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IS31LT3554 integrated silicon solution, inc. ? www.issi.com 1 rev. 00a, 08/01/2014 boost type led driver with 4-channel current source advanced information august 2014 general description the IS31LT3554 is a led driver based on a highly efficient boost controller operating over a wide input voltage range of 4.5v to 33v. the IS31LT3554 contains four regulated current sources with 1.5% (typ.) current matching between strings for a uniform led brightness. each current source can be programmed via an external resistor to drive from 20ma to 180ma. a fast slew rate current source allows high frequency and narrow pulse width dimming signals to achieve a very high contrast ratio. the device operating frequency can be adjusted from 0.1mhz to 1mhz. the IS31LT3554 has safety protection features to prevent damage during fault conditions. protection features include an internal soft-start circuit to prevent a high inrush current during startup, open/short led protection to automatically disable a faulty current source, over temperature protection (otp), cycle-by-cycle current limit, under voltage lockout (uvlo), programmable ovp, vout short / schottky diode open protection and schottky diode short-circuit protection. the IS31LT3554 is available in a thermally enhanced etssop-16 package. features ? input voltage range: 4.5v to 33v ? 4-channel led current sinks, 180ma per string ? led current adjustable from 20ma to 180ma ? string-to-string current matching accuracy: 1.5% ? adjustable operating frequency: 100khz to 1mhz ? external pwm dimming ? high contrast ratio ? less than 3a shutdown current ? programmable soft-start ? built-in protection features ? ocp, otp, uvlo ? open/short led protection ? programmable ovp ? schottky diode/inductor short-circuit protection applications ? lcd monitor/tv ? led lighting typical application circuit figure 1 typical application circuit
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 2 rev. 00a, 08/01/2014 pin configuration package pin configuration (top view) etssop-16 pin description no. pin description 1 rt a resistor value to ground sets the switching frequency from 100khz to 1mhz. 2 en enable input. the ic is enabled when this pin is above 2.4v. the ic is disabled and in shutdown mode when this pin is below 0.5v. in shutdown mode only a minimal 3a current is consumed. 3 cs current sense input from the switching converter. a sense resistor from the source of the external n-mosfet to gnd sets the switching current limit. 4 out switching n-mosfet gate drive output. this pin outputs a high voltage (5v/v cc -0.5v) to drive an external switching n-mosfet. 5 vcc 5v linear regulator output. bypass this pin to gnd with a ceramic capacitor as close as possible to the pin. 6 vin supply input (4.5v to 33v). bypass vin to gnd with a capacitor (typical 10f) to keep the dc input voltage constant. 7 status led operation status will output logic low if a fault is detected. 8 comp soft-start and control loop compensation. 9 dim pwm signal input for led dimming. if dimming is not implemented, connect it to vcc pin 5. 10,11 ch1,ch2 led current sink 1 and 2 (up to 180ma). if unused leave the pin open unconnected. 12 gnd ground. connect all grounds at a single point. 13,14 ch3,ch4 led current sink 3 and 4 (up to 180ma). if unused leave the pin open unconnected. 15 iset led current adjust input. connect a resistor r iset between iset pin and gnd to set the reference current through each led string. 16 ovp over voltage protection pin. connect a resistor-divider from the switching converter output to this pin. the ovp comparator reference is internally set to 2.0v. above 2.0v triggers ovp and shuts down switch power; switch resumes normal operation when the pin voltage drops below hysteresis voltage. thermal pad connect to gnd.
