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lt3592 1 3592fc typical application features applications description 500ma wide input voltage range step-down led driver with 10:1 dimming the lt ? 3592 is a ? xed frequency step-down dc/dc con- verter designed to operate as a constant-current source. an external sense resistor monitors the output current allowing accurate current regulation, ideal for driving high current leds. the output current can be dimmed by a factor of 10 using an external signal for nighttime brake lights. the high switching frequency offers several advantages by permitting the use of a small inductor and small ceramic capacitors. small components combined with the lt3592s 10-pin dfn leadless surface mount package save space and cost versus alternative solutions. the constant switching frequency combined with low-impedance ceramic capaci- tors result in low, predictable output ripple. a wide input voltage range of 3.6v to 36v makes the lt3592 useful in a variety of applications. current mode pwm architecture provides fast transient response and cycle-by-cycle current limiting. thermal shutdown provides additional protection. 50/500ma two series red led driver n wide input voltage range operation from 3.6v to 36v n resistor adjustable 400khzC2.2mhz switching frequency n shorted and open led protected n internal switch current sense resistor n external resistor programs led current, pin selects 10:1 ratio n 50ma/500ma led current settings n catch diode current sense to prevent runaway at high v in n small thermally enhanced 10-lead dfn (2mm 3mm) and msop-10 packages n automotive signal lighting n industrial lighting n constant-current, constant voltage supplies ef? ciency for 2 red leds, l = 10h, 900khz 10 h 51k 3592 ta01a 4.7 f 1 f 0.4 10k 0.1 f v in shdn bright r t boost sw da cap out v fb lt3592 gnd on brake 140k 900khz + 200/20mv C v in 7v to 32v l , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. input voltage (v) 4 efficiency (%) 100 95 90 60 75 70 65 80 85 55 50 20 12 24 3592 ta01b 28 16 8 bright 500ma
lt3592 2 3592fc absolute maximum ratings v in , bright voltages ................................ C0.3v to 36v boost voltage .........................................................60v boost above sw pin ...............................................30v cap , out voltages (out cap) ...............................30v v fb voltage .................................................................4v r t voltage ...................................................................6v (note 1) top view ddb package 10-lead (3mm s 2mm) plastic dfn 10 11 9 6 7 8 4 5 3 2 1 v fb out cap boost sw r t bright shdn v in da ja = 76c/w, jc = 13.5c/w exposed pad (pin 11) is gnd, must be soldered to pcb 1 2 3 4 5 r t bright shdn v in da 10 9 8 7 6 11 v fb out cap boost sw top view mse package 10-lead plastic msop ja = 38c/w, jc = 8c/w exposed pad (pin 11) is gnd, must be soldered to pcb pin configuration order information lead free finish tape and reel part marking * package description temperature range lt3592eddb#pbf lt3592eddb#trpbf ldcq 10-lead (3mm 2mm) plastic dfn C40c to 125c LT3592IDDB#pbf LT3592IDDB#trpbf ldcq 10-lead (3mm 2mm) plastic dfn C40c to 125c lt3592emse#pbf lt3592emse#trpbf ltdcr 10-lead plastic msop C40c to 125c lt3592imse#pbf lt3592imse#trpbf ltdcr 10-lead plastic msop C40c to 125c lead based finish tape and reel part marking package description temperature range lt3592eddb lt3592eddb#tr ldcq 10-lead (3mm 2mm) plastic dfn C40c to 125c LT3592IDDB LT3592IDDB#tr ldcq 10-lead (3mm 2mm) plastic dfn C40c to 125c lt3592emse lt3592emse#tr ltdcr 10-lead plastic msop C40c to 125c lt3592imse lt3592imse#tr ltdcr 10-lead plastic msop C40c to 125c consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/ shdn voltage ............................................................v in da pin current (average) ..................... C1.2a (sourcing) operating temperature range (notes 2, 3) lt3592e ............................................. C40c to 125c lt3592i .............................................. C40c to 125c storage temperature range ................... C65c to 150c
lt3592 3 3592fc electrical characteristics note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the lt3592e is guaranteed to meet performance speci? cations from 0c to 125c junction temperature. speci? cations over the C40c to 125c operating junction temperature range are assured by design, characterization and correlation with statistical process controls. the lt3592i is guaranteed over the full C40c to 125c operating junction temperature range. the operating lifetime is derated at junction temperatures greater than 125c. parameter conditions min typ max units minimum input voltage l 3.25 3.6 v input quiescent current in shutdown not switching v shdn = 0.3v 2 0.1 3 2 ma a cap to out voltage 0.4 cap to out bright = 1.4v bright = 0.3v l l 190 18 200 20 210 22 mv mv da pin current to stop osc C0.8 C1 C1.2 a switching frequency r t = 357k r t = 140k r t = 48.7k 350 800 1.9 400 900 2.2 450 1000 2.5 khz khz mhz maximum duty cycle r t = 140k 90 94 % shdn input high voltage 2.3 v shdn input low voltage 0.3 v bright input high voltage 1.4 v bright input low voltage 0.3 v switch current limit (note 4) l 0.85 1.25 1.5 a switch v cesat i sw = 500ma 300 mv boost pin current i sw = 500ma 20 30 ma switch leakage current 110 a minimum boost voltage (v boost C v in )v out = 4v 1.8 2.5 v boost diode forward voltage i dio = 50ma 800 mv v fb voltage out = cap = 4v, bright = 12v l 1.185 1.21 1.235 v v fb input leakage current v fb = 1.21v l C250 250 na the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 12v, v boost = 16v, v out = 4v unless otherwise noted. (note 2) note 3: this ic includes overtemperature protection that is intended to protect the device during momentary overload conditions. junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. continuous operation above the speci? ed maximum operating junction temperature may result in device degradation or failure. note 4: switch current measurements are performed when the outputs are not switching. slope compensation reduces current limit at higher duty cycles.
