forward voltage drop e 0.5v b. use the e # t value from the previous formula and b. e # t e 34.8 (v # m s) match it with the e # t number on the vertical axis of the c. i load (max) e 1a inductor value selection guide shown in figure 7. d. from the inductor value selection guide shown in fig- c. on the horizontal axis, select the maximum load cur- ure 7, the inductance region intersected by the 35 (v # rent. m s) horizontal line and the 1a vertical line is 100 m h, and d. identify the inductance region intersected by the e # t the inductor code is l29. value and the maximum load current value. each region e. from the table in figure 8, locate line l29, and select is identified by an inductance value and an inductor code an inductor part number from the list of manufacturers (lxx). part numbers. e. select an appropriate inductor from the four manufac- turer's part numbers listed in figure 8. 3. output capacitor selection (c out ) 3. output capacitor seiection (c out ) a. in the majority of applications, low esr electrolytic or a. see section on c out in application information sec- solid tantalum capacitors between 82 m f and 220 m f tion. provide the best results. this capacitor should be locat- b. from the quick design table shown in figure 3, locate ed close to the ic using short capacitor leads and short the output voltage column. from that column, locate the copper traces. do not use capacitors larger than 220 m f. output voltage closest to the output voltage in your appli- for additional information, see section on output ca- cation. in this example, select the 24v line. under the pacitors in application information section. output capacitor section, select a capacitor from the list b. to simplify the capacitor selection procedure, refer to of through hole electrolytic or surface mount tantalum the quick design table shown in figure 3. this table con- types from four different capacitor manufacturers. it is tains different output voltages, and lists various output recommended that both the manufacturers and the man- capacitors that will provide the best design solutions. ufacturers series that are listed in the table be used. c. the capacitor voltage rating should be at least 1.5 in this example, through hole aluminum electrolytic ca- times greater than the output voltage, and often much pacitors from several different manufacturers are avail- higher voltage ratings are needed to satisfy the low esr able. requirements needed for low output ripple voltage. 82 m f 35v panasonic hfq series 82 m f 35v nichicon pl series procedure continued on next page. example continued on next page. http://www.national.com 11
lm2598 series buck regulator design procedure (adjustable output) procedure (adjustable output voltage version) example (adjustable output voltage version) 4. feedforward capacitor (c ff ) (see figure 1 ) c. for a 20v output, a capacitor rating of at least 30v or more is needed. in this example, either a 35v or 50v for output voltages greater than approximately 10v, an capacitor would work. a 35v rating was chosen although additional capacitor is required. the compensation ca- a 50v rating could also be used if a lower output ripple pacitor is typically between 50 pf and 10 nf, and is wired voltage is needed. in parallel with the output voltage setting resistor, r 2 .it provides additional stability for high output voltages, low other manufacturers or other types of capacitors may input-output voltages, and/or very low esr output ca- also be used, provided the capacitor specifications (es- pacitors, such as solid tantalum capacitors. pecially the 100 khz esr) closely match the types listed in the table. refer to the capacitor manufacturers data c ff e 1 31 c 10 3 c r 2 sheet for this information. 4. feedforward capacitor (c ff ) this capacitor type can be ceramic, plastic, silver mica, the table shown in figure 3 contains feed forward ca- etc. (because of the unstable characteristics of ceramic pacitor values for various output voltages. in this exam- capacitors made with z5u material, they are not recom- ple,a1nf capacitor is needed. mended.) 5. catch diode selection (d1) 5. catch diode selection (d1) a. refer to the table shown in figure 11. schottky diodes a. the catch diode current rating must be at least 1.3 provide the best performance, and in this example a 3a, times greater than the maximum load current. also, if the 40v, 1n5822 schottky diode would be a good choice. power supply design must withstand a continuous output the 3a diode rating is more than adequate and will not short, the diode should have a current rating equal to the be overstressed even for a shorted output. maximum current limit of the lm2598. the most stressful condition for this diode is an overload or shorted output 6. input capacitor (c in ) condition. the important parameters for the input capacitor are the b. the reverse voltage rating of the diode should be at input voltage rating and the rms current rating. with a least 1.25 times the maximum input voltage. nominal input voltage of 28v, an aluminum electrolytic aluminum electrolytic capacitor with a voltage rating c. this diode must be fast (short reverse recovery time) greater than 42v (1.5 c v in ) would be needed. since and must be located close to the lm2598 using short the the next higher capacitor voltage rating is 50v, a 50v leads and short printed circuit traces. because of their capacitor should be used. the capacitor voltage rating of fast switching speed and low forward voltage drop, (1.5 c v in ) is a conservative guideline, and can be modi- schottky diodes provide the best performance and effi- fied somewhat if desired. ciency, and should be the first choice, especially in low output voltage applications. ultra-fast recovery, or high- the rms current rating requirement for the input capaci- efficiency rectifiers are also a good choice, but some tor of a buck regulator is approximately (/2 the dc load types with an abrupt turn-off characteristic may cause current. in this example, with a 1a load, a capacitor with instability or eml problems. ultra-fast recovery diodes a rms current rating of at least 500 ma is needed. typically have reverse recovery times of 50 ns or less. the curves shown in figure 15 can be used to select an rectifiers such as the 1n4001 series are much too slow appropriate input capacitor. from the curves, locate the and should not be used. 50v line and note which capacitor values have rms cur- 6. input capacitor (c in ) rent ratings greater than 500 ma. either a 100 m for 120 m f, 50v capacitor could be used. a low esr aluminum or tantalum bypass capacitor is needed between the input pin and ground to prevent for a through hole design, a 120 m f/50v electrolytic large voltage transients from appearing at the input. in capacitor (panasonic hfq series or nichicon pl series addition, the rms current rating of the input capacitor or equivalent) would be adequate. other types or other should be selected to be at least (/2 the dc load