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lt3492 1 3492fa features applications description triple output led driver with 3000:1 pwm dimming the lt ? 3492 is a triple output dc/dc converter designed to operate as a constant-current source and is ideal for driving leds. the lt3492 works in buck, boost or buck- boost mode. the lt3492 uses a ? xed frequency, current mode architecture resulting in stable operation over a wide range of supply and output voltages. a frequency adjust pin allows the user to program switching frequency between 330khz and 2.1mhz to optimize ef? ciency and external component size. the external pwm input provides 3000:1 led dimming on each channel. each of the three channels has a built-in gate driver to drive an external led-disconnect p-channel mosfet, allowing high dimming range. the output current range of each channel of the lt3492 is programmed with an external sense resistor. the ctrl pin is used to adjust the led current either for analog dimming or overtemperature protection. n true color pwm? dimming delivers up to 3000:1 dimming ratio n built-in gate driver for pmos led disconnect n three independent driver channels with 600ma, 60v internal switches n operates in buck, boost, buck-boost modes n ctrl pin accurately sets led current sense threshold over a range of 10mv to 100mv n adjustable frequency: 330khz to 2.1mhz n open led protection n wide input voltage range: operation from 3v to 30v transient protection to 40v n surface mount components n 28-lead (4mm 5mm) qfn and tssop packages n rgb lighting n billboards and large displays n automotive and avionic lighting n constant-current sources high dimming ratio triple output buck-mode led power supply 3000:1 pwm dimming at 100hz typical application 0.3a 0.47f 0.47f isn1 330m 330m tg1 pv in 58v 10 leds 33h isp1 0.3a isn2 tg2 33h isp2 0.3a 0.47f isn3 1f s 3 330m 10k 150k 49.9k 680pf 3492 ta01a tg3 33h isp3 sw1 sw2 lt3492 gnd sw3 tg1-3 v c1-3 v ref ctrl1-3 isp1-3 isn1-3 v in pwm1-3 shdn v in 3v to 24v 1f fadj ovp1-3 1.3mhz l , lt, ltc, ltm, linear technology and the linear logo are registered trademarks of linear technology corporation. true color pwm is a trademark of linear technology corporation. all other trademarks are the property of their respective owners. protected by u.s. patents, including 7199560, 7321203, and others pending. pwm 5v/div 1s/div 3492 ta01b i led 0.2a/div i l 0.5a/div
lt3492 2 3492fa absolute maximum ratings v in (note 4) ...............................................................40v sw1-sw3, isn1-isn3, isp1-isp3 ............................60v tg1-tg3 ...............................................isp C 10v to isp pwm1-pwm3 ...........................................................20v v ref , ctrl1-ctrl3, fadj, v c1 -v c3 , ovp1-ovp3 ....2.5v shdn (note 4) ...........................................................v in (note 1) pin configuration 1 2 3 4 5 6 7 8 9 10 11 12 13 14 top view fe package 28-lead plastic tssop 28 27 26 25 24 23 22 21 20 19 18 17 16 15 shdn pwm3 pwm2 pwm1 v ref ctrl3 ctrl2 ctrl1 fadj v c3 v c2 v c1 ovp3 ovp2 v in tg3 isn3 isp3 sw3 sw2 isp2 isn2 tg2 sw1 isp1 isn1 tg1 ovp1 gnd 29 t jmax = 125c, ja = 30c/w, jc = 10c/w exposed pad (pin 29) is gnd, must be soldered to pcb 9 10 top view gnd 29 ufd package 28-lead (4mm s 5mm) plastic qfn 11 12 13 28 27 26 25 24 14 23 6 5 4 3 2 1 pwm1 v ref ctrl3 ctrl2 ctrl1 fadj v c3 v c2 isp3 sw3 sw2 isp2 isn2 tg2 sw1 isp1 pwm2 pwm3 shdn v in tg3 isn3 v c1 ovp3 ovp2 ovp1 tg1 isn1 7 17 18 19 20 21 22 16 8 15 t jmax = 125c, ja = 34c/w, jc = 2.7c/w exposed pad (pin 29) is gnd, must be soldered to pcb lead free finish tape and reel part marking* package description temperature range lt3492efe#pbf lt3492efe#trpbf lt3492fe 28-lead plastic tssop C40c to 125c lt3492ife#pbf lt3492ife#trpbf lt3492fe 28-lead plastic tssop C40c to 125c lt3492eufd#pbf lt3492eufd#trpbf 3492 28-lead (4mm 5mm) plastic qfn C40c to 125c lt3492iufd#pbf lt3492iufd#trpbf 3492 28-lead (4mm 5mm) plastic qfn 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. consult ltc marketing for information on non-standard lead based ? nish parts. *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/ order information operating junction temperature range (note 2) .................................................. C40c to 125c max junction temperature .................................... 125c storage temperature range tssop ............................................... C65c to 150c ufd .................................................... C65c to 125c