lt3592 4 3592fc typical performance characteristics ef? ciency (2 red leds, l = 10h, 900khz) ef? ciency (1 red led, l = 6.8h, 900khz) ef? ciency (2 red leds, l = 22h, 400khz) minimum v in for 500ma output current vs v out , l = 22h, f = 400khz (led loads) minimum v in for 500ma output current vs v out , l = 6.8h, f = 900khz (led loads) switch voltage drop vs switch current switch voltage drop at 500ma vs temperature (t a = 25c, unless otherwise noted) input voltage (v) 4 efficiency (%) 100 95 90 60 50 75 70 65 80 85 55 45 40 20 12 24 3592 g01 28 16 8 bright (500ma) input voltage (v) 4 efficiency (%) 100 95 90 60 50 75 70 65 80 85 55 45 40 20 12 24 3592 g02 28 16 8 bright (500ma) input voltage (v) 4 efficiency (%) 100 95 90 60 75 70 65 80 85 55 50 20 12 24 3592 g03 28 16 8 bright (500ma) input voltage (v) 2 output voltage (v) 12 11 10 4 7 6 5 8 9 3 2 8 3592 g05 12 10 46 input voltage (v) 2 output voltage (v) 12 11 10 4 7 6 5 8 9 3 2 8 3592 g06 12 10 46 switch current (ma) 0 v in C v sw (mv) 500 450 400 100 250 200 150 300 350 50 0 600 3592 g08 800 700 100 200 300 400 500 temperature (c) C50 v in C v sw (mv) 400 375 300 350 325 275 250 100 3592 g09 150 050 ef? ciency (2 red leds, l = 4.7h, 2.2mhz) minimum v in for 500ma output current vs v out , l = 4.7h, f = 2.2mhz (led loads) input voltage (v) 4 efficiency (%) 100 95 90 60 75 70 65 80 85 55 50 20 12 24 3592 g04 28 16 8 bright (500ma) input voltage (v) 2 output voltage (v) 12 11 10 4 7 6 5 8 9 3 2 8 3592 g07 16 10 12 14 46
lt3592 5 3592fc undervoltage lockout vs temperature switching frequency vs temperature typical performance characteristics current limit during soft start frequency foldback switch current limit switch current limit operating waveforms operating waveforms, discontinuous mode (t a = 25c, unless otherwise noted) temperature (c) C50 undervoltage lockout (v) 3.4 3.3 3.2 3.1 100 3592 g10 150 050 temperature (c) C50 f sw (khz) 2300 1700 1900 2100 1500 1300 1100 900 700 500 300 110 3592 g11 130 C30 C10 90 70 50 30 10 r t = 357k r t = 140k r t = 48.7k v shdn (v) 0.5 switch current limit (ma) 1400 1200 1000 800 600 400 200 0 2 3592 g12 2.5 1.5 1 shdn voltage (mv) 600 frequency (khz) 2500 2000 1500 1000 500 0 2000 3592 g13 2200 1000 1200 1400 1600 1800 800 r t = 48.7k duty cycle (%) 0 switch current limit (a) 1.4 1.3 1.2 1.1 0.8 0.9 1 0.7 0.6 0.5 80 3592 g14 100 40 60 20 temperature (c) C50 switch current limit (a) 1.50 1.40 1.45 1.35 1.30 1.15 1.20 1.25 1.10 1.05 1.00 90 110 3592 g15 130 30 10 70 50 C30 C10 typical v sw 5v/div v cap 10mv/div ac-coupled i l 500ma/div 3592 g16 500ns/div v sw 5v/div v cap 10mv/div ac-coupled i l 500ma/div 3592 g17 500ns/div
lt3592 6 3592fc typical performance characteristics switching frequency vs r t v bright vs v out v dim vs v out boost diode voltage vs current v out (v) 0 v cap C v out (mv) 210 205 200 195 190 810 3592 g18 12 46 2 v bright (mv) v out (v) 0 v cap C v out (mv) 25 24 23 17 18 19 20 21 22 16 15 810 3592 g19 12 46 2 v dim (mv) diode current (ma) 0 diode forward voltage (v) 1.0 0.9 0.6 0.7 0.8 0.5 0.4 100 150 3592 g20 200 50 v bstdio r t (k) 30 frequency (khz) 3000 2500 1000 1500 2000 500 0 300 330 3592 g21 360 60 90 120 150 180 210 240 270 (t a = 25c, unless otherwise noted)
lt3592 7 3592fc pin functions r t (pin 1): programs the frequency of the internal oscillator. connect a resistor from r t to ground. refer to table 1 or the typical performance characteristics for resistor values that result in desired oscillator frequencies. bright (pin 2): used to program a 10:1 dimming ratio for the led current. drive this pin above 1.4v to command maximum intensity or below 0.3v to command minimum intensity. this pin can be pwmed at 150hz for brightness control between the 1x and 10x current levels. shdn (pin 3): used to shutdown the switching regulator and the internal bias circuits. this pin can be pwmed at 150hz for brightness control. v in (pin 4): supplies current to the lt3592s internal cir- cuitry and to the internal power switches. must be locally bypassed. for automotive applications, a pi network with a cap from v in to gnd, a series inductor connected between v in and the power source, and another cap from the far end of the inductor to gnd is recommended. da (pin 5): allows the external catch diode current to be sensed to prevent current runaway, such as when v in is high and the duty cycle is very low. connect this pin to the anode of the external catch schottky diode. sw (pin 6): the sw pin is the output of the internal power switch. connect this pin to the inductor and the cathode of the switching diode. boost (pin 7): provides a drive voltage, higher than the input voltage to the internal bipolar npn power switch. boost will normally be tied to the sw pin through a 0.1f capacitor. an internal schottky is provided for the boost function and an external diode is not needed. an external schottky diode should be connected between boost and cap for single led applications or whenever a higher boost voltage is desired. cap (pin 8): output of the step-down converter and also an input to the led current sense ampli? er. connect the ? lter capacitor, inductor, and the top of the external led current sense resistor to this pin. out (pin 9): drives the led or leds and is the other input to the led current sense ampli? er. connect this pin to the anode of the top led in the string, the bottom of the external led current sense resistor, and the top of the v fb resistor divider. v fb (pin 10): the feedback node for the output voltage control loop. tie this node to a resistor divider between out and gnd to set the maximum output voltage of the step- down converter according to the following formula: v out = 1.21? r1 + r2 r2 where r1 connects between out and v fb and r2 connects between v fb and gnd. exposed pad (pin 11): ground. the underside exposed pad metal of the package provides both electrical contact to ground and good thermal contact to the printed circuit board. the device must be soldered to the circuit board for proper operation.