current. manufacturers capacitors can be used provided the the capacitor manufacturers data sheet must be rms ripple current ratings are adequate. checked to assure that this current rating is not exceed- for surface mount designs, solid tantalum capacitors ed. the curve shown in figure 15 shows typical rms can be used, but caution must be exercised with regard current ratings for several different aluminum electrolytic to the capacitor surge current rating (see application in- capacitor values. formation or input capacitors in this data sheet). the tps this capacitor should be located close to the ic using series available from avx, and the 593d series from short leads and the voltage rating should be approxi- sprague are both surge current tested. mately 1.5 times the maximum input voltage. to further simplify the buck regulator design procedure, na- if solid tantalum input capacitors are used, it is recom- tional semiconductor is making available computer design ended that they be surge current tested by the manufac- software to be used with the simple switcher line ot switch- turer. ing regulators. switchers made simple (version 4.2 or later) is available on a 3 (/2 diskette for ibm compatible use caution when using a high dielectric constant ce- computers. ramic capacitor for input bypassing, because it may cause severe ringing at the v in pin. for additional information, see section on input capaci- tor in application information section. http://www.national.com 12
lm2598 series buck regulator design procedure (adjustable output) (continued) voltage output (v) through hole electrolytic output capacitor surface mount tantalum output capacitor panasonic nichicon pl feedforward avx tps sprague feedforward hfq series series capacitor series 595d series capacitor ( m f/v) ( m f/v) ( m f/v) ( m f/v) 1.2 330/50 330/50 0 330/6.3 330/6.3 0 4 220/25 220/25 4.7 nf 220/10 220/10 4.7 nf 6 220/25 220/25 3.3 nf 220/10 220/10 3.3 nf 9 180/25 180/25 1.5 nf 100/16 180/16 1.5 nf 12 120/25 120/25 1.5 nf 68/20 120/20 1.5 nf 15 120/25 120/25 1.5 nf 68/20 100/20 1.5 nf 24 82/35 82/35 1 nf 33/25 33/35 220 pf 28 82/50 82/50 1 nf 10/35 33/35 220 pf figure 3. output capacitor and feedforward capacitor selection table lm2598 series buck regulator design procedure inductor value selection guides (for continuous mode operation) tl/h/12593 25 figure 4. lm2598-3.3 tl/h/12593 26 figure 5. lm2598-5.0 tl/h/12593 27 figure 6. LM2598-12 tl/h/12593 28 figure 7. lm2598-adj http://www.national.com 13
lm2598 series buck regulator design procedure (continued) inductance ( m h) current (a) schott renco pulse engineering coilcraft through surface through surface through surface surface hole mount hole mount hole mount mount l4 68 0.32 67143940 67144310 rl-1284-68-43 rl1500-68 pe-53804 pe-53804-s do1608-68 l5 47 0.37 67148310 67148420 rl-1284-47-43 rl1500-47 pe-53805 pe-53805-s do1608-473 l6 33 0.44 67148320 67148430 rl-1284-33-43 rl1500-33 pe-53806 pe-53806-s do1608-333 l9 220 0.32 67143960 67144330 rl-5470-3 rl1500-220 pe-53809 pe-53809-s do3308-224 l10 150 0.39 67143970 67144340 rl-5470-4 rl1500-150 pe-53810 pe-53810-s do3308-154 l11 100 0.48 67143980 67144350 rl-5470-5 rl1500-100 pe-53811 pe-53811-s do3308-104 l12 68 0.58 67143990 67144360 rl-5470-6 rl1500-68 pe-53812 pe-53812-s do3308-683 l13 47 0.70 67144000 67144380 rl-5470-7 rl1500-47 pe-53813 pe-53813-s do3308-473 l14 33 0.83 67148340 67148450 rl-1284-33-43 rl1500-33 pe-53814 pe-53814-s do3308-333 l15 22 0.99 67148350 67148460 rl-1284-22-43 rl1500-22 pe-53815 pe-53815-s do3308-223 l16 15 1.24 67148360 67148470 rl-1284-15-43 rl1500-15 pe-53816 pe-53816-s do3308-153 l17 330 0.42 67144030 67144410 rl-5471-1 rl1500-330 pe-53817 pe-53817-s do3316-334 l18 220 0.55 67144040 67144420 rl-5471-2 rl1500-220 pe-53818 pe-53818-s do3316-224 l19 150 0.66 67144050 67144430 rl-5471-3 rl1500-150 pe-53819 pe-53819-s do3316-154 l20 100 0.82 67144060 67144440 rl-5471-4 rl1500-100 pe-53820 pe-53820-s do3316-104 l21 68 0.99 67144070 67144450 rl-5471-5 rl1500-68 pe-53821 pe-53821-s do3316-683 l22 47 1.17 67144080 67144460 rl-5471-6 e pe-53822 pe-53822-s do3316-473 l23 33 1.40 67144090 67144470 rl-5471-7 e pe-53823 pe-53823-s do3316-333 l24 22 1.70 67148370 67144480 rl-1283-22-43 e pe-53824 pe-53824-s do3316-223 l26 330 0.80 67144100 67144480 rl-5471-1 e pe-53826 pe-53826-s do5022p-334 l27 220 1.00 67144110 67144490 rl-5471-2 e pe-53827 pe-53827-s do5022p-224 l28 150 1.20 67144120 67144500 rl-5471-3 e pe-53828 pe-53828-s do5022p-154 l29 100 1.47 67144130 67144510 rl-5471-4 e pe-53829 pe-53829-s do5022p-104 l30 68 1.78 67144140 67144520 rl-5471-5 e pe-53830 pe-53830-s do5022p-683 l35 47 2.15 67144170 e rl-5473-1 e pe-53935 pe-53935-s e figure 8. inductor manufacturers part numbers coilcraft inc. phone (800) 322-2645 fax (708) 639-1469 coilcraft inc., europe phone a 11 1236 730 595 fax a 44 1236 730 627 pulse engineering inc. phone (619) 674-8100 fax (619) 674-8262 pulse engineering inc., phone a 353 93 24 107 europe fax a 353 93 24 459 renco electronics inc. phone (800) 645-5828 fax (516) 586-5562 schott corp. phone (612) 475-1173 fax (612) 475-1786 figure 9. inductor manufacturers phone numbers nichicon corp. phone (708) 843-7500 fax (708) 843-2798 panasonic phone (714) 373-7857 fax (714) 373-7102 avx corp. phone (803) 448-9411 fax (803) 448-1943 sprague/vishay phone (207) 324-4140 fax (207) 324-7223 figure 10. capacitor manufacturers phone numbers http://www.national.com 14
lm2598 series buck regulator design procedure (continued) vr 1a diodes 3a diodes surface mount through hole surface mount through hole schottky ultra fast schottky ultra fast schottky ultra fast schottky ultra fast recovery recovery recovery recovery 20v sk12 all of these 1n5817 all of these all of in5820 all of diodes are diodes are these these sr102 sk32 sr302 rated to at rated to at diodes diodes least 50v. least 50v. are rated are rated mbr320 to at least to at least 30v sk13 1n5818 1n5821 50v. 50v. mbrs130 sr103 sk33 mbr330 11dq03 31dq03 40v sk14 1n5822 mbrs140 1n5819 sk34 sr304 10bq040 sr104 mbrs340 mbr340 10mq040 murs120 11dq04 mur120 30wq04 murs320 31dq04 mur320 more 50v or mbrs160 10bf10 sr105 sk35 30wf10 sr305 30wf10 10bq050 mbr150 mbrs360 mbr350 10mq060 11dq05 30wq05 31dq05 figure 11. diode selection table block diagram tl/h/12593 29 figure 12 http://www.national.com 15