lt3492 3 3492fa electrical characteristics the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 5v, shdn = 5v, pwm1-3 = 5v, fadj = 0.5v, ctrl1-3 = 1.5v, ovp1-3 = 0v, unless otherwise noted. parameter conditions min typ max units v in operation voltage (note 4) 3 30 v v in undervoltage lockout 2.1 2.4 v full-scale led current sense voltage isp1-3 = 48v l 98 96 100 103 104 mv mv one-tenth scale led current sense voltage ctrl1-3 = 100mv, isp1-3 = 48v 7 10 13 mv ispn/isnn operating voltage 2.5 60 v v ref output voltage i ref = 200a, current out of pin l 1.96 2 2.04 v v ref line regulation 3v v in 40v, i ref = 10a 0.03 %/v quiescent current in shutdown shdn = 0v 0.1 10 a quiescent current idle pwm1-pwm3 = 0v 6 8 ma quiescent current active (not switching) v c1 -v c3 = 0v 11 15 ma switching frequency fadj = 1.5v fadj = 0.5v fadj = 0.1v 1800 1000 280 2100 1300 340 2400 1600 400 khz khz khz maximum duty cycle fadj = 1.5v (2.1mhz) fadj = 0.5v (1.3mhz) fadj = 0.1v (330khz) 73 80 78 87 97 % % % ctrl1-3 input bias current current out of pin, ctrl1-3 = 0.1v 20 100 na fadj input bias current current out of pin, fadj = 0.1v 20 100 na ovp1-3 input bias current current out of pin, ovp1-3 = 0.1v 10 100 na ovp1-3 threshold 0.95 1 1.05 v v c1-3 idle input bias current pwm1-3 = 0v C20 0 20 na v c1-3 output impedance isp1-3 = 48v 10 m eamp g m ( i vc / v cap-led ) isp1-3 = 48v 200 s sw1-3 current limit (note 3) 600 1000 1300 ma sw1-3 v cesat i sw = 500ma (note 3) 340 mv sw1-3 leakage current shdn = 0v, sw = 5v 2 a
lt3492 4 3492fa 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 lt3492e 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 lt3492i is guaranteed over the full C40c to 125c operating junction temperature range. electrical characteristics note 3: current ? ows into pin. current limit and switch v cesat is guaranteed by design and/or correlation to static test. note 4: absolute maximum voltage at the v in and shdn pins is 40v for nonrepetitive 1 second transients, and 30v for continuous operation. note 5: gate turn-on/turn-off delay is measured from 50% level of pwm voltage to 90% level of gate on/off voltage. the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 5v, shdn = 5v, pwm1-3 = 5v, fadj = 0.5v, ctrl1-3 = 1.5v, ovp1-3 = 0v, unless otherwise noted. parameter conditions min typ max units isp1-3 input bias current 180 250 a isp1-3, isn1-3 idle input bias current pwm1-3 = 0v 1 a isp1-3, isn1-3 input bias current in shutdown shdn = 0v 1 a shdn input low voltage 0.4 v shdn input high voltage 1.5 v shdn pin current shdn = 5v, current into pin 65 120 a pwm1-3 input low voltage 0.4 v pwm1-3 input high voltage 1.2 v pwm1-3 pin current current into pin 160 210 a gate off voltage (isp1-3Ctg1-3) isp1-3 = 60v, pwm1-3 = 0v 0.1 0.3 v gate on voltage (isp1-3Ctg1-3) isp1-3 = 60v 5.5 6.5 7.5 v gate turn-on delay c load = 300pf, isp1-3 = 60v (note 5) 110 ns gate turn-off delay c load = 300pf, isp1-3 = 60v (note 5) 110 ns
lt3492 5 3492fa quiescent current switch on voltage switch frequency vs fadj switch current limit vs duty cycle reference voltage vs temperature switch current limit vs temperature (t a = 25c unless otherwise noted) typical performance characteristics v in (v) 0 8 10 14 30 3492 g01 6 4 10 20 40 2 0 12 input current (ma) pwm1-3 = 5v v c = gnd, not switching pwm1-3 = 0v duty cycle (%) 0 switch current limit (ma) 600 800 1200 1000 80 3492 g03 400 200 0 20 40 60 100 temperature (c) C50 current limit (ma) 800 1000 1200 25 75 3492 g04 600 400 C25 0 50 100 125 200 0 temperature (c) C50 v ref (v) 2.03 25 3492 g05 2.00 1.98 C25 0 50 1.97 1.96 2.04 2.02 2.01 1.99 75 100 125 fadj (v) 0 0 switch frequency (khz) 250 750 1000 1250 0.8 2250 3492 g06 500 0.4 0.2 1.0 0.6 1.2 1500 1750 2000 switch current (ma) 0 switch voltage (mv) 300 400 600 500 800 3492 g02 200 100 0 200 400 600 1000
lt3492 6 3492fa (t a = 25c unless otherwise noted) typical performance characteristics v isp- v isn threshold vs temperature pmos turn on waveforms pmos turn off waveforms temperature (c) C50 v isp- v isn threshold (mv) 101 102 103 25 75 3492 g10 100 99 C25 0 50 100 125 98 97 ctrl = 1.2v v isp = 24v 5v 0v pwm 60v 50v tg 200ns/div v isp = 60v q g fet = 6nc 3492 g11 200ns/div v isp = 60v q g fet = 6nc 3492 g12 5v 0v pwm 60v 50v tg switch frequency vs temperature v isp -v isn threshold vs ctrl v isp- v isn threshold vs v isp ctrl (v) 0 0 v isp -v isn threshold (mv) 20 40 60 80 120 0.2 0.4 0.6 0.8 3492 g08 1 1.2 100 v isp = 24v temperature (c) C50 switch frequency (mhz) 1.2 1.3 1.4 25 75 3492 g07 1.1 C25 0 50 100 125 1.0 fadj = 0.5v v isp (v) 0 97 v isp- v isn trheshold (mv) 98 99 100 101 102 103 10 20 30 40 3492 g09 60 50 ctrl = 1.2v