lt3592 8 3592fc block diagram reg/uvlo 3592 bd r s q 7 6 boost batt 4 v in sw 5 da l1 l2 l3 c3 c1 c4 d1 1 r t r t 11 gnd shdn osc gnd q1 10r r3 bright brake cap out v in v fb 1.21v r sense v c r1 r2 led1 led2 C C + r 8 9 10 2 C + C + C + 3 c2a c2b c2c g m = 6 r
lt3592 9 3592fc operation the lt3592 is a constant frequency, current mode step- down led driver. an internal oscillator that is programmed by a resistor from the r t pin to ground enables an rs ? ip-? op, turning on the internal 1.25a power switch q1. an ampli? er and comparator monitor the current ? owing between the v in and sw pins, turning the switch off when this current reaches a level determined by the voltage at v c . an error ampli? er that servos the v c node has two inputs, one from a voltage measurement and one from a current measurement. an instrumentation ampli? er measures the drop across an external current sense resistor between the cap and out pins and applies a gain of 60 (bright low for dim mode) or 6 (bright high for bright mode) to this signal and presents it to one negative error amp input. the output of a external resistor divider between out and ground is tied to the v fb pin and presented to a second negative error amp input. whichever input is higher in voltage will end up controlling the loop, so a circuit in which current control is desired (as for driving a led) will be set up such that the output of the instrumentation amp will be higher than the v fb pin at the current level that is desired. the voltage feedback loop will act to limit the output voltage and prevent circuit damage if an led should go open circuit. the positive input to the error amp is a 1.21v reference, so the voltage loop forces the v fb pin to 1.21v and the current loop forces the voltage difference between cap and out to be 200mv for bright mode and 20mv for dim mode. a rise in the output of the error ampli? er results in a increase in output current, and a fall in the error ampli? er output means less output current. current limit is provided by an active clamp on the v c node, and this node is also clamped to the shdn pin. soft-start is implemented by ramping the shdn pin using an external resistor and capacitor. an internal regulator provides power to the control circuitry and also includes an undervoltage lockout to prevent switching when v in is less than 3.25v. if shdn is low, the output is disconnected and the input current is less than 2a. the switch driver operates from the input of the boost pin. an external capacitor and internal diode are used to generate a voltage at the boost pin that is higher than the input supply, which allows the driver to fully saturate the internal bipolar npn power switch for ef? cient operation. an external diode can be used to make the boost drive more effective at low output voltage. the oscillator reduces the lt3592s operating frequency when the voltage at the out pin is low. this frequency foldback helps to control the output current during startup and overload. the anode of the catch diode for the step-down circuit is connected to the da pin to provide a direct sense of the current in this device. if this diodes current goes above a level set by an internal catch diode current limit circuit, the oscillator frequency is slowed down. this prevents current runaway due to minimum on time limitations at high v in voltages. this function can easily be disabled by tying the da pin and the catch diode anode to ground.
lt3592 10 3592fc oscillator the frequency of operation is programmed by an external resistor from r t to ground. table 1 shows r t values for commonly used oscillator frequencies, and refer to the typi- cal performance characteristics curve for other values. applications information the bright mode current is given by: i bright = 200mv/r sense the dim mode current is 10% of the bright mode value. the maximum allowed dc value of the bright mode cur- rent is 500ma. when the recommended component values are used in a 900khz 2 led application, the transient from switching between bright and dim currents will be less than 50s in duration. the sense resistor used should exhibit a low tc to keep the led current from drifting as the operating temperature changes. the bright pin can tolerate voltages as high as 36v and can be safely tied to v in even in high voltage applications, but it also has a low threshold voltage (~0.7v) that allows it to interface to logic level control signals. input voltage range the maximum allowed input voltage for the lt3592 is 36v. the minimum input voltage is determined by either the lt3592s minimum operating voltage of 3.6v or by its maximum duty cycle. the duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: dc = v out + v d v in ?v sw + v d where v d is the forward voltage drop of the catch diode (~0.4v) and v sw is the voltage drop of the internal switch table 2. inductor vendor information supplier phone fax website panasonic (800) 344-2112 www.panasonic.com/industrial/components/components.html vishay (402) 563-6866 (402) 563-6296 www.vishay.com/resistors coilcraft (847) 639-6400 (847) 639-1469 www.coilcraft.com coev magnetics (800) 227-7040 (650) 361-2508 www.circuitprotection.com/magnetics.asp murata (814) 237-1431 (800) 831-9172 (814) 238-0490 www.murata.com sumida usa: (847) 956-0666 japan: 81(3) 3607-5111 usa: (847) 956-0702 japan: 81(3) 3607-5144 www.sumida.com tdk (847) 803-6100 (847) 803-6296 www.component.tdk.com toko (847) 297-0070 (847) 699-7864 www.tokoam.com fb resistor network the output voltage limit is programmed with a resistor divider between the output and the v fb pin. this is the voltage that the output will be clamped to in case the led goes open circuit. choose the resistors according to r1 = r2([v out /1.21v] C 1) be sure to choose v out such that it does not interfere with the operation of the current control loop; it should be set at least 10% above the maximum expected led voltage for the selected bright output current. r2 should be 20k or less to avoid bias current errors. an optional phase- lead capacitor of 22pf between v out and v fb reduces light-load ripple. output current selection the output current levels are programmed by the value of the external current sense resistor between cap and out. table 1. r t values for selector oscillator frequencies f osc r t 400khz 357k 900khz 140k 2.2mhz 48.7k