application information pin functions a v in (pin 2)ethis is the positive input supply for the ic switching regulator. a suitable input bypass capacitor must be present at this pin to minimize voltage transients and to supply the switching currents needed by the regulator. ground (pin 4)ecircuit ground. output (pin 1)einternal switch. the voltage at this pin switches between approximately ( a v in b v sat ) and ap- proximately b 0.5v, with a duty cycle of v out /v in . to mini- mize coupling to sensitive circuitry, the pc board copper area connected to this pin should be kept to a minimum. feedback (pin 6)esenses the regulated output voltage to complete the feedback loop. shutdown /soft-start (pin 7)ethis dual function pin pro- vides the following features: (a) allows the switching regula- tor circuit to be shut down using logic level signals thus dropping the total input supply current to approximately 85 m a. (b) adding a capacitor to this pin provides a soft-start feature which minimizes startup current and provides a con- trolled ramp up of the output voltage. error flag (pin 3)eopen collector output that provides a low signal (flag transistor on) when the regulated output voltage drops more than 5% from the nominal output volt- age. on start up, error flag is low until v out reaches 95% of the nominal output voltage and a delay time determined by the delay pin capacitor. this signal can be used as a reset to a microprocessor on power-up. delay (pin 5)eat power-up, this pin can be used to provide a time delay between the time the regulated output voltage reaches 95% of the nominal output voltage, and the time the error flag output goes high. special note if any of the above three features (shutdown / soft-start, error flag, or delay) are not used, the respective pins should be left open. external components soft-start capacitor c ss ea capacitor on this pin provides the regulator with a soft-start feature (slow start-up). when the dc input voltage is first applied to the regulator, or when the shutdown /soft- start pin is allowed to go high, a constant current (approxi- mately 5 m a begins charging this capacitor). as the capaci- tor voltage rises, the regulator goes through four operating regions (see the bottom curve in figure 13 ). 1. regulator in shutdown. when the sd /ss pin voltage is between 0v and 1.3v, the regulator is in shutdown, the out- put voltage is zero, and the ic quiescent current is approxi- mately 85 m a. 2. regulator on, but the output voltage is zero. with the sd /ss pin voltage between approximately 1.3v and 1.8v, the internal regulator circuitry is operating, the quiescent current rises to approximately 5 ma, but the output voltage is still zero. also, as the 1.3v threshold is exceeded, the soft-start capacitor charging current decreases from 5 m a down to approximately 1.6 m a. this decreases the slope of capacitor voltage ramp. 3. soft-start region. when the sd /ss pin voltage is be- tween 1.8v and 2.8v ( @ 25 c), the regulator is in a soft- start condition. the switch (pin 1) duty cycle initially starts out very low, with narrow pulses and gradually get wider as the capacitor sd /ss pin ramps up towards 2.8v. as the duty cycle increases, the output voltage also increases at a controlled ramp up. see the center curve in figure 13 . the input supply current requirement also starts out at a low level for the narrow pulses and ramp up in a controlled man- ner. this is a very useful feature in some switcher topolo- gies that require large startup currents (such as the inverting configuration) which can load down the input power supply. note: the lower curve shown in figure 13 shows the soft-start region from 0% to 100%. this is not the duty cycle percentage, but the output voltage percentage. also, the soft-start voltage range has a negative temperature coefficient associated with it. see the soft-start curve in the electrical characteristics section. 4. normal operation. above 2.8v, the circuit operates as a standard pulse width modulated switching regulator. the capacitor will continue to charge up until it reaches the inter- nal clamp voltage of approximately 7v. if this pin is driven from a voltage source, the current must be limited to about 1 ma. tl/h/12593 30 figure 13. soft-start, delay, error, output http://www.national.com 16
application information (continued) tl/h/12593 31 figure 14. timing diagram for 5v output delay capacitor c delay eprovides delay for the error flag output. see the upper curve in figure 13 , and also refer to timing diagrams in figure 14 . a capacitor on this pin provides a time delay between the time the regulated output voltage (when it is increasing in value) reaches 95% of the nominal output volt- age, and the time the error flag output goes high. a 3 m a constant current from the delay pin charges the delay ca- pacitor resulting in a voltage ramp. when this voltage reach- es a threshold of approximately 1.3v, the open collector error flag output (or power ok) goes high. this signal can be used to indicate that the regulated output has reached the correct voltage and has stabilized. if, for any reason, the regulated output voltage drops by 5% or more, the error output flag (pin 3) immediately goes low (internal transistor turns on). the delay capacitor provides very little delay if the regulated output is dropping out of regulation. the delay time for an output that is decreasing is approximately a 1000 times less than the delay for the rising output. for a 0.1 m f delay capacitor, the delay time would be approximately 50 ms when the output is rising and pass- es through the 95% threshold, but the delay for the output dropping would only be approximately 50 m s. r pull up ethe error flag output, (or power ok) is the collec- tor of a npn transistor, with the emitter internally grounded. to use the error flag, a pullup resistor to a positive voltage is needed. the error flag transistor is rated up to a maximum of 45v and can sink approximately 3 ma. if the error flag is not used, it can be left open. feedforward capacitor (adjustable output voltage version) c ff e a feedforward capacitor c ff , shown across r2 in figure 1 is used when the output voltage is greater than 10v or then c out has a very low esr. this capacitor adds lead compensation to the feedback loop and increases the phase margin for better loop stability. for c ff selection, see the design procedure section. if the output ripple is large ( l 5% of the nominal output voltage), this ripple can be coupled to the feedback pin through the feedforward capacitor and cause the error com- parator to trigger the error flag. in this situation, adding a resistor, r ff , in series with the feedforward capacitor, ap- proximately 3 times r1, will attenuate the ripple voltage at the feedback pin. input capacitor c in ea low esr aluminum or tantalum bypass capacitor is needed between the input pin and ground pin. it must be located near the regulator using short leads. this capacitor prevents large voltage transients from appearing at the in- put, and provides the instantaneous current needed each time the switch turns on. the important parameters for the input capacitor are the voltage rating and the rms current rating. because of the relatively high rms currents flowing in a buck regulator's input capacitor, this capacitor should be chosen for its rms current rating rather than its capacitance or voltage ratings, although the capacitance value and voltage rating are di- rectly related to the rms current rating. the rms current rating of a capacitor could be viewed as a capacitor's power rating. the rms current flowing through the capacitors internal esr produces power which causes the internal temperature of the capacitor to rise. the rms current rating of a capacitor is determined by the amount of current required to raise the internal temperature approxi- mately 10 c above an ambient temperature of 105 c. the ability of the capacitor to dissipate this heat to the surround- ing air will determine the amount of current the capacitor can safely sustain. capacitors that are physically large and have a large surface area will typically have higher rms current ratings. for a given capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltage capacitor, and thus be able to dissipate more heat to the surrounding air, and therefore will have a higher rms current rating. http://www.national.com 17
