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IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 1 40v led driver with te mperature compensation general description the IS31LT3352 is a continuous mode inductive step-down converter, designed for driving a single led or multiple series connected leds efficiently from a voltage source higher than the required led voltage. the chip operates from an input supply between 6v and 40v and provides an externally adjustable output current of up to 750ma. depending upon supply voltage and external components, this can provide up to 30 watts of output power. the IS31LT3352 includes an integrated output switch and a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current. the IS31LT3352 integrates temperature compensation function in order to maintain leds? stable and reliable operation. the IS31LT3352 m easures the thermistance mounted close to leds. when ambient temperature near leds goes too high and the negative temperature coefficient thermistors reac h the value of threshold resistance connected at r th pin, output current starts to reduce automatically. after the ambient temperature falls down to a safe temperature the current will return to the set value. the IS31LT3352 can be connected as led drivers? chain with the same temperature compensation percentage. in this chain, every IS31LT3352?s adjo output pin drives next stage?s IS31LT3352 adji input pin with temperature compensation information. so, only one thermistor is needed in the whole IS31LT3352 system. application circuit features ? simple low parts count ? internal 40v power switch ? wide input voltage range: 6v to 40v ? up to 750ma output current ? high efficiency (up to 95% ) ? 1200 1 dimming ratio ? typical 5% output current accuracy ? single pin on/off and brightness control using dc voltage or pwm ? up to 1mhz switching frequency ? inherent open-circuit led protection ? thermal shutdown to protect ic itself ? temperature compensation to protect leds applications ? low voltage halogen replacement leds ? automotive lighting ? low voltage industrial lighting ? led back-up lighting ? illuminated signs october 2011 copyright ? ? ? 2011 ? integrated ? silicon ? solution, ? inc. ? all ? rights ? reserved. ? issi ? reserves ? the ? right ? to ? make ? changes ? to ? this ? specification ? and ? its ? products ? at ? any ? time ? without ? notice. ? issi ? assumes ? no ? liability ? arising ? out ? of ? the ? application ? or ? use ? of ? any ? information, ? products ? or ? services ? described ? herein. ? customers ? are ? advised ? to ? obtain ? the ? latest ? version ? of ? this ? device ? specification ? before ? relying ? on ? any ? published ? information ? and ? before ? placing ? orders ? for ? products. ? integrated ? silicon ? solution, ? inc. ? does ? not ? recommend ? the ? use ? of ? any ? of ? its ? products ? in ? life ? support ? applications ? where ? the ? failure ? or ? malfunction ? of ? the ? product ? can ? reasonably ? be ? expected ? to ? cause ? failure ? of ? the ? life ? support ? system ? or ? to ? significantly ? affect ? its ? safety ? or ? effectiveness. ? products ? are ? not ? authorized ? for ? use ? in ? such ? applications ? unless ? integrated ? silicon ? solution, ? inc. ? receives ? written ? assurance ? to ? its ? satisfaction, ? that: ? a.) ? the ? risk ? of ? injury ? or ? damage ? has ? been ? minimized; ? b.) ? the ? user ? assume ? all ? such ? risks; ? and ? c.) ? potential ? liability ? of ? integrated ? silicon ? solution, ? inc ? is ? adequately ? protected ? under ? the ? circumstances IS31LT3352
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 2 system application note: each IS31LT3352 can driver up to three slav e chips in the next stages, and it is recommend no more than three stages are used f or the current coherence, therefore, up to thirteen IS31LT3352s are allowed in one system IS31LT3352 IS31LT3352 IS31LT3352