lt3492 7 3492fa ctrl1, ctrl2, ctrl3: led current adjustment pins. sets voltage across external sense resistor between isp and isn pins of the respective converter. setting ctrl voltage to be less than 1v will control the current sense voltage to be one-tenth of ctrl voltage. if ctrl voltage is higher than 1v, the default current sense voltage is 100mv. the ctrl pin must not be left ? oating. fadj: switching frequency adjustment pin. setting fadj voltage to be less than 1v will adjust switching frequency up to 2.1mhz. if fadj voltage is higher than 1v, the default switching frequency is 2.1mhz. the fadj pin must not be left ? oating. gnd: signal ground and power ground. solder exposed pad directly to ground plane. isn1, isn2, isn3: noninverting input of current sense error ampli? er. connect directly to led current sense resistor terminal for current sensing of the respective converter. isp1, isp2, isp3: inverting input of current sense error ampli? er. connect directly to other terminal of led current sense resistor terminal of the respective converter. ovp1, ovp2, ovp3: open led protection pins. a voltage higher than 1v on ovp turns off the internal main switch of the respective converter. tie to ground if not used. pwm1, pwm2, pwm3: pulse width modulated input. signal low turns off the respective converter, reduces quiescent supply current and causes the v c pin for that converter to become high impedance. pwm pin must not be left ? oating; tie to v ref if not used. shdn : shutdown pin. used to shut down the switching regulator and the internal bias circuits for all three convert- ers. tie to 1.5v or greater to enable the device. tie below 0.4v to turn off the device. sw1, sw2, sw3: switch pins. collector of the internal npn power switch of the respective converter. connect to external inductor and anode of external schottky recti- ? er of the respective converter. minimize the metal trace area connected to this pin to minimize electromagnetic interference. tg1, tg2, tg3: the gate driver output pin for discon- nect p-channel mosfet. one for each converter. when the pwm pin is low, the tg pin pulls up to isp to turn off the external mosfet. when the pwm pin is high, the external mosfet turns on. ispn-tgn is limited to 6.5v to protect the mosfet. leave open if the external mosfet is not used. v c1 , v c2 , v c3 : error ampli? er compensation pins. connect a series rc from these pins to gnd. v in : input supply pin. must be locally bypassed. powers the internal control circuitry. v ref : reference output pin. can supply up to 200a. the nominal output voltage is 2v. pin functions
lt3492 8 3492fa C + C + + eamp a1 C + v1 pwm1 v c 1v pwm comparator slope r1 2k C + v sense i led led array m1 r sense a8 ctrl buffer q3 1v ctrl1 v c1 q1 gnd r2 20k r6 r5 C + C + a3 sr latch isense replicated for each channel shared components s rq a2 C + a9 a6 npn driver C + a10 a4 a7 mosfet driver a5 v in v ref fadj 3492 bd shdn ovp1 r3 r4 r c pwm1 tg1 isn1 isp1 sw1 d1 l1 v in c2 c1 c c v in v in c3 c4 internal regulator and uvlo 2v reference ramp generator oscillator q2 v in 200a figure 1. lt3492 block diagram working in boost con? guration block diagram
lt3492 9 3492fa applications information operation the lt3492 uses a ? xed frequency, current mode control scheme to provide excellent line and load regulation. op- eration can be best understood by referring to the block diagram in figure 1. the oscillator, ramp generator, refer- ence, internal regulator and uvlo are shared among the three converters. the control circuitry, power switch etc., are replicated for each of the three converters. figure 1 shows the shared circuits and only converter 1 circuits. if the shdn pin is logic low, the lt3492 is shut down and draws minimal current from v in . if the shdn pin is logic high, the internal bias circuits turn on. the switching regulators start to operate when their respective pwm signal goes high. the main control loop can be understood by following the operation of converter 1. the start of each oscillator cycle sets the sr latch, a3, and turns on power switch q1. the signal at the noninverting input (slope node) of the pwm comparator a2 is proportional to the sum of the switch current and oscillator ramp. when slope exceeds v c (the output of the error ampli? er a1), a2 resets the latch and turns off the power switch q1 through a4 and a5. in this manner, a10 and a2 set the correct peak current level to keep the output in regulation. ampli? er a8 has two noninverting inputs, one from the 1v internal voltage reference and the other one from the ctrl1 pin. whichever input is lower takes precedence. a8, q3 and r2 force v1, the voltage across r1, to be one tenth of either 1v or the voltage of ctrl1 pin, whichever is lower. v sense is the voltage across the sensing