lt3592 11 3592fc applications information (~0.4v at maximum load). this leads to a minimum input voltage of: v in(min) = v out + v d dc max ?v d + v sw with dc max = 0.90. the maximum input voltage is determined by the absolute maximum ratings of the v in and boost pins. the con- tinuous mode operation, the maximum input voltage is determined by the minimum duty cycle, which is dependent upon the oscillator frequency: dc min = f osc ? 70nsec v in(max) = v out + v d dc min ?v d + v sw note that this is a restriction on the operating input voltage for continuous mode operation. the circuit will tolerate transient inputs up to the absolute maximum of the v in and boost pins. the input voltage should be limited to the v in absolute maximum range (36v) during overload conditions (short circuit or startup). minimum on time the lt3592 will still regulate the output properly at input voltages that exceed v in(max) (up to 36v); however, the output voltage ripple increases as the input voltage is increased. figure 1 illustrates switching waveforms in a 2.2mhz single red led application near v in(max) = 24v. as the input voltage is increased, the part is required to switch for shorter periods of time. delays associated with turning off the power switch dictate the minimum on time of the part. the minimum on time for the lt3592 is ~70ns. figure 2 illustrates the switching waveforms when the input voltage is increased to v in = 26v. now the required on time has decreased below the mini- mum on time of 70ns. instead of the switch pulse width becoming narrower to accommodate the lower duty cycle requirement, the switch pulse width remains ? xed at 70ns. in figure 2, the inductor current ramps up to a value exceeding the load current and the output ripple increases to about 70mv. the part then remains off until the output voltage dips below the programmed value before it switches again. provided that the load can tolerate the increases output voltage ripple and the the components have been properly selected, operation about v in(max) is safe and will not dam- age the part. figure 3 illustrates the switching waveforms when the input voltage is increased to 36v. figure 1. figure 2. figure 3. 1s/div v sw 20v/div 3592 f01 v out 50mv/div i l 500ma/div 1s/div v sw 20v/div 3592 f02 v out 50mv/div i l 500ma/div 1s/div v sw 20v/div 3592 f03 v out 50mv/div i l 500ma/div
lt3592 12 3592fc applications information as the input voltage increases, the inductor current ramps up more quickly, the number of skipped pulses increases, and the output voltage ripple increases. for operation above v in(max) , the only component requirement is that they be adequately rated for operation at the intended voltage levels. the lt3592 is robust enough to survive prolonged opera- tion under these conditions as long as the peak inductor current does not exceed 1.2a. inductor saturation due to high current may further limit performance in this operat- ing regime. inductor selection and maximum output current a good ? rst choice for the inductor value is: l = 1.2a ? v out + 0.2v + v d () ? where v d is the forward voltage drop of the catch diode (~0.4v), f is the switching frequency in mhz and l is in h. with this value, there will be no subharmonic oscillation for applications with 50% or greater duty cycle. for low duty cycle applications in which v in is more than three times v out , a good guide for the minimum inductor value is l = 1.7 ? v in v out 0.2v () v in v sw + v d ? v out + 0.2v + v d () ? where v sw is the switch voltage drop (about 0.3v at 500ma). the inductors rms current rating must be greater than your maximum load current and its saturation current should be about 30% higher. for robust operation in fault conditions, the saturation current should be above 1.5a. to keep ef? ciency high, the series resistance (dcr) should be less than 0.1. table 2 lists several inductor vendors. of course, such a simple design guide will not always re- sult in the optimum inductor for your application. a larger value provides a higher maximum load current and reduces output voltage ripple at the expense of a slower transient response. if your load is lower than 500ma, then you can decrease the value of the inductor and operate with higher ripple current. this allows you to use a physically smaller inductor, or one with a lower dcr resulting in higher ef? - ciency. there are several graphs in the typical performance characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. low inductance may result in discontinuous mode operation, which is acceptable, but further reduces maximum load current. for details of the maximum output current and discontinuous mode operation, see linear technology application note 44. catch diode depending on load current, a 500ma to 1a schottky di- ode is recommended for the catch diode, d1. the diode must have a reverse voltage rating equal to or greater than the maximum input voltage. the on semiconductor mbra140t3 and central semiconductor cmmsh1-40 are good choices, as they are rated for 1a continuous forward current and a maximum reverse voltage of 40v. input filter network for applications that only require a capacitor, bypass v in with a 1f or higher ceramic capacitor of x7r or x5r type. y5v types have poor performance over tempera- ture and applied voltage and should not be used. a 1f ceramic capacitor is adequate to bypass the lt3592 and will easily handle the ripple current. however, if the input power source has high impedance, or there is signi? cant inductance due to long wires or cables, additional bulk capacitance might be necessary. the can be provided with a low performance (high esr) electrolytic capacitor in parallel with the ceramic device. some applications, such as those in automobiles, may require extra ? ltering due to emi/emc requirements. in these applications, very effective emi ? ltering can be pro- vided by a capacitor to ground right at the source voltage, a series ferrite bead, and a pi ? lter composed of a capacitor to ground, a series inductor, and another capacitor directly from the device pin to ground (see the block diagram for an example). typical values for the ? lter components are 10nf for c2c, a ferrite bead that is ~220 at 100mhz for l2, 3.3f for c2b, 10h for l3, and 1f for c2a. step-down regulators draw current from the input sup- ply in pulses with very fast rise and fall times. the input capacitor is required to reduce the resulting voltage ripple
