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| IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 1 40v led driver with internal switch general description the IS31LT3350 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 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 IS31LT3350 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. 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 and sot89-5 package. sot23-5 is availabl e for 350ma output current application. sot89-5 is available for 750ma output current application. features ? simple low parts count ? internal 40v power switch ? wide input voltage range: 6v to 40v ? up to 750ma output current sot89-5 package ? high efficiency (up to 95% ) ? typical 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 protection circuitry ? two packages available for different current applications ? low voltage halogen replacement leds ? automotive lighting ? low voltage industrial lighting ? led back-up lighting ? illuminated sign application circuit 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. ) ? p otential ? liabilit y ? of ? inte g rated ? silicon ? solution , ? inc ? is ? ade q uatel y ? p rotected ? under ? the ? circumstances october 2011
IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 2 pin configurations package pin configurations (top view) sot23-5 sot89-5 pin description pin name no. description lx 1 drain of power switch 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 (0.3v IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 4 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 adjust pin input voltage -0.3v to +6v i lx switch output current 800ma@sot89-5;400ma@ sot23-5 p tot power dissipation 1.2w @sot89-5;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; 45c/w @sot89-5 esd susceptibility(human body mode) 2kv electrical characteristics (v in =12v, t a =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 adj pin grounded 40 60 80 a i inqon quiescent supply current with output switching adj pin floating 450 600 a v sense mean current sense threshold voltage IS31LT3350-v1 91 95 101 mv IS31LT3350-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 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 ? i lxmean continuous lx switch current sot23-5 package 0.35 a sot89-5 package 0.65 IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 5 electrical characteristics (continued) (v in =12v, t a =25c unless otherwise stated) (note 2) symbol parameter conditions min. typ. max. unit i lx ( leak ) lx switch leakage current 1 a r lx lx switch ?on? resistance 0.9 1.5 ? 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 lx 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 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 hyst eresis 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-ma ximum-rated conditions for extended per iods may affect device reliability. 2. all parts are production tested at t a =25c. other temperature limits are guaranteed by design. IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 6 typical performance characteristics for typical application circuit and 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 50 60 70 80 90 100 5 10152025303540 vin(v) efficiency (%) 1led 2led 3led 4led 5led 6led 7led 8led 9led 10led 50 60 70 80 90 100 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 IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 7 typical performance characteristics (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) 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) IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 8 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 3. the above values assume that the adj 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 adj pin is driven from an external voltage. output current adjustment 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 . 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 the internal 500k ? pull-up resistor. 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 i nput 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 60 a nominal. 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. adj gnd gnd IS31LT3350 mcu dc adj gnd gnd IS31LT3350 10 k IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 9 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 , the 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 IS31LT3350 are in the range of 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 ( ? ) v d 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 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 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: IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 10 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. two packages are available for different application because of thermal consideration. sot23-5 is only available for 350ma current output application. the sot89-5 package?s thermal resistance is much lower so that it can handle 700ma current and suitable for 700ma application. note that the chip power 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. 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 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 clam p 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. gnd adj v in i sense lx l 1 d1 c1 IS31LT3350 led cle d vin rs IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 11 package information sot23-5 IS31LT3350 integrated silicon solution, inc. ? www.issi.com rev. a, 10/18/2011 12 sot89-5 |
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