resistor, r sense , which is connected in series with the leds. v sense is compared to v1 by a1. if v sense is higher than v1, the output of a1 will decrease, thus reducing the amount of current delivered to leds. in this manner the current sensing voltage v sense is regulated to v1. converters 2 and 3 are identical to converter 1. pwm dimming control the led array can be dimmed with pulse width modulation using the pwm1 pin and an external p-channel mosfet, m1. if the pwm1 pin is pulled high, m1 is turned on by internal driver a7 and converter 1 operates nominally. a7 limits isp1-tg1 to 6.5v to protect the gate of m1. if the pwm1 pin is pulled low, q1 is turned off. converter 1 stops operating, m1 is turned off, disconnects the led array and stops current draw from output capacitor c2. the v c1 pin is also disconnected from the internal circuitry and draws minimal current from the compensation capacitor c c . the v c1 pin and the output capacitor store the state of the led current until pwm1 is pulled up again. this leads to a highly linear relationship between pulse width and output light, and allows for a large and accurate dim- ming range. a p-channel mosfet with smaller total gate charge (q g ) improves the dimming performance, since it can be turned on and off faster. use a mosfet with a q g lower than 10nc, and a minimum v th of C1v to C2v. dont use a low v th pmos. to optimize the pwm control of all the three channels, the rising edge of all the three pwm signals should be synchronized. in the applications where high dimming ratio is not required, m1 can be omitted to reduce cost. in these conditions, tg1 should be left open. the pwm dimming range can be further increased by using ctrl1 pin to linearly adjust the current sense threshold during the pwm1 high state. loop compensation loop compensation determines the stability and transient performance. the lt3492 uses current mode control to regulate the output, which simpli? es loop compensation. to compensate the feedback loop of the lt3492, a series resistor-capacitor network should be connected from the v c pin to gnd. for most applications, the compensation capacitor should be in the range of 100pf to 2.2nf. the com- pensation resistor is usually in the range of 5k to 50k. to obtain the best performance, tradeoffs should be made in the compensation network design. a higher value of compensation capacitor improves the stability and dim- ming range (a larger capacitance helps hold the v c voltage when the pwm signal is low). however, a large compen- sation capacitor also increases the start-up time and the time to recover from a fault condition. similarly, a larger compensation resistor improves the transient response but may reduce the phase margin. a practical approach is to start with one of the circuits in this data sheet that
lt3492 10 3492fa is similar to your application and tune the compensation network to optimize the performance. the stability, pwm dimming waveforms and the start-up time should be checked across all operating conditions. open-led protection the lt3492 has open-led protection for all the three converters. as shown in figure 1, the ovp1 pin receives the output voltage (the voltage across the output capacitor) feedback signal from an external resistor divider. ovp1 voltage is compared with a 1v internal voltage reference by comparator a6. in the event the led string is disconnected or fails open, converter 1 output voltage will increase, caus- ing ovp1 voltage to increase. when ovp1 voltage exceeds 1v, the power switch q1 will turn off, and cause the output voltage to decrease. eventually, ovp1 will be regulated to 1v and the output voltage will be limited. in the event one of the converters has an open-led protection, the other converters will continue functioning properly. switching frequency and soft-start the lt3492 switching frequency is controlled by fadj pin voltage. setting fadj voltage to be less than 1v will reduce switching frequency. if fadj voltage is higher than 1v, the default switch- ing frequency is 2.1mhz. in general, a lower switching frequency should be used where either very high or very low switch duty cycle is required or higher ef? ciency is desired. selection of a higher switching frequency will allow use of low value external components and yield a smaller solution size and pro? le. as a cautionary note, operation of the lt3492 at a com- bination of high switching frequency with high output voltage and high switch current may cause excessive internal power dissipation. consideration should be given to selecting a switching frequency less than 1mhz if these conditions exist. connecting fadj pin to a lowpass ? lter (r5 and c4 in figure 1) from the ref pin provides a soft-start function. during start-up, fadj voltage increases slowly from 0v to the setting voltage. as a result, the switching frequency increases slowly to the setting frequency. this function limits the inrush current during start-up. input capacitor selection for proper operation, it is necessary to place a bypass capacitor to gnd close to the v in pin of the lt3492. a 1f or greater capacitor with low esr should be used. a ceramic capacitor is usually the best choice. in the buck mode con? guration, the capacitor at pv in has large pulsed currents due to the current returned though the schottky diode when the switch is off. for the best reliability, this capacitor should have low esr and esl and have an adequate ripple current rating. the rms input current is: i in(rms) = i led ?1?d () ?d where d is the switch duty cycle. a 1f ceramic type ca- pacitor placed close to the schottky diode and the ground plane is usually suf? cient for each channel. output capacitor selection the selection of output ? lter capacitor depends on the load and converter con? guration, i.e., step-up or step-down. for led applications, the equivalent resistance of the led is typically low, and the output ? lter capacitor should be large enough to attenuate the current ripple. to achieve the same led ripple current, the required ? lter capacitor value is larger in the boost and buck-boost mode applications than that in the buck mode applications. for the led buck mode applications at 1.3mhz, a 0.22f ce- ramic capacitor is usually suf? cient for each channel. for the led boost and buck-boost applications at 1.3mhz, a 1f ceramic capacitor is usually suf? cient for each chan- nel. lower switching frequency requires proportionately higher capacitor values. if higher led current ripple can be tolerated, a lower output capacitance can be selected to reduce the capacitors cost and size. use only ceramic capacitors with x7r or x5r dielectric, as they are good for temperature and dc bias stability of the capacitor value. all ceramic capacitors exhibit loss of capacitance value with increasing dc voltage bias, so it may be necessary to choose a higher value capacitor to get the required capacitance at the operation voltage. always check that the voltage rating of the capacitor is suf? cient. table 1 shows some recommended capacitor vendors. applications information
lt3492 11 3492fa table 1. ceramic capacitor manufacturers vendor type series taiyo yuden ceramic x5r, x7r avx ceramic x5r, x7r murata ceramic x5r, x7r kemet ceramic x5r, x7r tdk ceramic x5r, x7r inductor selection inductor value is selected based on switching frequency and desired transient response. the data sheet applica- tions show appropriate selections for a 1.3mhz switching frequency. proportionately higher values may be used if a lower switching frequency is selected. several inductors that work well with the lt3492 are listed in table 2. however, there are many other manufacturers and devices that can be used. consult each manufacturer for more detailed information and their entire range of parts. ferrite core inductors should be used to obtain the best ef? ciency. choose an inductor that can handle the necessary peak current without saturating, and ensure that the inductor has a low dcr (copper-wire resistance) to minimize i 2 r power losses. an inductor with a magnetic shield should be used to prevent noise radiation and cross coupling among the three channels. diode selection the schottky diode conducts current during the interval when the switch is turned off. select a diode v r rated for the maximum sw voltage. it is not necessary that the forward current rating of the diode equal the switch current limit. the average current, i f , through the diode is a function of the switch duty cycle. select a diode with forward current rating of: i f = i l ? (1 C d) where i l is the inductor current. if using the pwm feature for dimming, it is important to consider diode leakage, which increases with the tem- perature from the output during the pwm low interval. therefore, choose the schottky diode with suf? cient low leakage current at hot temperature. table 3 shows several schottky diodes that work well with the lt3492. applications information table 2. surface mount inductors part number value (h) dcr ( max) i rms (a) size w l h (mm3) sumida cdrh4d28 15 0.149 0.76 5.0 5.0 3.0 cdrh5d28 22 0.122 0.9 6.0 6.0 3.0 33 0.189 0.75 cooperet sd20 15 0.1655 1.25 5.0 5.0 2.0 22 0.2053 1.12 sd25 33 0.2149 1.11 5.0 5.0 2.5 taiyo yuden np04szb 15 0.180 0.95 4.0 4.0 1.8 22 0.210 0.77 tdk vlf5014a 15 0.32 0.97 4.5 4.7 1.4 22 0.46 