lt3592 13 3592fc at the lt3592 and to force this very high frequency switch- ing current into a tight local loop, minimizing emi. a 1f capacitor is capable of this task, but only if it is placed close to the lt3592 and catch diode (see the pcb layout section). a second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the lt3592. a ceramic input capacitor combined with trace or cable inductance forms a high quality (underdamped) tank circuit. if the lt3592 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the lt3592s voltage rating. this situation can easily be avoided, as discussed in the hot plugging safety section. for more details, see linear technology application note 88. output capacitor for most 2.2mhz led applications, a 3.3f or higher output capacitor is suf? cient for stable operation. a 900khz application should use a 4.7f or higher output capacitor. 400khz applications require a 22f or higher output capacitor. the minimum recommended values should provide an acceptable (if somewhat underdamped) transient response, but larger values can always be used when extra damping is required or desired. the output capacitor ? lters the inductor current to generate an output with low voltage ripple. it also stores energy in order to satisfy transient loads and stabilizes the lt3592s control loop. because the lt3592 operates at a high fre- quency, minimal output capacitance is necessary. in addition, the control loop operates well with or without the presence of signi? cant output capacitor equivalent series resistance (esr). ceramic capacitors, which achieve very low output ripple and small circuit size, are therefore an option. applications information you can estimate output ripple with the following equation: v ripple = i lp ? p 8???c out where i lp-p is the peak-to-peak ripple current in the in- ductor. the rms content of this ripple is very low, so the rms current rating of the output capacitor is usually not a concern. it can be estimated with the formula: i c(rms) = i l 12 the low esr and small size of ceramic capacitors make them the preferred type for lt3592 applications. not all ceramic capacitors are the same, though. many of the higher value ceramic capacitors use poor dielectrics with high temperature and voltage coef? cients. in particular, y5v and z5u types lose a large fraction of their capacitance with applied voltage and at temperature extremes. because loop stability and transient response depend on the value of c out , this loss may be unacceptable. use x7r and x5r types. table 3 lists several capacitor vendors. figure 4 shows the transient response of the lt3592 when switching between dim and bright current levels with two output capacitor choices. the output load is two series luxeon k2 red leds, the dim current is 50ma and the bright current is 500ma, and the circuit is running at 900khz. the upper photo shows the recommended 4.7f value. the second photo shows the improved response resulting from a larger output capacitor. table 3. capacitor vendor information supplier phone fax website avx (803) 448-9411 (803) 448-1943 www.avxcorp.com sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com taiyo yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com tdk (847) 803-6100 (847) 803-6296 www.component.tdk.com
lt3592 14 3592fc applications information boost pin considerations the capacitor c3 and an internal schottky diode from the cap to the boost pin are used to generate a boost voltage that is higher than the input voltage. an external fast switching schottky diode (such as the bas40) can be used in parallel with the internal diode to make this boost circuit even more effective. in most cases, a 0.1f capacitor works well for the boost circuit. the boost pin must be at least 2.5v above the sw pin for best ef? ciency. for output voltages above 12v, use a 0.1f cap and an external boost diode (such as a bas40) connected in parallel with the internal schottky diode, anode to cap and cathode to boost. for outputs between 3.3v and 12v, the 0.1f cap and the internal boost diode will be effective. for 3v to 3.3v outputs, use a 0.22f capacitor. for output between 2.5v and 3v, use a 0.47f capacitor and an external schottky diode connected as shown in figure 5a. for lower output voltages, the external boost diodes anode can be tied to the input voltage. this con- nection is not as ef? cient as the others because the boost pin current comes from a higher voltage. the user must also be sure that the maximum voltage rating of the boost pin is not exceeded. figure 4. transient load response of the lt3592 with different output capacitors figure 5. two circuits for generating the boost voltage v in cap boost gnd sw da batt lt3592 (5a) d2 optional c3 3592 f05a v in cap boost gnd sw batt lt3592 (5b) d2 c3 3592 f05b da 100s/div v sw v out i led 100s/div v sw 3592 f04 v out i led c = 4.7f c = 10f
lt3592 15 3592fc applications information the minimum operating voltage of an lt3592 application is limited by the undervoltage lockout (uvlo, ~3.25v) and by the maximum duty cycle as outlined above. for proper startup, the minimum input voltage is also limited by the boost circuit. if the input voltage is ramped slowly, or the lt3592 is turned on with its shdn pin when the output is already in regulation, then the boost capacitor might not be fully charged. because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. this minimum load generally goes to zero once the circuit has started. figure 6 shows a plot of minimum input voltage needed to start with a 500ma output current versus output voltage with led loads. for led applications, the output voltage will typically drop rapidly after start due to diode heating, but this is not a concern because the voltage to run is lower than the voltage to start. the plots show the worst case situation when v in is ramping very slowly. for a lower startup voltage, the boost diodes anode can be tied to v in , but this restricts the input range to one-half of the absolute maximum rating of the boost pin. at light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. this reduces the minimum input voltage to about 400mv above v cap . at higher load currents, the inductor current