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 3 rev. 00a, 08/01/2014 ordering information industrial range: -40c to +125c order part no. package qty IS31LT3554-zls4-tr IS31LT3554-zls4 etssop-16, lead-free 2500/reel 96/tube copyright ? ? ? 2014 ? 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
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 4 rev. 00a, 08/01/2014 absolute maximum ratings input voltage, v in -0.3v ~ +42v ch1 to ch4 voltage, v ch -0.3v ~ +65v gnd pin voltage, v gnd -0.3v ~ +0.3v voltage in other pins, v pin -0.3v ~ +7.0v thermal resistance, ja thermal simulation @25c ambient te mperature, still air convection, 2s2p boards according to jesd51. 39.9c/w operating junction temperature, t j 150c lead temperature(soldering, 10s), t lead 260c storage temperature range, t stg - 65c ~ +150c operating ambient temperature range, t a =t j - 40c ~ +125c esd(hbm) esd(cdm) 2kv 750v note: stresses beyond those listed under ?absolute maximum ratings? ma y cause permanent damage to the device. these are stress rating s only and functional operation of the device at t hese or any other condition beyond those i ndicated in the operational sections of th e specifications is not implied. exposure to absolute maximum rating condi tions for extended periods may affect device reliability. recommended operating conditions symbol parameter condition min. typ. max. unit v in input voltage 4.5 33 v f osc switch frequency 0.1 1 mhz i ledx led channel current 20 180 ma f pwm pwm dimming frequency (note 1) 0.1 20 khz t a operating ambient temperature t a = t j -40 125 c electrical characteristics v in =12v, v en =5v, typical values are at t a = 25c, unless otherwise noted. symbol parameter condition min. typ. max. unit input supply v in input voltage 4.5 33 v i cc quiescent current no switching 3 5 ma i sd shutdown current v en = v dim = 0v 0.1 3 a v uvlo uvlo v in rising 3.8 4.2 v v uvlo_hys uvlo hysteresis 0.2 v vcc section v cc vcc voltage v in 5.5v 5 v v in < 5.5v, i load = 10ma v in -0.1 v t rising out pin rising time 1nf load (note 1) 30 50 ns t falling out pin falling time 1nf load (note 1) 30 50 ns load regulation v in = 12v, i load = 0ma ~ 30ma 5 10 mv/ma line regulation v in = 6.5v ~ 12v, i load = 1ma 3 10 mv/v
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 5 rev. 00a, 08/01/2014 electrical characteristics (continued) v in =12v, v en =5v, typical values are at t a = 25c, unless otherwise noted. symbol parameter condition min. typ. max. unit high frequency oscillator t on-time minimum on-time f = 500khz (note 1) 200 ns f osc switch frequency (10% variation) r t = 100k ? 440 520 570 khz d max maximum duty cycle f = 500khz 88 90 % enable logic and dimming logic v en_h en high voltage 2.4 v v en_l en low voltage 0.5 v v dim_h pwm logic for external dimming 2.5 v v dim_l 0.3 v t pwm_min pwm dimming minimum pulse width (note 1) 3/f osc s power switch drive v limit current limit threshold voltage 480 560 640 mv v limit2 d/i short threshold voltage 720 800 930 mv t leb current sense leb time (note 1) 80 100 150 ns compensation and soft start (comp pin) g ea error amplifier trans-conductance 2300 a/v i o_h sourcing current v comp =0.5v 55 120 200 a i o_l sinking current v comp =2v 60 120 200 a over-voltage protection v ovp ovp threshold voltage v out rising 1.8 2.0 2.2 v v ovp_hys ovp hysteresis 200 250 330 mv v ovp_sd shutdown under abnormal condition 3.0 3.2 3.6 v
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 6 rev. 00a, 08/01/2014 electrical characteristics (continued) v in =12v, v en =5v, typical values are at t a = 25c, unless otherwise noted. symbol parameter condition min. typ. max. unit current source i ch_match led current matching between each string i led = 100ma (note 2) 1.5 3 % i led regulation current per channel r set = 12k ? 92 100 108 ma v led_reg minimum led regulation voltage i led = 100ma 230 400 mv v ledx led channel voltage 60 v i led_leak ch1 to ch4 leakage current v en = 0v, v led = 60v 0.1 1 a v led_s led short protection threshold 6.6 7.3 8.0 v over-temperature protection t otsd thermal shutdown temperature (note 1) 160 oc t hys thermal shutdown recovery (note 1) 140 oc note 1: guaranteed by design and characte rization, not production tested. note 2: % 100 2 _ ? ? ? ? avg min max match st i i i i .