0.51 wrth electronics 7447789133 33 0.24 1.22 7.3 7.3 3.2 coilcraft m556132 22 0.19 1.45 6.1 6.1 3.2 table 3. schottky diodes part number v r (v) i f (a) package zetex zlls350 40 0.38 sod523 zlls400 40 0.52 sod323 diodes b1100 100 1.0 sma rohm rb160m-60 60 1.0 pmdu/sod-123 undervoltage lockout the lt3492 has an undervoltage lockout circuit that shuts down all the three converters when the input volt- age drops below 2.1v. this prevents the converter from switching in an erratic mode when powered from a low supply voltage. programming the led current an important consideration when using a switch with a ? xed current limit is whether the regulator will be able to supply the load at the extremes of input and output voltage range. several equations are provided to help determine
lt3492 12 3492fa applications information this capability. some margin to data sheet limits is included, along with provision for 200ma inductor ripple current. for boost mode converters: i out(max) ? 0.4a v in(min) v out(max) for buck mode converters: i led(max) ? 0.4a for sepic and buck-boost mode converters: i out(max) ? 0.4a v in(min) (v out(max) + v in(min) ) if some level of analog dimming is acceptable at minimum supply levels, then the ctrl pin can be used with a resistor divider to v in (as shown in the block diagram) to provide a higher output current at nominal v in levels. the led current of each channel is programmed by con- necting an external sense resistor r sense in series with the led load, and setting the voltage regulation threshold across that sense resistor using ctrl input. if the ctrl voltage, v ctrl , is less than 1v, the led current is: i led = v ctrl 10 ? r sense if v ctrl is higher than 1v, the led current is: i led = 100mv r sense the ctrl pins should not be left open. the ctrl pin can also be used in conjunction with a ptc thermistor to provide overtemperature protection for the led load as shown in figure 2. thermal considerations the lt3492 is rated to a maximum input voltage of 30v for continuous operation, and 40v for nonrepetitive one second transients. careful attention must be paid to the internal power dissipation of the lt3492 at higher input voltages and higher switching frequencies/output voltage to ensure that a junction temperature of 125c is not exceeded. this is especially important when operating at high ambient temperatures. consider driving v in from 5v or higher to ensure the fastest switching edges, and minimize one source of switching loss. the exposed pad on the bottom of the package must be soldered to a ground plane. this ground should then be connected to an internal copper ground plane with thermal vias placed directly under the package to spread out the heat dissipated by the lt3492. board layout the high speed operation of the lt3492 demands careful attention to board layout and component placement. the exposed pad of the package is the only gnd terminal of the ic and is important for thermal management of the ic. therefore, it is crucial to achieve a good electrical and thermal contact between the exposed pad and the ground plane of the board. also, in boost con? guration, the schottky recti? er and the capacitor between gnd and the cathode of the schottky are in the high frequency switching path where current ? ow is discontinuous. these elements should be placed so as to minimize the path between sw and the gnd of the ic. to reduce electromagnetic interfer- ence (emi), it is important to minimize the area of the sw node. use the gnd plane under sw to minimize interplane coupling to sensitive signals. to obtain good current regulation accuracy and eliminate sources of channel to channel coupling, the isp and isn inputs of each channel of the lt3492 should be run as separate lines back to the terminals of the sense resistor. any resistance in series with isp and isn inputs should be minimized. avoid ex- tensive routing of high impedance traces such as ovp and v c . make sure these sensitive signals are star coupled to the gnd under the ic rather than a gnd where switching currents are ? owing. finally, the bypass capacitor on the v in supply to the lt3492 should be placed as close as possible to the v in terminal of the device. figure 2 50k 3492 f02 45k 2v v ref 470 ptc ctrl1-3
lt3492 13 3492fa typical applications minimum bom buck mode led driver 300:1 pwm dimming at 100hz ef? ciency 0.3a c4 0.22f c5 0.22f c6 0.22f i sn1 330m 330m pv in 58v 10 leds l1 33h l2 33h l3 33h i sp1 0.3a i sn2 i sp2 0.3a isn3 c1-c3 1f s 3 330m isp3 d1 d2 d3 c1-c3, c7: murata grm31cr72a105ka01l c4-c6: murata grm21br71h224ka01 d1-d3: diodes b1100 l1-l3: tdk vlf5014at-330mr50 10k 150k 49.9k 470pf 3492 ta07a sw1 sw2 lt3492 gnd sw3 tg1-3 v c1-3 v ref ctrl1-3 isp1-3 isn1-3 v in pwm1-3 shdn v in 5v c7 1f fadj ovp1-3 1.3mhz pwm 5v/div i l 0.5a/div i led 0.5a/div 5s/div 3492 ta07b pwm duty cycle (%) 0 50 efficiency (%) 55 65 70 75 40 80 100 95 3492 ta07c 60 20 60 80 85 90