is continu- ous and the duty cycle is limited by the maximum duty cycle of the lt3592, requiring a higher input voltage to maintain regulation. soft-start the shdn pin can be used to soft-start the lt3592, reducing the maximum input current during startup. the shdn pin is driven through an external rc ? lter to create a voltage ramp at this pin. figure 7 shows the startup waveforms with and without the soft-start circuit. by choosing a large rc time constant, the peak startup current can be reduced to programmed led current, with no overshoot. choose the value of the resistor so that it can supply 20a when the shdn pin reaches 2.3v. figure 6. input voltage needed to start at 500ma output current vs led voltage 3592 f06a input voltage (v) 2 led voltage (v) 12 11 10 4 7 6 5 8 9 3 2 8 12 10 46 400khz, l = 22h 3592 f06b input voltage (v) 2 led voltage (v) 12 11 10 4 7 6 5 8 9 3 2 8 12 10 46 900khz, l = 6.8h input voltage (v) 2 led voltage (v) 12 11 10 4 7 6 5 8 9 3 2 8 3592 f06c 16 10 12 14 46 2.2mhz, l = 4.7h
lt3592 16 3592fc applications information shorted and open led protection in case of a shorted led string or the out pin being shorted to ground by any means, the current loop will help to limit the output current for many conditions, but the switch current may still reach the switch current limit on some cycles despite the actions of the current loop. for some conditions (especially cold), the output current for shorted out will only be limited by the switch current limit (which can be as high as 1.5a) and the switching frequency foldback that occurs when out is close to ground, and the current control loop will have little to no effect. the total power dissipation will be quite low in either case due to the frequency foldback and the fact that the small current sense resistor will effectively be the output load for shorted out. peak switch and inductor currents will be high, but the peaks will be brief and well separated due to the lowered operating frequency. the main concern in this condition is that the output inductor not saturate and force the switch into an unsafe operating condition of simultaneous high current and high voltage drop. if the current sense resistor between cap and out becomes shorted or the cap pin is shorted to ground, the peak output current will be limited by the internal switch current limit, which could be as high as 1.5a. if an led goes open circuit, the voltage control loop through the r1-r2 resistor divider to fb will take control and prevent the output voltages from ? ying up close to v in . program the desired open circuit voltage to a value below the absolute maximum for the cap and out pins but well above the maximum possible forward drop of the led at the programmed bright current. reversed input protection in some systems, the output will be held high when the input to the lt3592 is absent. this may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode ored with the lt3592s output. if the v in pin is allowed to ? oat and the shdn pin is held high (either by a logic signal or because it is tied to v in ), then the lt3592s internal circuitry will draw its quiescent current through its sw pin. this is ? ne if the system can tolerate a few ma in this state. if you run v sw 10v/div v out 5v/div 50s/div i l 500ma/div v sw 10v/div v out 5v/div 50s/div i l 500ma/div shdn gnd lt3592 lt3592 3592 f07a run 15k 0.1f shdn gnd 3592 f07b figure 7. to soft-start the lt3592, add a resistor and capacitor to the shdn pin
lt3592 17 3592fc applications information figure 9. a well chosen input network prevents input voltage overshoot and ensures reliable operation when the lt3592 is connected to a live supply + + lt3592 v in gnd lt3592 v in gnd 1 f v in 20v/div i in 10a/div 5 s/div v in closing switch simulates hot plug i in low impedance energized 32v supply stray inductance due to 6 feet (2 meters) of twisted pair + + 2.2 f 10 f 50v 2.2 f 0.1 f 1 3493 f09 v in 20v/div i in 10a/div 5 s/div v in 20v/div i in 10a/div 5 s/div lt3592 v in gnd + + + (9a) (9b) (9c) figure 8. circuit to address reversed input and backpowering issues v in gnd fb shdn sw d4 v in lt3592 3592 f08 v out backup d4: mbr0540 +
lt3592 18 3592fc applications information ground the shdn pin, the sw pin current will drop to es- sentially zero. however, if the v in pin is grounded while the output is held high, then parasitic diodes inside the lt3592 can pull large currents from the output through the sw pin and the v in pin. figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. hot plugging safely the small size, robustness, and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of lt3592 circuits. however, these capaci- tors can cause problems if the lt3592 is plugged into a live supply (see linear technology application note 88 for a complete discussion). the low loss ceramic capacitor combined with stray inductance in series with the power source forms an underdamped tank circuit, and the volt- age at the v in pin of the lt35392 can ring to twice the nominal input voltage, possibly exceeding the lt3592s rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the lt3592 into an energized supply, the input network should be designed to prevent this overshoot. figure 9 shows the waveforms that result when an lt3592 circuit is connected to a 32v supply through six feet of 24 gauge twisted pair. the ? rst plot is the response with a 1f ceramic capacitor at the input. the input voltage rings as high as 56v and the input current peaks at 16a. one method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. in figure 9b, a tantalum chip capacitor has been added. this capacitors high equivalent series resistance (esr) damps the circuit and eliminates the voltage overshoot. the extra capacitor improves low frequency ripple ? ltering and can slightly improve the ef? ciency of the circuit, thought it is likely to be the largest component in the circuit. an alternate solution is shown in figure 9c. a 1 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). a 0.1f capacitor improves high frequency ? ltering. this solution is smaller and less expensive than the tantalum capacitor. for high input voltages, the impact of the 1 resistor on ef? ciency is minor, reducing it by less than one half percent for a two red series led load in bright mode operating from 32v. frequency