application information (continued) tl/h/12593 32 figure 15. rms current ratings for low esr electrolytic capacitors (typical) tl/h/12593 33 figure 16. capacitor esr vs capacitor voltage rating (typical low esr electrolytic capacitor) the consequences of operating an electrolytic capacitor above the rms current rating is a shortened operating life. the higher temperature speeds up the evaporation of the capacitor's electrolyte, resulting in eventual failure. selecting an input capacitor requires consulting the manu- facturers data sheet for maximum allowable rms ripple cur- rent. for a maximum ambient temperature of 40 c, a gener- al guideline would be to select a capacitor with a ripple cur- rent rating of approximately 50% of the dc load current. for ambient temperatures up to 70 c, a current rating of 75% of the dc load current would be a good choice for a conserva- tive design. the capacitor voltage rating must be at least 1.25 times greater than the maximum input voltage, and of- ten a much higher voltage capacitor is needed to satisfy the rms current requirements. a graph shown in figure 15 shows the relationship between an electrolytic capacitor value, its voltage rating, and the rms current it is rated for. these curves were obtained from the nichicon ``pl'' series of low esr, high reliability electrolytic capacitors designed for switching regulator ap- plications. other capacitor manufacturers offer similar types of capacitors, but always check the capacitor data sheet. ``standard'' electrolytic capacitors typically have much high- er esr numbers, lower rms current ratings and typically have a shorter operating lifetime. because of their small size and excellent performance, sur- face mount solid tantalum capacitors are often used for in- put bypassing, but several precautions must be observed. a small percentage of solid tantalum capacitors can short if the inrush current rating is exceeded. this can happen at turn on when the input voltage is suddenly applied, and of course, higher input voltages produce higher inrush cur- rents. several capacitor manufacturers do a 100% surge current testing on their products to minimize this potential problem. if high turn on currents are expected, it may be necessary to limit this current by adding either some resist- ance or inductance before the tantalum capacitor, or select a higher voltage capacitor. as with aluminum electrolytic ca- pacitors, the rms ripple current rating must be sized to the load current. output capacitor c out ean output capacitor is required to filter the output and provide regulator loop stability. low impedance or low esr electrolytic or solid tantalum capacitors designed for switching regulator applications must be used. when select- ing an output capacitor, the important capacitor parameters are; the 100 khz equivalent series resistance (esr), the rms ripple current rating, voltage rating, and capacitance value. for the output capacitor, the esr value is the most important parameter. the output capacitor requires an esr value that has an upper and lower limit. for low output ripple voltage, a low esr value is needed. this value is determined by the maxi- mum allowable output ripple voltage, typically 1% to 2% of the output voltage. but if the selected capacitor's esr is extremely low, there is a possibility of an unstable feedback loop, resulting in an oscillation at the output. using the ca- pacitors listed in the tables, or similar types, will provide design solutions under all conditions. if very low output ripple voltage (less than 15 mv) is re- quired, refer to the section on output voltage ripple and transients for a post ripple filter. an aluminum electrolytic capacitor's esr value is related to the capacitance value and its voltage rating. in most cases, higher voltage electrolytic capacitors have lower esr val- ues (see figure 16 ). often, capacitors with much higher voltage ratings may be needed to provide the low esr val- ues required for low output ripple voltage. http://www.national.com 18
application information (continued) the output capacitor for many different switcher designs often can be satisfied with only three or four different capac- itor values and several different voltage ratings. see the quick design component selection tables in figures 2 and 3 for typical capacitor values, voltage ratings, and manufac- turers capacitor types. electrolytic capacitors are not recommended for tempera- tures below b 25 c. the esr rises dramatically at cold tem- peratures and typically rises 3x @ b 25 c and as much as 10x at b 40 c. see curve shown in figure 17. solid tantalum capacitors have a much better esr spec for cold temperatures and are recommended for temperatures below b 25 c. catch diode buck regulators require a diode to provide a return path for the inductor current when the switch turns off. this must be a fast diode and must be located close to the lm2598 using short leads and short printed circuit traces. because of their very fast switching speed and low forward voltage drop, schottky diodes provide the best perform- ance, especially in low output voltage applications (5v and lower). ultra-fast recovery, or high-efficiency rectifiers are also a good choice, but some types with an abrupt turnoff characteristic may cause instability or emi problems. ultra- fast recovery diodes typically have reverse recovery times of 50 ns or less. rectifiers such as the 1n5400 series are much too slow and should not be used. tl/h/12593 34 figure 17. capacitor esr change vs temperature inductor selection all switching regulators have two basic modes of operation; continuous and discontinuous. the difference between the two types relates to the inductor current, whether it is flow- ing continuously, or if it drops to zero for a period of time in the normal switching cycle. each mode has distinctively dif- ferent operating characteristics, which can affect the regula- tors performance and requirements. most switcher designs will operate in the discontinuous mode when the load cur- rent is low. the lm2598 (or any of the simple switcher family) can be used for both continuous or discontinuous modes of opera- tion. in many cases the preferred mode of operation is the con- tinuous mode. it offers greater output