lt3492 14 3492fa 1000:1 pwm dimming at 100hz typical applications triple boost 100ma 12 led driver ef? ciency vs pwm duty cycle 12 leds c2 1f c3 1f c4 1f pv in 12v 100ma 1m 20k ovp1 isn1 tg1 1 2.2nf 3492 ta03a l1 22h l2 22h l3 22h isp1 d1 d2 d3 m1 m2 m3 sw1 sw2 lt3492 gnd sw3 tg1-3 ovp1-3 v c1-3 v ref ctrl1-3 isp1-3 isn1-3 v in pwm1-3 shdn v in 5v c1: murata grm31mr71c225ka35 c2-c4: murata grm31cr72a105ka01l c5: murata grm31mr71h105ka88 d1-d3: diodes b1100 l1-l3: tdk vlf5014at-220mr62 m1-m3: zetex zxmp6a13f c5 1f 12 leds 100ma 1m 20k ovp2 isn2 tg2 1 isp2 12 leds 100ma 1m 20k ovp3 150k 18.2k 49.9k 1.3mhz isn3 tg3 1 isp3 c1 2.2f s 3 fadj pwm 5v/div i l 0.5a/div i led 0.1a/div 2s/div 3492 ta03b pwm duty cycle (%) 0 75 80 85 80 3492 ta03c 70 65 20 40 60 100 60 55 50 efficiency (%)
lt3492 15 3492fa typical applications dual boost led driver c2 1f c3 1f c4 1f pv in 12v isn1 m1 1 2.2nf open 3492 ta04 l1 22h l2 22h l3 22h isp1 d1 d2 d3 sw1 tg1 sw2 lt3492 gnd sw3 tg2 fadj ovp1-3 tg3 v c1-3 v ref ctrl1-3 isp1-3 isn1-3 v in pwm1-3 shdn v in 3v to 12v c5 1f c1 2.2f s 3 isn2 1 isp2 12 leds 200ma 20k ovp2-3 150k 18.2k 49.9k 1.3mhz isn3 m2 1 isp3 12 leds 100ma 1m 1m 20k ovp1 c1: murata grm31mr71c225ka35 c2-c4: murata grm31cr72a105ka01l c5: murata grm31mr71h105ka88 d1-d3: diodes b1100 l1-l3: tdk vlf5014at-220mr62 m1, m2: zetex zxmp6a13f 1000:1 pwm dimming at 100hz for 200ma leds pwm 5v/div i l2 i l3 0.5a/div i led 0.2a/div 2s/div 3492 ta04b ef? ciency vs pwm duty cycle for 200ma leds pwm duty cycle (%) 0 75 80 85 80 3492 ta04c 70 65 20 40 60 100 60 55 50 efficiency (%)
lt3492 16 3492fa triple boost 100ma 9 led driver with v in controlled dimming 9 leds c2 1f c3 1f c4 1f 100ma 750k 20k ovp1 isn1 tg1 1 2.2nf 3492 ta08 l1 15h l2 15h l3 15h isp1 d1 d2 d3 m1 m2 m3 sw1 sw2 lt3492 gnd sw3 tg1-3 ovp1-3 v c1-3 isp1-3 isn1-3 v in pwm1-3 shdn v in 5v to 16v c1: murata grm31mr71c225ka35 c2-c5: murata grm31mr71h105ka88 d1-d3: zetex zlls400ta l1-l3: taiyo yuden np04szb 150m m1-m3: zetex zxmp6a13f c5 1f 9 leds 100ma 750k 20k ovp2 isn2 tg2 1 isp2 9 leds 100ma 750k 430k 18.2k ovp3 20k 100k 1mhz isn3 tg3 1 ctrl1-3 357k 40.2k isp3 c1 2.2f s 3 v ref fadj ctrl1-3 typical applications led current decreasing with v in ef? ciency vs v in v in (v) 2 i led (ma) 70 80 90 18 3492 ta08b 60 50 20 6 10 14 40 30 110 100 v in (v) 4 0 efficiency (%) 10 30 40 50 12 90 3492 ta08c 20 816 60 70 80
lt3492 17 3492fa typical applications triple led driver driving led strings in buck, boost and buck-boost modes 2 leds c3 1f v in 10v to 16v 0.3a isn1 tg1 330m 2.2nf 3492 ta05 l1 6.8h l2 22h l3 33h isp1 d2 m1 m2 m3 1 sw1 sw2 lt 3 4 9 2 gnd sw3 tg1-3 ovp2-3 v c1-3 isp1-3 isn1-3 v in pwm1-3 shdn c1: murata grm55dr71h335ka0193 c2: murata grm21br71h474ka88 c3, c5: murata grm31mr71h105ka88 c4: murata grm21br71h104ka01b d1: diodes dfls130 d2, d3: rohm rb160m-60 l1: tdk vlf5014at-6r8mr99 l2: tdk vlf5014at-229mr62 l3: tdk vlf5014at-330mr50 m1: zetex zxmp3a13f m2, m3 zetex zxmp6a13f 10 leds 0.1a 825k 20k ovp2 isn2 tg2 c2 0.47f d1 1 isp2 d3 4 leds 100k 0.1a 18.2k 150k 49.9k 1.3mhz isn3 tg3 3.9m v in isp3 c4 0.1f c5 1f ovp3 c1 3.3f s 3 v ref ctrl1-3 fadj ovp1 3000:1 pwm dimming at 100hz for ch1 (buck mode) 3000:1 pwm dimming at 100hz for ch2 (boost mode) 3000:1 pwm dimming at 100hz for ch3 (buck-boost mode) pwm 5v/div i l 0.5a/div i led 0.5a/div 1s/div 3492 ta05b pwm 5v/div i l 0.5a/div i led 0.1a/div 1s/div 3492 ta05c pwm 5v/div i l 0.5a/div i led 0.1a/div 1s/div 3492 ta05d
lt3492 18 3492fa triple buck mode led driver with open led protection 0.3a c4 0.47f c5 0.47f c6 0.47f d1 d2 d3 5.6k 5.6k 80.6k 80.6k 2k 2k isn1 m1 m2 m3 330m 330m tg1 pv in 48v l1 22h l2 22h l3 22h m4 ovp1 ovp2 m5 m6 isp1 0.3a isn2 tg2 isp2 0.3a isn3 c1-c3 1f s 3 330m 3492 ta02 tg3 isp3 5.6k 80.6k 2k ovp1 c1-c3, c7: murata grm31cr72a105ka01l c4-c6: murata grm21br72a474ka73 d1-d3: rohm rb160m-60 l1-l3: tdk vlf5014at-220mr62 m1-m3: zetex zxmp6a13f m4-m6: philips bc858b 470pf sw1 sw2 lt3492 gnd sw3 fadj tg1-3 ovp1-3 v c1-3 v ref ctrl1-3 isp1-3 isn1-3 v in pwm1-3 shdn v in 5v c7 1f 430k 10k 100k 1mhz 10 leds 10 leds 10 leds typical applications 2000:1 pwm dimming at 100hz ef? ciency vs pwm duty cycle for 200ma leds pwm 5v/div i l 0.5a/div i led 0.5a/div 1s/div 3492 ta02b pwm duty cycle (%) 0 50 efficiency (%) 55 65 70 75 40 80 100 95 3492 ta02c 60 20 60 80 85 90