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 7 rev. 00a, 08/01/2014 functional block diagram
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 8 rev. 00a, 08/01/2014 application information description the IS31LT3554 is a highly integrated hbled driver. the device operates from an input voltage up to 33v. advanced features include detection and string disconnect for open led strings, partial or fully shorted strings and unused strings. overvoltage protection clamps the converter output voltage to the programmed ovp threshold in the event of an open led string condition. the status pin outputs string open or short circuit conditions and over voltage or over temperature conditions. enable when the en pin connected to a logic-low the IS31LT3554 will completely shut down, reducing its current consumption to less than 3a. the device is enabled when the logic threshold at en exceeds 2.4v and it is disabled when it is lower than 0.5v. led current setting the maximum led current per channel can be adjusted up to 180ma via iset pin. when 180ma current is needed for an application, two or more channels can be paralleled to provide larger drive current. connect resistor r set between iset pin and gnd to set the reference current i set . the led current can be expressed as below equation (1): ] [ 1200 ] [ ? ? k r ma i set led (1) the current regulator of the IS31LT3554 has a fast response so that it can allow a very high contrast ratio. dimming control a pwm signal applied to the dim pin will adjust the led current to all enabled channels. during the ?high level? period of the pwm signal, the led is turned on and 100% of the current flows, while during the ?low level? period the led is turned off and almost no current flows. this on/off operation generates an average current flow that will set the led brightness between 1%i chx_max to 100% i chx_max . a 100hz (or higher) pwm signal frequency can be applied to pwm pin. frequency selection the IS31LT3554?s switching frequency can be adjusted between 100khz to 1mhz by using an external resistor r t , placed between rt pin and gnd. a low frequency operation can help to reduce switch loss for a higher efficiency while a high frequency operation will minimize the external component size. the approximate operating frequency can be expressed as below equation (2): ] [ 52 ] [ ? ? k r mhz f t osc (2) status output the status output provides a warning of extreme operating or fault conditions. after initial power up, the status pin will output a logi c low (normally high) if any of the following conditions exists: (1) any string is open (2) led short circuit protection (3) shut down under abnormal condition (4) over voltage protection (5) over temperature protection (6) schottky diode short protection (7) vout short/open schottky diode protection protection features the IS31LT3554 is designed with integrated safety features for a reliable and stable operation in the normal operating range. in the event of more than one fault condition occurring, the higher priority condition will take precedence. soft start the internal soft start circuit prevents a high inrush current during startup. over voltage protection the IS31LT3554 integrates an ovp circuit to prevent system damage should the output voltage become excessive. to maintain a safe output level, the integrated ovp circuit continuously monitors the voltage output level. the ovp pin is connected to the center tap of voltage-divider (r ov1 and r ov2 ) connected between high voltage output and gnd. if the voltage on ovp pin exceeds 2.0v the IS31LT3554 stops switching, which causes the output voltage to drop. when the ovp pin voltage drops below the threshold the device begins oscillating once again, which causes the output voltage to rise. this ovp hysteresis is 250mv (v ovp_hys ). the formula to calculate v ovp can be expressed as below equation (3): ? ? 2 2 1 0 . 2 ov ov ov ovp r v r r v ? ? ? (3) v ovp = ovp voltage v ovp_hys = ovp hysteresis voltage