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 3 pin configurations package pin configurations sop8 pin description pin name no. description v in 1 input voltage (6v to 40v). decouple to ground with 1 f or higher x7r ceramic capacitor close to device i sense 2 connect resistor r s from this pin to v in to define nominal average output current i outnom =0.1/r s r th 3 the pin set the starting temperature of temperature compensat ion by connecting an external resistor. r ntc 4 the output currents reduction slope set pin by connecting an external thermistor in temperature compensation mode. adjo 5 led drivers chain application pin. * when r 3 (ntc)>r 2 ,v adjo =v adji * when r 3 (ntc) IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 4 ordering information industrial range: -40 c to +85 c order number quantity per reel package v sense IS31LT3352-v1grls2-tr 2,500 sop-8, lead-free 91mv to 101mv IS31LT3352-v2grls2-tr 2,500 sop-8, lead-free 99mv to 110mv
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 5 absolute maximum ratings (note 1) symbol parameter rating v in input voltage -0.3v to +50v v isense i sense voltage v in +0.3v to v in -5v ,v in >5v v in +0.3v to -0.3v,v in <5v v lx lx output voltage -0.3v to +50v v adj ,v adjo , rth, r ntc pin input voltage -0.3v to +6v i lx switch output current 800ma p tot power dissipation 1.2w t op operating temperature -40 to 85c t st storage temperature -55 to 150c t j max junction temperature 150c r ja junction to ambient 80c/w esd susceptibility(human body mode) 2kv electrical characteristics (v in =12v, t amb =25c unless otherwise stated) (note 2) symbol parameter conditions min. typ. max. unit v in input voltage 6 40 v i inqoff quiescent supply current with output off adji pin grounded 40 60 80 a i inqon quiescent supply current with output switching adji pin floating 450 600 a v isense mean current sense threshold voltage measured on i sense pin with respect to v in adji pin floating IS31LT3352-v1 91 95 101 mv IS31LT3352-v2 99 105 110 v sensehys sense threshold hysteresis 15 % i sense i sense pin input current v sense =0.1v 8 10 a v ref internal reference voltage measured on adji pin with pin floating 1.2 v v adji external control voltage range on adji pin for dc brightness control 0.3 1.2 v v adjioff dc voltage on adji pin to switch chip from active (on) state to quiescent (off) state v adji falling 0.15 0.2 0.25 v v adjion dc voltage on adji pin to switch chip from quiescent (off) state to active (on) state v adji rising 0.2 0.25 0.3 v v os r th and r ntc pin offset voltage 10 mv
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 6 electrical characteristics (vin=12v, tamb=25c unless otherwise stated) (note 2) (continued) symbol parameter conditions min. typ. max. unit i lx(leak) lx switch leakage current 1 a v adjo adjo terminal voltage no temperature compensation adji pin floating i adjo =30 a 1.20 v r lx lx switch ?on? resistance 0.9 1.5 ? i lxmean continuous lx switch current 0.65 a r adji resistance between adji pin and vref 500 k ? d pwm(lf) brightness control range at low frequency pwm signal pwm frequency =100hz pwm amplitude=5v,vin=15v, l=27uh, driving 1 led 1200:1 d pwm(hf) brightness control range at high frequency pwm signal pwm frequency =10khz pwm amplitude=5v,vin=15v, l=27uh, driving 1 led 13:1 f lx operating frequency adji pin floating l=100 h (0.82 ? ) i out =350ma @ v led =3.4v driving 1 led 154 khz v adjo to v adji offset adji pin to adjo pin drift voltage -38 38 mv t onmin minimum switch ?on? time lx switch ?on? 200 ns t offmin minimum switch ?off? time lx switch ?off? 200 ns f lxmax recommended maximum operating frequency 1 mhz d lx recommended duty cycle range of output switch at f lxmax 0.3 0.7 0.9 t pd internal comparator propagation delay 50 ns t sd thermal shutdown temperature 140 c t sd-hys thermal shutdown hysteresis 20 c notes: 1. stresses beyond those listed under absolute maximum ratings ma y cause permanent damage to the device. these are stress rati ngs only, and functional operation of the device at thes e or any other conditions beyond those indicated under recommended operating conditio ns is not implied. exposure to absolute-maxi mum-rated conditions for extended peri ods may affect device reliability. 2. all parts are production tested at ta=25c. other temperature limits are guaranteed by design.