compensation the lt3592 uses current mode control to regulate the loop, whether the current control or voltage control loop is active. this simpli? es loop compensation. in particular, the lt3592 does not require the esr of the output capaci- tor for stability, allowing the use of ceramic capacitors to achieve low output ripple and small circuit size. a low esr output capacitor will typically provide for a greater margin of circuit stability than an otherwise equivalent capacitor with higher esr, although the higher esr will tend to provide a faster loop response. figure 10 shows an equivalent circuit for the lt3592 control loops, both for current and voltage mode. both use the same error ampli? er and power section, but an additional voltage gain amp is used in conjuction with the external current sense resistor to implement output current control. the error ampli? er is a transconductance type with ? nite output impedance. the power section, consisting of the modulator, power switch, and inductor, is modeled as a transconductance ampli? er generating an output current proportional to the voltage at the v c node. note that the output capacitor integrates this current, and that the capacitor on the v c node (c c ) integrates the error ampli? er output current, resulting in figure 10. model for loop response 3592 f10 esr c1 + c1 C + 0.7v gnd sw g m = 0.7a/v 300k g m = 300 a/v g m = 1/5k bright cap out v c r c c c v fb 1.2v r sense r1 r2 C C + 30k C + r l
lt3592 19 3592fc applications information two poles in the loop. rc provides a zero. with the recom- mended output capacitor, the loop crossover occurs above the r c c c zero. this simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. with a larger ceramic capacitor that will have lower esr, crossover may be lower and a phase lead capacitor (c pl ) across the feedback divider may improve the transient response. large electrolytic capacitors may have an esr large enough to create an additional zero, and the phase lead might not be necessary. if the output capacitor is different than the recommended capacitor, stability should be checked across all operating conditions, including dim and bright current modes, voltage control via fb, input voltage, and temperature. pcb layout for proper operation and minimum emi, care must be taken during printed circuit board layout. figure 11 shows the recommended component placement with trace, ground plane, and via locations. note that large, switched currents ? ow in the lt3592s v in and sw pins, the catch diode (d1), and the input capacitor (c2). the loop formed by these components should be as small as possible and tied to system ground in only one place. these components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. place a local, unbroken ground plane below these components, and tie this ground plane to system ground at one location (ideally at the ground terminal of the output capacitor c1). the sw and boost nodes should be as small as possible. finally, keep the fb node small so that the ground pin and ground traces will shield it from the sw and boost nodes. include vias near the exposed gnd pad of the lt3592 to help remove heat from the lt3592 to the ground plane. high temperature considerations the die temperature of the lt3592 must be lower than the maximum rating of 125c. this is generally not a concern unless the ambient temperature is above 85c. for higher temperatures, extra care should be taken in the layout of the circuit to ensure good heat sinking at the lt3592. the maximum load current should be derated as the ambient temperature approaches 125c. the die temperature is calculated by multiplying the lt3592 power dissipation by the thermal resistance from junction to ambient. power dissipation within the lt3592 can be estimated by calculating the total power loss from an ef? ciency measurement and subtracting the catch diode loss. the resulting temperature rise at full load is nearly independent of input voltage. thermal resistance depends upon the layout of the circuit board, but 76c/w is typical for the 3mm 2mm dfn (ddb10) package, and 38c/w is typical for the ms10e package. higher output voltages at higher output voltages, the choice of output capacitor becomes especially critical. many small case size ceramic capacitors lose much of their rated capacitance well below figure 11. a good pcb layout ensures proper, low emi operation 3592 f11 shdn bright v in sys gnd
lt3592 20 3592fc applications information figure 12. switching transient when going from 50ma to 500ma current and back in voltage mode their maximum voltage capability. if a capacitor with a lower voltage rating is found to not be stable in a design, it will often result in a smaller solution to choose a larger capacitor value of the same voltage rating than to choose one of the same capacitance and higher voltage rating. for example, a 10f, 10v ceramic capacitor might be smaller than a 4.7f, 16v part, if a 4.7f, 10v capacitor is found to not be adequate in a given application. the lt3592hv can tolerate sustained output voltages of up to 25v. for output voltages above 12v, use an external schottky diode for the boost circuit with the anode tied to cap and the cathode tied to boost (as shown in figure 13). transient performance with voltage control loop the voltage control loop transient characteristics are similar to, but not identical to the current control loop. figure 12 shows the transient for a 12v input application running at 900khz with a 6.8h inductor and a 4.7f ceramic output capacitor. the lt3592 is in bright (500ma) mode but the current load is switched from 50ma to 450ma and back, so the current control loop is not active for either current level and the output voltage is regulated through the resistive voltage divider to the fb pin. other linear technology publications application notes an19, an35, and an44 contain more detailed descriptions and design information for step-down regulators and other switching regulators. the lt1376 data sheet has an extensive discussion of output ripple, loop compensation, and stability testing. design note dn100 shows how to generate a bipolar output supply using a step-down regulator. 10s/div v sw 10v/div 3592 f12 v out 1v/div i led 200ma/div figure 13. boost circuit with external schottky diode for output voltages above 12v v in cap boost gnd sw da batt lt3592 d2 c3 3592 f13