power, lower peak switch, inductor and diode currents, and can have lower out- put ripple voltage. but it does require larger inductor values to keep the inductor current flowing continuously, especially at low output load currents and/or high input voltages. to simplify the inductor selection process, an inductor se- lection guide (nomograph) was designed (see figures 3 through 6 ). this guide assumes that the regulator is operat- ing in the continuous mode, and selects an inductor that will allow a peak-to-peak inductor ripple current to be a certain percentage of the maximum design load current. this peak- to-peak inductor ripple current percentage is not fixed, but is allowed to change as different design load currents are se- lected. (see figure 18 .) tl/h/12593 35 figure 18. ( d i ind ) peak-to-peak inductor ripple current (as a percentage of the load current) vs load current by allowing the percentage of inductor ripple current to in- crease for low load currents, the inductor value and size can be kept relatively low. when operating in the continuous mode, the inductor cur- rent waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage), with the aver- age value of this current waveform equal to the dc output load current. inductors are available in different styles such as pot core, toroid, e-core, bobbin core, etc., as well as different core materials, such as ferrites and powdered iron. the least ex- pensive, the bobbin, rod or stick core, consists of wire wound on a ferrite bobbin. this type of construction makes for an inexpensive inductor, but since the magnetic flux is not completely contained within the core, it generates more electro-magnetic interference (eml). this magnetic flux can induce voltages into nearby printed circuit traces, thus caus- ing problems with both the switching regulator operation and nearby sensitive circuitry, and can give incorrect scope readings because of induced voltages in the scope probe. also see section on open core inductors. when multiple switching regulators are located on the same pc board, open core magnetics can cause interference be- tween two or more of the regulator circuits, especially at high currents. a torroid or e-core inductor (closed magnetic structure) should be used in these situations. the inductors listed in the selection chart include ferrite e-core construction for schott, ferrite bobbin core for renco and coilcraft, and powdered iron toroid for pulse engineer- ing. exceeding an inductor's maximum current rating may cause the inductor to overheat because of the copper wire losses, or the core may saturate. if the inductor begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive (the dc resistance of the winding). this can cause the switch current to rise very rapidly and force the switch into a cycle-by-cycle current limit, thus reducing http://www.national.com 19
application information (continued) the dc output load current. this can also result in overheat- ing of the inductor and/or the lm2598. different inductor types have different saturation characteristics, and this should be kept in mind when selecting an inductor. the inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation. discontinuous mode operation the selection guide chooses inductor values suitable for continuous mode operation, but for low current applications and/or high input voltages, a discontinuous mode design may be a better choice. it would use an inductor that would be physically smaller, and would need only one half to one third the inductance value needed for a continuous mode design. the peak switch and inductor currents will be higher in a discontinuous design, but at these low load currents (200 ma and below), the maximum switch current will still be less than the switch current limit. discontinuous operation can have voltage waveforms that are considerable different than a continuous design. the output pin (switch) waveform can have some damped sinus- oidal ringing present. (see photo titled; discontinuous mode switching waveforms) this ringing is normal for discontinu- ous operation, and is not caused by feedback loop instabili- ties. in discontinuous operation, there is a period of time where neither the switch or the diode are conducting, and the inductor current has dropped to zero. during this time, a small amount of energy can circulate between the inductor and the switch/diode parasitic capacitance causing this characteristic ringing. normally this ringing is not a problem, unless the amplitude becomes great enough to exceed the input voltage, and even then, there is very little energy pres- ent to cause damage. different inductor types and/or core materials produce dif- ferent amounts of this characteristic ringing. ferrite core in- ductors have very little core loss and therefore produce the most ringing. the higher core loss of powdered iron induc- tors produce less ringing. if desired, a series rc could be placed in parallel with the inductor to dampen the ringing. the computer aided design software switchers made sim- ple (version 4.2) will provide all component values for con- tinuous and discontinuous modes of operation. tl/h/12593 36 figure 19. post ripple filter waveform output voltage ripple and transients the output voltage of a switching power supply operating in the continuous mode will contain a sawtooth ripple voltage at the switcher frequency, and may also contain short volt- age spikes at the peaks of the sawtooth waveform. the output ripple voltage is a function of the inductor saw- tooth ripple current and the esr of the output capacitor. a typical output ripple voltage can range from approximately 0.5% to 3% of the output voltage. to obtain low ripple volt- age, the esr of the output capacitor must be low, however, caution must be exercised when using extremely low esr capacitors because they can affect the loop stability, result- ing in oscillation problems. if very low output ripple voltage is needed (less than 20 mv), a post ripple filter is recom- mended. (see figure 1 .) the inductance required is typically between 1 m h and 5 m h, with low dc resistance, to main- tain good load regulation. a low esr output filter capacitor is also required to assure good dynamic load response and ripple reduction. the esr of this capacitor may be as low as desired, because it is out of the regulator feedback loop. the photo shown in figure 19 shows a typical output ripple voltage, with and without a post ripple filter. when observing output ripple with a scope, it is essential that a short, low inductance scope probe ground connection be used. most scope probe manufacturers provide a special probe terminator which is soldered onto the regulator board, preferable at the output capacitor. this provides a very short scope ground thus eliminating the problems associat- ed with the 3 inch ground lead normally provided with the probe, and provides a much cleaner and more accurate pic- ture of the ripple voltage waveform. the voltage spikes are caused by the fast switching action of the output switch, the diode, and the parasitic inductance of the output filter capacitor, and its associated wiring. to minimize these voltage spikes, the output capacitor should be designed for switching regulator applications, and the lead lengths must be kept very short. wiring inductance, stray capacitance, as well as the scope probe used to eval- uate these transients, all contribute to the amplitude of these spikes. tl/h/12593 37 figure 20. peak-to-peak inductor ripple current vs load current http://www.national.com 20