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 7 typical operating conditions for typical application circuit ,at t amb =25c unless otherwise stated. efficiency vs. no. of leds l=100uh, rs=0.33ohm efficiency vs. no. of leds l=47uh, rs=0.33ohm output current variation with supply voltage l=100uh,rs=0.33ohm output current variation with supply voltage l=47uh, rs=0.33ohm 0.5 0.6 0.7 0.8 0.9 1 5 10152025303540 vin(v) efficiency (%) 1led 2led 3led 4led 5led 6led 7led 8led 9led 10led 0.5 0.6 0.7 0.8 0.9 1 5 10152025303540 vin(v) effiency (%) 1led 2led 3led 4led 5led 6led 7led 8led 9led 10led -2 -1 0 1 2 3 4 5 6 5 10152025303540 vin(v) devilation from nominal current(%) 1led 2led 3led 4led 5led 6led 7led 8led 9led 10led -3 -2 -1 0 1 2 3 4 5 6 7 5 10152025303540 vin(v) devilation from nominal current(%) 1led 2led 3led 4led 5led 6led 7led 8led 9led 10led
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 8 typical operating conditions (continued) vref vs. vin over nominal supply voltage range supply current vs. vin (operating) vref vs. vin at low supply voltage shutdown current vs. vin (quiescent) vadjo vs. rntc rntc falling, rth=1kohm 1.1985 1.199 1.1995 1.2 1.2005 1.201 5 10152025303540 vin(v) vref(v) 0 100 200 300 400 500 600 0 5 10 15 20 25 30 35 40 vin(v) iin(ua) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0246810 vin(v) vref(v) 0 20 40 60 80 100 120 0 5 10 15 20 25 30 35 40 vin(v) iin(ua) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 300 600 900 1200 rntc(ohm) vadjo(v)
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 9 application information setting nominal average output current with external resistor r s the nominal average output current in the led(s) is determined by the value of the external current sense resistor (r s ) connected between v in and i sense and is given by: i out nom = 0.1/r s [for r s >0.13 ? ] the table below gives values of nominal average output current for several preferred values of current setting resistor (r s ) in the typical application circuit shown on page 1: r s ( ? ) nominal average output current (ma) 0.13 769 0.15 667 0.27 370 0.3 333 vsense is divided into two ranges to improve current accuracy, please refer to bin information on page 4. the above values assume that the adji pin is floating and at a nominal voltage of v ref =1.2v. note that r s =0.13 ? is the minimum allowed value of sense resistor under these conditions to maintain switch current below the specified maximum value. it is possible to use different values of r s if the adji pin is driven from an external voltage. output current adjustment by external dc control voltage the adji pin can be driven by an external dc voltage (v adji ), as shown, to adjust the output current to a value below the nominal average value defined by r s . the nominal average output current in this case is given by: i outdc = 0.083*v adji /r s [for 0.3v< v adji <1.2v] note that 100% brightness setting corresponds to v adji = v ref . when driving the adji pi n above 1.2v, the current will be clamped to 100% brightness automatically. the input impedance of the adji pin is 500k 25%. output current adjustment by pwm control directly driving adji input a pulse width modulated (pwm) signal with duty cycle d pwm can be applied to the adji pin, as shown below, to adjust the output current to a value below the nominal average value set by resistor r s , the signal range from 0v~5v. the pwm signal must have the driving ability to drive an internal 500k ? pull-up resistor. driving the adji input from a microcontroller another possibility is to drive the chip from the open drain output of a microcontroller. the diagram below shows one method of doing this: the diode and resistor suppress possible high amplitude negative spikes on the adji input resulting from the drain-source capacitance of the fet. negative spikes at the input to the chip should be avoided as they may cause errors in output current or erratic device operation. shutdown mode taking the adji pin to a voltage below 0.2v will turn off the output and supply current will fall to a low standby level of 60 a nominally. inherent open-circuit led protection if the connection to the led(s) is open-circuited, the coil is isolated from the lx pin of the chip, so the chip will not be damaged, unlike in many boost converters, where the back emf may damage the internal switch by forcing the drain above its breakdown voltage. capacitor selection a low esr capacitor should be used for input decoupling, as the esr of this capacitor appears in series with the supply source impedance and lowers overall efficiency. this capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. adji gnd IS31LT3352 gnd mcu k a dji gnd IS31LT3352 gnd dc 10