lt3492 19 3492fa package description fe28 (eb) tssop 0204 0.09 C 0.20 (.0035 C .0079) 0 o C 8 o 0.25 ref 0.50 C 0.75 (.020 C .030) 4.30 C 4.50* (.169 C .177) 134 5 6 7 8910 11 12 13 14 19 20 22 21 15 16 18 17 9.60 C 9.80* (.378 C .386) 4.75 (.187) 2.74 (.108) 28 2726 25 24 23 1.20 (.047) max 0.05 C 0.15 (.002 C .006) 0.65 (.0256) bsc 0.195 C 0.30 (.0077 C .0118) typ 2 recommended solder pad layout exposed pad heat sink on bottom of package 0.45 p 0.05 0.65 bsc 4.50 p 0.10 6.60 p 0.10 1.05 p 0.10 4.75 (.187) 2.74 (.108) millimeters (inches) *dimensions do not include mold flash. mold flash shall not exceed 0.150mm (.006") per side note: 1. controlling dimension: millimeters 2. dimensions are in 3. drawing not to scale see note 4 4. recommended minimum pcb metal size for exposed pad attachment 6.4 0 (.25 2 bsc fe package 28-lead plastic tssop (4.4mm) (reference ltc dwg # 05-08-1663) exposed pad variation eb
lt3492 20 3492fa 4.00 p 0.10 (2 sides) 2.50 ref 5.00 p 0.10 (2 sides) note: 1. drawing proposed to be made a jedec package outline mo-220 variation (wxxx-x). 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package pin 1 top mark (note 6) 0.40 p 0.1 0 27 28 1 2 bottom viewexposed pad 3.50 ref 0.75 p 0.05 r = 0.115 typ r = 0.05 typ pin 1 notch r = 0.20 or 0.3 5 s 45 o chamfer 0.25 p 0.05 0.50 bsc 0.200 ref 0.00 C 0.05 (ufd28) qfn 0506 rev b recommended solder pad pitch and dimensions apply solder mask to areas that are not soldered 0.70 p 0.05 0.25 p 0.05 0.50 bsc 2.50 ref 3.50 ref 4.10 p 0.05 5.50 p 0.05 2.65 p 0.05 3.10 p 0.05 4.50 p 0.05 package outline 2.65 p 0.10 3.65 p 0.10 3.65 p 0.05 package description ufd package 28-lead plastic qfn (4mm 5mm) (reference ltc dwg # 05-08-1712 rev b)
lt3492 21 3492fa 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. revision history rev date description page number a 04/10 corrected pin names for fe package in pin con? guration section 2
lt3492 22 3492fa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2009 lt 0410 rev a ? printed in usa typical application triple buck-boost mode 100ma 4 led driver 3000:1 pwm dimming at 100hz pv in 10v to 16v 2.2nf 3492 ta06 sw1 sw2 pv in lt3492 gnd sw3 isp1-3 isn1-3 v in pwm1-3 shdn v in 5v to 16v c8 1f 4 leds 100ma 3.9m 100k ovp3 18.2k isn3 tg3 1 l1 22h l2 22h l3 22h isp3 isn2 1 isp2 c2 0.1f c4 0.1f c6 0.1f c5 1f pv in c3 1f pv in c7 1f isn1 d1 d2 d3 1 isp1 3.9m 100k ovp2 3.9m 100k ovp1 c1 2.2f 4 leds 100ma tg2 tg1 4 leds 100ma m1 m2 m3 c1: murata grm31mr71e225ka93 c2, c4, c6: murata grm21br71h104ka01b c3, c5, c7: murata grm31mr71h105ka88 c8: murata grm31mr71e105ka93 d1-d3: rohm rb160m-60 l1-l3: tdk vlf5014at-220mr62 m1-m3: zetex zxmp6a13f fadj tg1-3 ovp1-3 v c1-3 v ref ctrl1-3 150k 49.9k 1.3mhz related parts part number description comments lt3496 triple 0.75a, 2.1mhz, 45v led driver v in : 3v to 30v, v out(max) = 45v, dimming = 3000:1, i sd < 1a, 4mm 5mm qfn and tssop16e packages lt3474 36v, 1a (i led ), 2mhz, step-down led driver v in : 4v to 36v, v out(max) = 13.5v, true color pwm dimming = 400:1, i sd < 1a, tssop16e package lt3475 dual 1.5a (i led ), 36v, 2mhz step-down led driver v in : 4v to 36v, v out(max) = 13.5v, true color pwm dimming = 3000:1, i sd < 1a, tssop20e package lt3476 quad output 1.5a, 36v, 2mhz high current led driver with 1000:1 dimming v in : 2.8v to 16v, v out(max) = 36v, true color pwm dimming = 1000:1, i sd < 10a, 5mm 7mm qfn package lt3477 3a, 42v, 3mhz boost, buck-boost, buck led driver v in : 2.5v to 25v, v out(max) = 40v, dimming = analog/pwm, i sd < 1a, qfn and tssop20e packages lt3478/lt3478-1 4.5a, 42v, 2.5mhz high current led driver with 3000:1 dimming v in : 2.8v to 36v, v out(max) = 42v, true color pwm dimming = 3000:1, i sd < 3a, tssop16e package lt3486 dual 1.3a, 2mhz high current led driver v in : 2.5v to 24v, v out(max) = 36v, true color pwm dimming = 1000:1, i sd < 1a, 5mm 3mm dfn and tssop16e packages lt3517 1.5a, 2.5mhz, 45v led driver v in : 3v to 30v, v out(max) = 45v, dimming = 3000:1, i sd < 1a, 4mm 4mm qfn and tssop16e packages lt3518 2.3a, 2.5mhz, 45v led driver v in : 3v to 30v, v out(max) = 45v, dimming = 3000:1, i sd < 1a, 4mm 4mm qfn and tssop16e packages lt3755/lt3755-1 40v in , 75v out , full featured led controller v in : 4.5v to 40v, v out(max) = 75v, true color pwm dimming = 3000:1, i sd < 1a, 3mm 3mm qfn-16 and ms16e packages lt3756-1 100v high current led controller v in : 6v to 100v, v out(max) = 100v, true color pwm dimming = 3000:1, i sd < 1a, 3mm 3mm qfn-16 and ms16e packages lt c ? 3783 high current led controller v in : 3v to 36v, v out(max) = ext fet, true color pwm dimming = 3000:1, i sd < 20a, 5mm 4mm qfn10 and tssop16e packages pwm 5v/div i l 0.5a/div i led 0.1a/div 1s/div 3492 ta06b

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