lt3592 21 3592fc typical applications single red led driver with boost diode to v in due to low v out 15 h 30k 3592 ta02 22 f 1 f 0.4 10k 357k 400khz 0.1 f v in shdn bright r t boost sw da cap out v fb lt3592 1n4148 mbra120 luxeon lxk2-pd12-s00 gnd v in 5v to 16v fault on off 6.8 h 51k 3592 ta03 4.7 f 1 f 0.4 10k 48.7k 2.2mhz 10 h bead 0.1 f cmmshi-40 v in shdn bright r t boost sw da cap out v fb lt3592 gnd 3.3 f 10nf v in 8v to 32v brake + 200/20mv C on off luxeon lxk2-pd12-s00 50ma/500ma two series red led driver 5v supply with 500ma current limit 6.8 h 31.6k 3592 ta04 4.7 f 1 f 0.4 10k 48.7k 2.2mhz 0.1 f mbra140 v in shdn bright r t boost sw da cap out v fb lt3592 gnd v in 8v to 32v 5v on
lt3592 22 3592fc package description ddb package 10-lead plastic dfn (3mm 2mm) (reference ltc dwg # 05-08-1722 rev ?) 2.00 0.10 (2 s ide s ) note: 1. drawing conform s to ver s ion (wecd-1) in jedec package outline m0-229 2. drawing not to s cale 3. all dimen s ion s are in millimeter s 4. dimen s ion s of expo s ed pad on bottom of package do not include mold fla s h. mold fla s h, if pre s ent, s hall not exceed 0.15mm on any s ide 5. expo s ed pad s hall be s older plated 6. s haded area i s only a reference for pin 1 location on the top and bottom of package 0.40 0.10 bottom view?expo s ed pad 0.64 0.05 (2 s ide s ) 0.75 0.05 r = 0.115 typ r = 0.05 typ 2.39 0.05 (2 s ide s ) 3.00 0.10 (2 s ide s ) 1 5 10 6 pin 1 bar top mark ( s ee note 6) 0.200 ref 0 ? 0.05 (ddb10) dfn 0905 rev ? 0.25 0.05 2.39 0.05 (2 s ide s ) recommended s older pad pitch and dimen s ion s 0.64 0.05 (2 s ide s ) 1.15 0.05 0.70 0.05 2.55 0.05 package outline 0.25 0.05 0.50 b s c pin 1 r = 0.20 or 0.25 45 chamfer 0.50 b s c
lt3592 23 3592fc information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. package description mse package 10-lead plastic msop (reference ltc dwg # 05-08-1664) msop (mse) 0908 rev c 0.53 p 0.152 (.021 p .006) seating plane 0.18 (.007) 1.10 (.043) max 0.17 C 0.27 (.007 C .011) typ 0.86 (.034) ref 0.50 (.0197) bsc 12 3 45 4.90 p 0.152 (.193 p .006) 0.497 p 0.076 (.0196 p .003) ref 8 9 10 10 1 7 6 3.00 p 0.102 (.118 p .004) (note 3) 3.00 p 0.102 (.118 p .004) (note 4) note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 0.254 (.010) 0 o C 6 o typ detail a detail a gauge plane 5.23 (.206) min 3.20 C 3.45 (.126 C .136) 0.889 p 0.127 (.035 p .005) recommended solder pad layout 0.305 p 0.038 (.0120 p .0015) typ 2.083 p 0.102 (.082 p .004) 2.794 p 0.102 (.110 p .004) 0.50 (.0197) bsc bottom view of exposed pad option 1.83 p 0.102 (.072 p .004) 2.06 p 0.102 (.081 p .004) 0.1016 p 0.0508 (.004 p .002) detail b detail b corner tail is part of the leadframe feature. for reference only no measurement purpose 0.05 ref 0.29 ref
lt3592 24 3592fc linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2008 lt 0409 rev c ? printed in usa related parts part number description comments lt1932 constant current, 1.2mhz, high ef? ciency white led boost regulator v in(min) = 1v, v in(max) = 10v, v out(max) = 34v, dimming analog/pwm, i sd < 1a, thinsot? package lt3465/ lt3465a constant current, 1.2mhz/2.7mhz, high ef? ciency white led boost regulator with integrated schottky diode v in(min) = 2.7v, v in(max) = 16v, v out(max) = 34v, dimming analog/pwm, i sd < 1a, thinsot package lt3466/ lt3466-1 dual constant current, 2mhz, high ef? ciency white led boost regulator with integrated schottky diode v in(min) = 2.7v, v in(max) = 24v, v out(max) = 40v, dimming 5ma, i sd < 16a, 3mm 3mm dfn-10 package lt3474/ lt3474-1 36v, 1a (i led ), 2mhz,step-down led driver v in(min) = 4v, v in(max) = 36v, v out(max) = 13.5v, dimming 400:1 true color pwm, i sd < 1a, tssop-16e package lt3475/ lt3475-1 dual 1.5a(i led ), 36v, 2mhz,step-down led driver v in(min) = 4v, v in(max) = 36v, v out(max) = 13.5v, dimming 3,000:1 true color pwm, i sd < 1a, tssop-20e package lt3476 quad output 1.5a, 2mhz high current led driver with 1,000:1 dimming v in(min) = 2.8v, v in(max) = 16v, v out(max) = 36v, dimming 1,000:1 true color pwm, i sd < 10a, 5mm 7mm qfn-10 package lt3478/ lt3478-1 4.5a, 2mhz high current led driver with 3,000:1 dimming v in(min) = 2.8v, v in(max) = 36v, v out(max) = 40v, dimming 1,000:1 true color pwm, i sd < 10a, 5mm 7mm qfn-10 package lt3486 dual 1.3a , 2mhz high current led driver v in(min) = 2.5v, v in(max) = 24v, v out(max) = 36v, dimming 1,000:1 true color pwm, i sd < 1a, 5mm 3mm dfn, tssop-16e package lt3491 constant current, 2.3mhz, high ef? ciency white led boost regulator with integrated schottky diode v in(min) = 2.5v, v in(max) = 12v, v out(max) = 27v, dimming 300:1 true color pwm, i sd < 8a, 2mm 2mm dfn-6, sc70 package lt3496 triple output 750ma, 2.1mhz high current led driver with 3,000:1 dimming v in(min) = 3v, v in(max) = 30v, v out(max) = 40v, dimming 3,000:1 true color pwm, i sd < 1a, 4mm 5mm qfn-28 package lt3497 dual 2.3mhz, full function led driver with integrated schottkys and 250:1 true color pwm dimming v in(min) = 2.5v, v in(max) = 10v, v out(max) = 32v, dimming 250:1 true color pwm, i sd < 12a, 2mm 3mm dfn-10 package lt3498 20ma led driver and oled driver integrated schottkys v in(min) = 2.5v, v in(max) = 12v, v out(max) = 32v, dimming analog/pwm, i sd < 8.5a, 2mm 3mm dfn-10 package lt3517 1.3a, 2.5mhz high current led driver with 3,000:1 dimming v in(min) = 3v, v in(max) = 30v, dimming 3,000:1 true color pwm, i sd < 1a, 4mm 4mm qfn-16 package lt3518 2.3a, 2.5mhz high current led driver with 3,000:1 dimming v in(min) = 3v, v in(max) = 30v, dimming 3,000:1 true color pwm, i sd < 1a, 4mm 4mm qfn-16 package lt3590 48v, 850khz, 50ma step-down led driver v in(min) = 4.5v, v in(max) = 50v, dimming 0.4, i sd < 15a, 2mm 2mm dfn-6, sc70 package lt3591 constant current, 1mhz, high ef? ciency white led boost regulator with integrated schottky diode and 80:1 true color pwm dimming v in(min) = 2.5v, v in(max) = 12v, v out(max) = 40v, dimming 80:1 true color pwm, i sd < 9a, 3mm 2mm dfn-8 package thinsot is a trademark of linear technology corporation. 33 h 158k 3592 ta05 4.7 f white leds 1 f 0.4 10k 0.1 f mbra140 bas40 v in shdn bright r t boost sw da cap out v fb lt3592 gnd bright 140k 900khz v in 24v to 36v on off typical applications five white led driver with external booste diode

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