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 10 if the source is a dc supply, the capacitor is decided by ripple of the source, the value is given by: max on f u t i c ? ? * min i f is the value of output current max u ? is the ripple of power supply. ton is the ?on? time of mosfet. the value is normally 2 times of the minimum value. if the source is an ac supply, typical output voltages ripple from a nominal 12v ac transformer can be 10%.if the input capacitor value is lower than 200 f, the ac input waveform is distorted, some times the lowest value will be lower than the forward voltage of led strings. this will lower the average current of the leds. so it is recommended to set the value of the capacitor bigger than 200uf. for maximum stability over temperature and voltage, capacitors with x7r, x5r, or better dielectric are recommended. capacitors with y5v dielectric are not suitable for decoupling in this application and should not be used. inductor selection recommended inductor values for the IS31LT3352 are in the range 47 h to 220 h. higher values of inductance are recommended at higher supply voltages and low output current in order to minimize errors due to switching delays, which result in increased ripple and lower efficiency. higher values of inductance also result in a smaller change in output current over the supply voltage range. (see graphs). the inductor should be mounted as close to the chip as possible with low resistance connections to the lx and v in pins. the chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current. it is recommended to use inductor with saturation current bigger than 1.2a for 700ma output current and inductor with saturation current bigger than 500ma for 350ma output current. the inductor value should be chosen to maintain operating duty cycle and switch 'on/off' times within the specified limits over the supply voltage and load current range. the following equations can be used as a guide. lx switch 'on' time ) ( lx s avg led in on r rl r i v v i l t ? ? ? ? ? ? note: t onmin >200ns lx switch 'off' time ) ( s avg d led off r rl i v v i l t ? ? ? ? ? note: t offmin >200ns where: l is the coil inductance (h) r l is the coil resistance ( ? ) i avg is the required led current (a) ? i is the coil peak-peak ripple current (a) {internally set to 0.3 i avg } v in is the supply voltage (v) v led is the total led forward voltage (v) r lx is the switch resistance ( ? ) vd is the diode forward voltage at the required load current (v) example: for v in =12v, l=47 h, r l =0.64 ? , v led =3.4v, i avg =333ma and vd =0.36v t on = (47e-6 0.105)/(12 - 3.4 - 0.612) = 0.62 s t off = (47e-6 0.105)/(3.4 + 0.36 + 0.322)= 1.21 s this gives an operating frequency of 546khz and a duty cycle of 0.34. optimum performance will be achieved by setting the duty cycle close to 0.5 at t he nominal supply voltage. this helps to equalize the undershoot and overshoot and improves temperature stability of the output current. diode selection for maximum efficiency and performance, the rectifier (d1) should be a fast low capacitance schottky diode with low reverse leakage at the maximum operating voltage and temperature. if alternative diodes are used, it is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. it is very important to consider the reverse leakage of the diode when operating above 85c. excess leakage will increase the power dissipation in the device. the higher forward voltage and overshoot due to reverse recovery time in silicon di odes will increase the peak voltage on the lx output. if a silicon diode is used, care should be taken to ensure that the total voltage appearing on the lx pin including supply ripple, does not exceed the specified maximum value. reducing output ripple peak to peak ripple current in the led can be reduced, if required, by shunting a capacitor c led across the led(s) as shown below:
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 11 a value of 1 f will reduce nominal ripple current by a factor three (approx.). propor tionally lower ripple can be achieved with higher capacitor values. note that the capacitor will not affect opera ting frequency or efficiency, but it will increase start-up delay , by reducing the rate of rise of led voltage. operation at low supply voltage the internal regulator disables the drive to the switch until the supply has risen above the startup threshold set internally which makes power mosfet on-resistance small enough. above this thres hold, the chip will start to operate. however, with the supply voltage below the specified minimum value, the switch duty cycle will be high and the chip power dissipation will be at a maximum. care should be taken to avoid operating the chip under such conditions in the application, in order to minimize the risk of exceeding the maximum allowed die temperature. (see next section on thermal considerations ). note that when driving loads of two or more leds, the forward drop will normally be su fficient to prevent the chip from switching below appro ximately 6v. this will minimize the risk of damage to the chip. thermal considerations when operating the chip at high ambient temperatures, or when driving maximum load current, care must be taken to avoid exceeding the package power dissipation limits. note that the chip pow er dissipation will most often be a maximum at minimum supply voltage. it will also increase if the efficiency of the circuit is low. this may result from the use of unsuitable