application information (continued) when a switching regulator is operating in the continuous mode, the inductor current waveform ranges from a triangu- lar to a sawtooth type of waveform (depending on the input voltage). for a given input and output voltage, the peak-to- peak amplitude of this inductor current waveform remains constant. as the load current increases or decreases, the entire sawtooth current waveform also rises and falls. the average value (or the center) of this current waveform is equal to the dc load current. if the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and the switcher will smoothly change from a continuous to a dis- continuous mode of operation. most switcher designs (irre- gardless how large the inductor value is) will be forced to run discontinuous if the output is lightly loaded. this is a perfectly acceptable mode of operation. in a switching regulator design, knowing the value of the peak-to-peak inductor ripple current ( d i ind ) can be useful for determining a number of other circuit parameters. pa- rameters such as, peak inductor or peak switch current, minimum load current before the circuit becomes discontin- uous, output ripple voltage and output capacitor esr can all be calculated from the peak-to-peak d i ind . when the induc- tor nomographs shown in figures 4 through 7 are used to select an inductor value, the peak-to-peak inductor ripple current can immediately be determined. the curve shown in figure 20 shows the range of ( d i ind ) that can be expected for different load currents. the curve also shows how the peak-to-peak inductor ripple current ( d i ind ) changes as you go from the lower border to the upper border (for a given load current) within an inductance region. the upper border represents a higher input voltage, while the lower border represents a lower input voltage (see inductor selection guides). these curves are only correct for continuous mode opera- tion, and only if the inductor selection guides are used to select the inductor value consider the following example: v out e 5v, maximum load current of 800 ma v in e 12v, nominal, varying between 10v and 14v. the selection guide in figure 5 shows that the vertical line for a 0.8a load current, and the horizontal line for the 12v input voltage intersect approximately midway between the upper and lower borders of the 68 m h inductance region. a 68 m h inductor will allow a peak-to-peak inductor current ( d i ind ) to flow that will be a percentage of the maximum load current. referring to figure 20, follow the 0.8a line approximately midway into the inductance region, and read the peak-to-peak inductor ripple current ( d i ind ) on the left hand axis (approximately 300 ma p-p). as the input voltage increases to 14v, it approaches the upper border of the inductance region, and the inductor rip- ple current increases. referring to the curve in figure 20, it can be seen that for a load current of 0.8a, the peak-to- peak inductor ripple current ( d i ind ) is 300 ma with 12v in, and can range from 340 ma at the upper border (14v in) to 225 ma at the lower border (10v in). once the d i ind value is known, the following formulas can be used to calculate additional information about the switch- ing regulator circuit. 1. peak inductor or peak switch current e # i load a d i ind 2 j e # 0.8a a 0.3 2 j e 0.95a 2. minimum load current before the circuit becomes discon- tinuous e d i ind 2 e 0.3 2 e 0.15a 3. output ripple voltage e ( d i ind ) c (esr of c out ) e 0.3a c 0.16 x e 48 mv p-p 4. esr of c out e output ripple voltage ( d v out ) d i ind e 0.048v 0.30a e 0.16 x open core inductors another possible source of increased output ripple voltage or unstable operation is from an open core inductor. ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end. these magnetic lines of flux will induce a voltage into any wire or pc board copper trace that comes within the induc- tor's magnetic field. the strength of the magnetic field, the orientation and location of the pc copper trace to the mag- netic field, and the distance between the copper trace and the inductor, determine the amount of voltage generated in the copper trace. another way of looking at this inductive coupling is to consider the pc board copper trace as one turn of a transformer (secondary) with the inductor winding as the primary. many millivolts can be generated in a copper trace located near an open core inductor which can cause stability problems or high output ripple voltage problems. if unstable operation is seen, and an open core inductor is used, it's possible that the location of the inductor with re- spect to other pc traces may be the problem. to determine if this is the problem, temporarily raise the inductor away from the board by several inches and then check circuit operation. if the circuit now operates correctly, then the magnetic flux from the open core inductor is causing the problem. substituting a closed core inductor such as a tor- roid or e-core will correct the problem, or re-arranging the pc layout may be necessary. magnetic flux cutting the ic device ground trace, feedback trace, or the positive or neg- ative traces of the output capacitor should be minimized. sometimes, locating a trace directly beneath a bobbin in- ductor will provide good results, provided it is exactly in the center of the inductor (because the induced voltages cancel themselves out), but if it is off center one direction or the other, then problems could arise. if flux problems are pres- ent, even the direction of the inductor winding can make a difference in some circuits. this discussion on open core inductors is not to frighten the user, but to alert the user on what kind of problems to watch out for when using them. open core bobbin or ``stick'' induc- tors are an inexpensive, simple way of making a compact efficient inductor, and they are used by the millions in many different applications. http://www.national.com 21
