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IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 1 40v led driver with external switch general description the IS31LT3354 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 device operates from an input supply between 6v and 40v and provides an externally adjustable output current of up to 2a or even higher, which is determined by the external mosfet and inductor. the IS31LT3354 includes a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current. output current can be adjusted linearly by applying an external control signal to the adj pin. the adj pin will accept either a dc voltage or a pwm waveform. this will provide either a continuous or a gated output current. applying a voltage of 0.2v or lower to the adj pin turns the output off and switches the chip into a low current standby state. the chip is assembled in sot23-5 package. features ? simple low parts count ? wide input voltage range: 6v to 40v ? output current only limited by external component selection ? high efficiency (up to 98% ) ? typical 1200:1 dimming ratio ? typical 3% 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 protection circuitry applications ? low voltage halogen replacement leds ? automotive lighting ? low voltage industrial lighting ? led back-up lighting ? illuminated sign typical application circuit gnd adj v in i sense gate rs l1 d1 sn3354 c2 5 4 1 2 3 up to 10 leds vin(dc)=6v~40v ap2306 c1 0.1uf c3 figure 1 typical IS31LT3354 application schematic ? ? ? ? ? ? ? ? ? ? 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 september 2011 IS31LT3354
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 2 pin configuration package pin configuration (top view) sot23-5 pin description pin name no. description gate 1 output gate driver for an external nmosfet gnd 2 ground (0v) adj 3 multi-function on/off and brightness control pin: * leave floating for normal operation.(v adj = v ref = 1.2v giving nominal average output current i out nom =0.1/r s ) * drive to voltage below 0.2v to turn off output current * drive with dc voltage (0.3vIS31LT3354?stls2-tr sot23-5, lead-free 3000
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 3 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 gate gate pin voltage -0.3v to +6v v adj adjust pin input voltage -0.3v to +6v p tot power dissipation 600mw @sot23-5 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 108c/w @sot23-5 esd susceptibility (human body model) 4kv electrical characteristics t a = 25c (note 2) symbol parameter conditions min. typ. max. unit v in input voltage 6 40 v i inqoff quiescent supply current with output off adj pin grounded 80 95 110 a i inqon quiescent supply current with output switching adj pin floating 450 600 a v sense mean current sense threshold voltage 91 96 101 mv 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 adj pin with pin floating 1.2 v v adj external control voltage range on adj pin for dc brightness control 0.3 1.2 v v adjoff dc voltage on adj pin to switch chip from active (on) state to quiescent (off) state v adj falling 0.15 0.2 0.25 v v adjon dc voltage on adj pin to switch chip from quiescent (off) state to active (on) state v adj rising 0.2 0.25 0.3 v r adj resistance between adj pin and v ref 500 k ?
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 4 electrical characteristics t a = 25c, v in = 12v (note 3) symbol parameter conditions min. typ. max. unit i source gate sourcing current 80 ma i sin k gate sinking current 100 ma 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 low frequency pwm signal pwm frequency =10khz pwm amplitude=5v,vin=15v, l=27uh, driving 1 led 13:1 f sw operating frequency adj pin floating l=100 h (0.82 ? ) i out =350ma @ v led =3.4v driving 1 led 154 khz t onmin minimum switch ?on? time 200 ns t offmin minimum switch ?off? time 200 ns f swmax recommended maximum operating frequency 1 mhz d sw recommended duty cycle range of output switch at f swmax 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 may cause permanent damage to the device. these are stress ratin gs only, and functional operation of the device at these or any other conditions beyond those indicated under recommended operating cond itions is not implied. exposure to absolute-maximum-rated conditions for extended periods may affect device reliability. 2. all parts are production tested at t a = 25c. other temperature limits are guaranteed by design 3. guaranteed by design
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 5 50% 60% 70% 80% 90% 100% 0 5 10 15 20 25 30 efficiency vout(v) 18v 24v 30v vin l=47uh, ? rs=0.05 ? mos=ap2306 1800 1850 1900 1950 2000 2050 2100 2150 2200 0 5 10 15 20 25 30 iout(ma ) vin(v) 1led 2leds 3leds 4leds 5leds 6leds 7leds 8leds l=47uh, ? rs=0.05 ? mos=ap2306 1800 1850 1900 1950 2000 2050 2100 2150 2200 0 5 10 15 20 25 30 iout(ma) vout(v) 24v 30v 18v vin l=47uh, ? rs=0.05 ? mos=ap2306 typical performance characteristics for typical application circuit and t amb =25c unless otherwise stated. efficiency vs. no. of leds efficiency vs. output voltage output current variation with supply voltage output current variation with output voltage
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 6 0 100 200 300 400 500 0 5 10 15 20 25 30 35 40 iin ? (ua) vin ? (v) 0 20 40 60 80 100 120 0 5 10 15 20 25 30 35 40 vin(v) iin(ua) supply current vs. vin (operating) 1.1985 1.199 1.1995 1.2 1.2005 1.201 5 10152025303540 vin(v) vref(v) vref vs. vin shutdown current vs. vin (quiescent)