coils, or excessive parasitic output capacitance on the switch output. temperature compensation of output current high luminance leds often need to be supplied with a temperature compensated current in order to maintain stable and reliable operation at all drive levels. the leds are usually mounted remotely from the chip. for this reason, the temperature coe fficients of the internal circuits for the IS31LT3352 have been optimized to minimize the change in output current when no compensation is employed. if output current compensation is required, it is possible to use an external temperature sensing network - normally using negative temperature coefficient (ntc) thermistors and/or diodes, mounted very close to the led(s). the output of the sensing network can reduce output current with increasing temperature through internal circuit. as shown in the figure below, the temperature compensation curve is decided by r1, ntc thermistor r2 and resistor r3. when led(s) temperature increases, thermistance of r2 starts to reduce. as r2 reduces to the point that r2?s thermistance plus r3 resistance equaling to r1 resistance, temperature compensation function starts to work and iout starts to reduce. the iout current with temperature compensation?s equation is: in the case that 0.3< vadji <1.2v: i outdc = 0.083*v adji (r2+r3)/r1*rs in the case that vadji >1.2v: i outdc = 0.1*(r2+r3)/r1*rs r2 and r3 decide the temperature compensation slope, if r3 is just 0ohm, slope is only decided by thermistor r2?s parameter b-constant. and larger r3?s resistance results in slope more even. if the temperature compensation threshold is selected, larger r2 and r3 selected need larger r1 to match and vice versa. too large r1 make rth pin more sensitive to noise, too small r1 will make ic current consumption larger. from 1k to 100k of r1 is recommended. an IS31LT3352 calculator is available from the issi to assist with temperature compensation design and here are some detail examples as below: IS31LT3352 gnd r th r n tc adjo r 2 ( n tc ) r 1 r 3 v IS31LT3352 i sense in lx l1 d 1 c led led vin rs
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 12 b=4485, r1=48.6k, r2=100k, r3=0r b=4485, r1=20.6k, r2=100k, r3=0r b=4485, r1=22.1k, r2=220k, r3=0r b=4485, r1=58.6k, r2=100k, r3=10k 0 50 100 150 200 250 300 350 400 0 20 40 60 80 100 120 140 160 led ambient temp () current (ma) 0 50 100 150 200 250 300 350 400 0 20406080100120140160 led ambient temp () current (ma) 0 50 100 150 200 250 300 350 400 0 20 40 60 80 100 120 140 160 led ambient temp () current (ma) 0 50 100 150 200 250 300 350 400 0 20 40 60 80 100 120 140 160 led ambient temp () current (ma)
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 13 layout considerations lx pin the lx pin of the chip is a fast switching node, so pcb traces should be kept as short as possible. to minimize ground 'bounce', the ground pin of the chip should be soldered directly to the ground plane. coil and decoupling capacitors it is particularly important to mount the coil and the input decoupling capacitor close to the chip to minimize parasitic resistance and indu ctance, which will degrade efficiency. it is also important to take account of any trace resistance in series with current sense resistor rs. high voltage traces avoid running any high voltage traces close to the adji pin, to reduce the risk of leakage due to board contamination. any such leakage may raise the adji pin voltage and cause excessive output current. a ground ring placed around the adji pin will minimize changes in output current under these conditions adji pin the adji pin is a high impedance input, so when left floating, pcb traces to this pin should be as short as possible to reduce noise pickup. the adji pin is a high impedance input, so when left floating, pcb traces to this pin should be as short as possible to reduce noise pickup. adji pin can also be connected to a voltage between 1.2v~5v. in this case, the internal circuit will clamp the output current at the value which is set by adji=1.2v . rth, rntc pin the pcb trace from r1 to rth pin should be as short as possible to reduce noise pick up. because ntc thermistor r2 is mounted close to the leds and remote from IS31LT3352, the pcb trace from r2 to r ntc pin will be longer and pick up noise more easily. a 0.1uf capacitor from r ntc pin to ground and close to the r ntc pin is recommended to filter the frequency noise and provide protection against high voltage transients. adjo pin because adjo pin drives next stages, adji pins and the pcb trace may be longer which picks up noise easily. in this case 200pf (max) capacitor is needed to connect from adjo trace to ground to filter out the noise. best practice is to connect one capacitor respectively close to adjo output pin and the next stage adji input pins, but the total capacitance besides the parasitic capacitance from adjo pin to ground must be less than 200pf. please refer to the connection as below.
IS31LT3352 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 14 package information sop-8

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