application information (continued) tl/h/12593 38 circuit data for temperature rise curve to-220 package (t) capacitors through hole electrolytic inductor through hole, schott, 68 m h diode through hole, 3a 40v, schottky pc board 3 square inches single sided 2 oz. copper (0.0028 ) figure 21. junction temperature rise, to-220 tl/h/12593 39 circuit data for temperature rise curve to-263 package (s) capacitors surface mount tantalum, molded ``d'' size inductor surface mount, schott, 68 m h diode surface mount, 3a 40v, schottky pc board 3 square inches single sided 2 oz. copper (0.0028 ) figure 22. junction temperature rise, to-263 thermal considerations the lm2598 is available in two packages, a 7-pin to-220 (t) and a 7-pin surface mount to-263 (s). the to-220 package can be used without a heat sink for ambient temperatures up to approximately 50 c (depending on the output voltage and load current). the curves in fig- ure 21 show the lm2598t junction temperature rises above ambient temperature for different input and output voltages. the data for these curves was taken with the lm2598t (to- 220 package) operating as a switching regulator in an ambi- ent temperature of 25 c (still air). these temperature rise numbers are all approximate and there are many factors that can affect these temperatures. higher ambient temper- atures require some heat sinking, either to the pc board or a small external heat sink. the to-263 surface mount package tab is designed to be soldered to the copper on a printed circuit board. the cop- per and the board are the heat sink for this package and the other heat producing components, such as the catch diode and inductor. the pc board copper area that the package is soldered to should be at least 0.4 in 2 , and ideally should have 2 or more square inches of 2 oz. (0.0028) in) copper. additional copper area improves the thermal characteristics, but with copper areas greater than approximately 3 in 2 , only small improvements in heat dissipation are realized. if fur- ther thermal improvements are needed, double sided or multilayer pc-board with large copper areas are recom- mended. the curves shown in figure 22 show the lm2598s (to-263 package) junction temperature rise above ambient tempera- ture with a 1a load for various input and output voltages. this data was taken with the circuit operating as a buck switching regulator with all components mounted on a pc board to simulate the junction temperature under actual operating conditions. this curve can be used for a quick check for the approximate junction temperature for various conditions, but be aware that there are many factors that can affect the junction temperature. for the best thermal performance, wide copper traces and generous amounts of printed circuit board copper should be used in the board layout. (one exception to this is the output (switch) pin, which should not have large areas of copper.) large areas of copper provide the best transfer of heat (low- er thermal resistance) to the surrounding air, and moving air lowers the thermal resistance even further. package thermal resistance and junction temperature rise numbers are all approximate, and there are many factors that will affect these numbers. some of these factors in- clude board size, shape, thickness, position, location, and even board temperature. other factors are, trace width, total printed circuit copper area, copper thickness, single- or dou- ble-sided, multilayer board and the amount of solder on the board. the effectiveness of the pc board to dissipate heat also depends on the size, quantity and spacing of other components on the board, as well as whether the surround- ing air is still or moving. furthermore, some of these compo- nents such as the catch diode will add heat to the pc board and the heat can vary as the input voltage changes. for the inductor, depending on the physical size, type of core mate- rial and the dc resistance, it could either act as a heat sink taking heat away from the board, or it could add heat to the board. http://www.national.com 22
application information (continued) shutdown /soft-start the circuit shown in figure 23 is a standard buck regulator with 24v in, 12v out, 280 ma load, and using a 0.068 m f soft-start capacitor. the photo in figures 24 and 25 show the effects of soft-start on the output voltage, the input cur- rent, with, and without a soft-start capacitor. figure 24 also shows the error flag output going high when the output volt- age reaches 95% of the nominal output voltage. the re- duced input current required at startup is very evident when comparing the two photos. the soft-start feature reduces the startup current from 1a down to 240 ma, and delays and slows down the output voltage rise time. this reduction in start up current is useful in situations where the input power source is limited in the amount of current it can deliver. in some applications soft-start can be used to replace undervoltage lockout or delayed startup functions. if a very slow output voltage ramp is desired, the soft-start capacitor can be made much larger. many seconds or even minutes are possible. if only the shutdown feature is needed, the soft-start capac- itor can be eliminated. tl/h/12593 40 figure 24. output voltage, input current, error flag signal, at start-up, with soft-start tl/h/12593 41 figure 25. output voltage, input current, at start-up, without soft-start tl/h/12593 42 figure 23. typical circuit using shutdown /soft-start and error flag features http://www.national.com 23
application information (continued) tl/h/12593 43 figure 26. inverting b 5v regulator with shutdown and soft-start lnverting regulator the circuit in figure 26 converts a positive input voltage to a negative output voltage with a common ground. the circuit operates by bootstrapping the regulators ground pin to the negative output voltage, then grounding the feedback pin, the regulator senses the inverted output voltage and regu- lates it. this example uses the lm2598-5 to generate a b 5v out- put, but other output voltages are possible by selecting oth- er output voltage versions, including the adjustable version. since this regulator topology can produce an output voltage that is either greater than or less than the input voltage, the maximum output current greatly depends on both the input and output voltage. the curve shown in figure 27 provides a guide as to the amount of output load current possible for the different input and output voltage conditions. the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and this must be limited to a maximum of 40v. in this example, when con- verting a 20v to b 5v, the regulator would see 25v be- tween the input pin and ground pin. the lm2598 has a max- imum input voltage rating of 40v. tl/h/12593 44 figure 27. maximum load current for inverting regulator circuit an additional diode is required in this regulator configura- tion. diode d1 is used to isolate input voltage ripple or noise from coupling through the c in capacitor to the output, under light or no load conditions. also, this diode isolation chang- es the topology to closely resemble a buck configuration thus providing good closed loop stability. a schottky diode is recommended for low input voltages, (because of its low- er voltage drop) but for higher input voltages, a 1n5400 diode could be used. because of differences in the operation of the inverting reg- ulator, the standard design procedure is not used