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 7 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 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) 0.05 2000 0.083 1200 0.15 667 0.3 333 vsense is divided into two ranges to improve current accuracy, please refer to bin information on page 3. the above values assume that the adj pin is floating and at a nominal voltage of v ref =1.2v. it is possible to use different values of r s if the adj pin is driven from an external voltage. rs need to be chosen 1% accuracy resistor with enough power tolerance and good temperature characteristic to ensure stable output current. output current adjust ment by external dc control voltage the adj pin can be driven by an external dc voltage (v adj ), as shown, to adjust the output current to a value above or below the nominal average value defined by r s . adj gnd sn3354 dc the nominal average output current in this case is given by: i outdc = 0.083*v adj /r s [for 0.3v< v adj <1.2v] note that 100% brightness setting corresponds to v adj = v ref . when driving the adj pin above 1.2v, the current will be clamped to 100% brightness automatically. the input impedance of the adj pin is 500k ? 25%. output current adjustment by pwm control directly driving adj input a pulse width modulated (pwm) signal with duty cycle d pwm can be applied to the adj 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 is from 0v~5v. the pwm signal must have the driving ability to drive internal 500k ? pull-up resistor. adj gnd sn3354 0v 5v driving the adj 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 adj 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. see the section on pwm dimming for more details of the various modes of control using high frequency and low frequency pwm signals. shutdown mode taking the adj pin to a voltage below 0.2v will turn off the output and supply current will fall to a low standby level of 95 a nominal. IS31LT3354 IS31LT3354 IS31LT3354
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 8 inherent open-circuit led protection if the connection to the led(s) is open-circuited, the coil is isolated from the switch, so neither the chip nor the switch will be damaged, unlike in many boost converters, where the back emf may damage the 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. if the source is 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, sometimes the lowest value will be lower than the forward voltage of led strings. this 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. switch mosfet selection the IS31LT3354 demands a power n-mosfet as a switch. the voltage and current rating of the mosfet must be higher than the application output voltage and the inductor peak current. the v gs(th) of mosfet should be lower than 3v and the r dson should be as low as possible for maximum efficiency and performance. ap2306 and ap2310 are recommended. note: for the recommended mosfets, the maximum load current is about 2a. for high current applications, the operating input voltage, the led current, and the switching frequency will determine the operating temperature of the mosfet. switching frequency can be lowered by choosing a larger value of inductance, however, the mosfet specifications must be carefully analyzed first. the key specifications to consider are r dson and c ds , both should be as low as possible. inductor selection recommended inductor values for the IS31LT3354 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 gate 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 an 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. switch 'on' time ) ( lx s avg led in on r rl r i v v i l t ? ? ? ? ? ? note: t onmin >200ns 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 ( ? ) v d is the diode forward voltage at the required load current (v)
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 9 example: for v in =12v, l=47 h, r l =0.64 ? , v led =3.4v, i avg =333ma and v d =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 the 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 diodes will increase the peak voltage on the switch. if a silicon diode is used, care should be taken to ensure that the total voltage appearing across the switch 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: gnd adj v in i sense gate rs l1 d1 sn3354 c2 5 4 1 2 3 ap2306 c1 0.1uf cled a value of 1 f will reduce nominal ripple current by a factor three (approx.). proportionally, lower ripple can be achieved with higher capacitor values. note that the capacitor will not affect operating 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 driving to the switch until the supply has risen above the startup threshold set internally which makes the power mosfet?s on-resistance small enough. above this threshold, 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 sufficient to prevent the chip from switching below approximately 6v. this will minimize the risk of damage to the chip. thermal considerations the IS31LT3354 utilizes an external mosfet to switch the inductor current, and thus dissipates very little power. the thermal characteristics of the mosfet dominate in typical application circuits for the IS31LT3354. care should be taken to ensure a large copper ground plane and a good thermal conductivity between the mosfet and the ground plane. note that the switch power dissipation increases with increasing supply voltage. this is caused primarily by two things, the resulting increase in switching frequency and the higher voltage across the switch during the off time. this may result from the use of unsuitable coils, or excessive parasitic output capacitance on the switch output. IS31LT3354
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 10 layout considerations external mosfet drain the drain of the external mosfet 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 inductance, which will degrade efficiency. it is also important to take account of any trace resistance in series with current sense resistor r s . adj pin the adj 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. adj 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 adj=1.2v. high voltage traces avoid running any high voltage traces close to the adj pin, to reduce the risk of leakage due to board contamination. any such leakage may raise the adj pin voltage and cause excessive output current. a ground ring placed around the adj pin will minimize changes in output current under these conditions.
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 11 classification reflow profiles profile feature pb-free assembly preheat & soak temperature min (tsmin) temperature max (tsmax) time (tsmin to tsmax) (ts) 150c 200c 60-120 seconds average ramp-up rate (tsmax to tp) 3c/second max. liquidous temperature (tl) time at liquidous (tl) 217c 60-150 seconds peak package body temperature (tp)* max 260c time (tp)** within 5c of the specified classification temperature (tc) max 30 seconds average ramp-down rate (tp to tsmax) 6c/second max. time 25c to peak temperature 8 minutes max. figure 19 classification profile
IS31LT3354 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 12 packaging information sot23-5

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