to select the inductor value. in the majority of designs, a 68 m h, 1.5 amp inductor is the best choice. capacitor selection can also be narrowed down to just a few values. using the val- ues shown in figure 26 will provide good results in the ma- jority of inverting designs. this type of inverting regulator can require relatively large amounts of input current when starting up, even with light loads. input currents as high as the lm2598 current limit (approximately 1.5a) are needed for 2 ms or more, until the output reaches its nominal output voltage. the actual time depends on the output voltage and the size of the output capacitor. input power sources that are current limited or sources that can not deliver these currents without getting loaded down, may not work correctly. because of the rela- tively high startup currents required by the inverting topolo- gy, the soft-start feature shown in figure 26 is recommend- ed. also shown in figure 26 are several shutdown methods for the inverting configuration. with the inverting configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but is now at the nega- tive output voltage. the shutdown methods shown accept ground referenced shutdown signals. http://www.national.com 24
application information (continued) undervoltage lockout some applications require the regulator to remain off until the input voltage reaches a predetermined voltage. figure 28 contains a undervoltage lockout circuit for a buck config- uration, while figures 29 and 30 are for the inverting types (only the circuitry pertaining to the undervoltage lockout is shown). figure 28 uses a zener diode to establish the threshold voltage when the switcher begins operating. when the input voltage is less than the zener voltage, resis- tors r1 and r2 hold the shutdown /soft-start pin low, keep- ing the regulator in the shutdown mode. as the input voltage exceeds the zener voltage, the zener conducts, pulling the shutdown /soft-start pin high, allowing the regulator to be- gin switching. the threshold voltage for the undervoltage lockout feature is approximately 1.5v greater than the zener voltage. tl/h/12593 45 figure 28. undervoltage lockout for a buck regulator figures 29 and 30 apply the same feature to an inverting circuit. figure 29 features a constant threshold voltage for turn on and turn off (zener voltage plus approximately one volt). since the sd /ss pin has an internal 7v zener clamp, r2 is needed to limit the current into this pin to approximate- ly 1 ma when q1 is on. if hysteresis is needed, the circuit in figure 30 has a turn on voltage which is different than the turn off voltage. the amount of hysteresis is approximate- ly equal to the value of the output voltage. tl/h/12593 47 figure 29. undervoltage lockout without hysteresis for an inverting regulator tl/h/12593 46 figure 30. undervoltage lockout with hysteresis for an inverting regulator negative voltage charge pump occasionally a low current negative voltage is needed for biasing parts of a circuit. a simple method of generating a negative voltage using a charge pump technique and the switching waveform present at the out pin, is shown in figure 31. this unregulated negative voltage is approxi- mately equal to the positive input voltage (minus a few volts), and can supply up to a 200 ma of output current. there is a requirement however, that there be a minimum load of several hundred ma on the regulated positive output for the charge pump to work correctly. also, resistor r1 is required to limit the charging current of c1 to some value less than the lm2598 current limit (typically 1.5a). this method of generating a negative output voltage without an additional inductor can be used with other members of the simple switcher family, using either the buck or boost topology. tl/h/12593 48 figure 31. charge pump for generating a low current, negative output voltage http://www.national.com 25
application information (continued) typical through hole pc board layout, fixed output (1x size), double sided, through hole plated tl/h/12593 51 c in e150 m f/50v aluminum electrolytic, panasonic ``hfq series'' c out e120 m f/25v aluminum electrolytic, panasonic ``hfq series'' d1e3a, 40v schottky rectifier, 1n5822 l1e68 m h, l30, renco, through hole r pull-up e10 k x c delay e0.1 m f c sd /ss e0.1 m f typical through hole pc board layout, adjustable output (1x size), double sided, through hole plated tl/h/12593 52 c in e150 m f/50v, aluminum electrolytic, panasonic ``hfq series'' c out e120 m f/25v aluminum electrolytic, panasonic ``hfq series'' d1e3a, 40v schottky rectifier, 1n5822 l1e68 m h, l30, renco, through hole r1e1 k x ,1% r2euse formula in design procedure c ff esee figure 4 . r ff esee application information section (c ff section) r pull-up e10 k x c delay e0.1 m f c sd /ss e0.1 m f figure 32. pc board layout http://www.national.com 26
physical dimensions inches (millimeters) unless otherwise noted 7-lead to-220 (t) order number lm2598t-3.3, lm2598t-5.0, lm2598t-12 or lm2598t-adj ns package number ta07b http://www.national.com 27
lm2598 simple switcher power converter 150 khz 1a step-down voltage regulator, with features physical dimensions inches (millimeters) unless otherwise noted (continued) 7-lead to-263 surface mount package (s) order number lm2598s-3.3, lm2598s-5.0, lm2598s-12 or lm2598s-adj ns package number ts7b life support policy national's products are not authorized for use as critical components in life support devices or systems without the express written approval of the president of national semiconductor corporation. as used herein: 1. life support devices or systems are devices or 2. a critical component is any component of a life systems which, (a) are intended for surgical implant support device or system whose failure to perform can into the body, or (b) support or sustain life, and whose be reasonably expected to cause the failure of the life failure to perform, when properly used in accordance support device or system, or to affect its safety or with instructions for use provided in the labeling, can effectiveness. be reasonably expected to result in a significant injury to the user. national semiconductor national semiconductor national semiconductor national semiconductor corporation europe hong kong ltd. japan ltd. 1111 west bardin road fax: a 49 (0) 180-530 85 86 13th floor, straight block, tel: 81-043-299-2308 arlington, tx 76017 email: europe.support @ nsc.com ocean centre, 5 canton rd. fax: 81-043-299-2408 tel: 1(800) 272-9959 deutsch tel: a 49 (0) 180-530 85 85 tsimshatsui, kowloon fax: 1(800) 737-7018 english tel: a 49 (0) 180-532 78 32 hong kong fran 3 ais tel: a 49 (0) 180-532 93 58 tel: (852) 2737-1600 http://www.national.com italiano tel: a 49 (0) 180-534 16 80 fax: (852) 2736-9960 national does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and national reserves the right at any time without notice to change said circuitry and specifications.

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