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 9 rev. 00a, 08/01/2014 over current protection the IS31LT3554 integrates an ocp circuit. the cs pin is connected to the external voltage-sense resistor (r cs ) that is placed between the drain of mos and gnd. if the voltage on cs pin exceeds 0.56v (v limit ), it is turned off immediately and will not turn on until the next cycle begins. schottky diode/inductor short circuit protection the IS31LT3554 features a schottky diode/inductor short-circuit protection circuit. when cs pin voltage exceeds 0.8v (v limit2 ) for more than 16 switching clocks, the ic will latch. the voltage of cs is monitored after a short delay of leading edge blanking signal. led open circuit protection the IS31LT3554 integrates an led open-circuit protection circuit. when any led string is open, v out will boost up until the voltage at ovp pin reaches an approximate 2.0v threshold. the ic will automatically ignore the open string whose corresponding pin voltage is less than 100mv and the remaining string will continue operation. if all the strings are open and the voltage at ovp reaches a threshold of 2.0v, the mosfet drive gate will turn off and ic will shut down and latch. led short circuit protection the IS31LT3554 integrates an led short-circuit protection circuit. if the voltage at any of the two channel pins exceeds a threshold of approximately 7.3v (v led_s ) during normal operation, the corresponding string is turned off and is latched off. to reset the latch and start operation, the v in and/or en must be toggled off/on. the priority of the led short detecting logic is lower than the open led and ovp logic. the led short circuit detection logic is triggered when v led_min is <0.1v under dimming on mode. it is disabled when an led open occurs or when the output voltage resumes to the regulated output level. v out short / open schottky diode protection the IS31LT3554 monitors the ovp pin, if the ovp pin voltage is less than 0.1v, mosfet drive output will turn off. this protects the converter if the output schottky diode is open or v out is shorted to ground. under voltage lockout the IS31LT3554 provides an under voltage lockout circuit with built in hysteresis to prevent an undefined status during startup. the uvlo circuit shuts down the device when v cc drops below 3.6v (typical) and will turn when v cc rises above 3.8v. the uvlo circuit has a 200mv hysteresis (v uvlo_hys ), which means the device will start up when v cc rises above 3.8v. over temperature protection the thermal overload prot ection prevents excessive power dissipation from overheating and damaging the IS31LT3554. if the junction temperature exceeds approximately 160oc (t otsd ), the ic will shut down to allow it to cool down. the device will begin a soft-start process when the junction temperature (t j ) of the die falls below approximately 140oc (t hys ). shut down under abnormal condition the IS31LT3554 integrates a shutdown under abnormal condition protection circuit. when the ovp pin voltage exceeds 3.2v (v ovp_sd ), the ic will latch. the en pin must then be toggled to restart the ic. this feature can be used for any other protection to shut down the ic. application information design example for IS31LT3554 this section describes a component calculation method for selecting component values when designing with the IS31LT3554 (figure 1). for the purposes of this example, the following assumptions are given as application requirements: v in = 12v led loading = 10 4 led current per channel, i led = 120ma led v f1 = 3.2v at 120ma f osc = 1mhz efficiency, = 90% once the initial operating parameters have been identified, the next step is to sequentially calculate the individual parameters in an ordered manner starting with step 1 and continuing step by step until the final step 5. step 1: calculate the led current setting resistor r set using equation (4): ] [ 1200 ] [ ? ? k r ma i set led (4) the i led = 120ma, so the r set should be: ? ? ? ? ? k ma i k r led set 10 120 1200 ] [ 1200 ] [ choose a 10k ? resistor for r set . step 2: calculate the frequency setting resistor r t using equation (5): ] [ 52 ] [ ? ? k r mhz f t osc (5) therefore, for an f osc = 1mhz, r t is calculated as:
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 10 rev. 00a, 08/01/2014 ? ? ? ? ? k mhz f k r osc t 52 1 52 ] [ 52 ] [ choose a standard resistor value of 51k ? for r t . note: a high f osc operating frequency will result in a smaller pwm duty cycle enabling high contrast dimming results. step 3: calculate ovp resistors r ov1 and r ov2 , using equation (6): ?? 2 2 1 0 . 2 ] [ ov ov ov ovp r r r v v ? ? ? (6) step 3a: calculate the maximum required led output voltage, v out . v v led series number v f out 32 2 . 3 10 ) ( 1 ? ? ? ? ? (7) step 3b: use v ovp equation (3) to calculate r ov1 and r ov2 . ?? v v v r r r v v out ov ov ov ovp 4 . 38 32 2 . 1 2 . 1 0 . 2 ] [ 2 2 1 ? ? ? ? ? ? ? ? (3) therefore, 2 1 2 . 18 ov ov r r ? ? pick a standard resistor value of 56k ? for r ov2 then calculate for r ov1 = 1.0m ? . note: multiplier 1.2x is added for design margin to cover noise and output ripple voltage. step 4: choose the input filter capacitor. the input capacitor (c in ) filters the current peaks drawn from the input supply and reduces noise injection into the IS31LT3554. a 22 f/63v electrolytic capacitor is recommended for most applications. step 5: choose the output capacitors. the output capacitors provide filtering for both the boost converter and for the pwm dimming function. the biggest factors that contribute to the size of the output capacitor are: pwm dimming frequency and pwm duty cycle. another major contri butor is leakage current, i lk; the reverse current of the switching diode. in this design the pwm dimming frequency is to be 100hz and the minimum duty cycle is 0.1%. typically, the voltage variation on the output, v cout , during pwm dimming must be less than 250mv, to minimize any audible hum. the output capacitance is calculated using equation (8): f v f d i c cout pwm min lk out ? 96 . 39 25 . 0 100 001 . 0 1 1000 1 ? ? ? ? ? ? ? ? ? (8) where i lk = 1ma. because capacitors tend to degrade when subjected to high voltages; a capacitance value greater than 39.96 f should be selected. two 22 f 63v capacitors connected in parallel are a good choice to fulfill this requirement. step 6: calculate duty cycle d on using equation (9): % 5 . 62 32 12 32 ? ? ? ? ? out in out on v v v d (9) the turn on time t on is calculated using equation (10) below: s f d t osc on on ? 625 . 0 1 625 . 0 ? ? ? (10) step 6a: calculate the average input current i in(avg), using equation (11): a v i v i in led out avg in 42 . 1 9 . 0 12 4 12 . 0 32 4 ) ( ? ? ? ? ? ? ? ? ? ? (11) equation (12) is used to calculate the maximum ripple current i ripple(max) : a i i avg in max ripple 84 . 2 42 . 1 2 2 ) ( ) ( ? ? ? ? ? (12) step 6b: calculate the minimum inductor value, l 1 . using equation (13): h i t v l max ripple on in min ccm ? 64 . 2 84 . 2 625 . 0 12 ) ( ) ( ? ? ? ? ? (13) the selected value for l 1 should be higher than the l ccm(min) . a lower inductance for l 1 will result in a narrower pwm duty cycle for high contrast dimming. the trade off for a low inductance is higher output current ripple. therefore choose a 10h inductor for l 1 .
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 11 rev. 00a, 08/01/2014 step 6c: determining the peak input current i peak, calculated as follows: first calculate the ripple current i ripple using equation (14): a l v t i in on ripple 75 . 0 10 12 625 . 0 1 ? ? ? ? ? (14) then calculate the inductor peak current using equation (15): a i i i ripple avg in peak 795 . 1 75 . 0 5 . 0 42 . 1 5 . 0 ) ( ? ? ? ? ? ? ? (15) step 6d: the peak current protect resistor r cs is shown in equation (16): ? ? ? ? ? ? ? 24 . 0 795 . 1 54 . 0 8 . 0 ] [ 54 . 0 8 . 0 ] [ a i r peak cs (16) choose a 0.235 ? resistor for r cs . generally a constant term, 0.8x, is added when calculating the r cs value. a lower r cs value will result in a higher i peak to prevent over current protection errors. step 7: the boost converter requires a diode to carry the inductor current during the mosfet off time. schottky diodes are recommended due to their fast recovery time and low forward voltage. d 1 should be rated to handle the maximum output voltage (plus switching node ringing) and the peak switch current. the conduction loss of the diode is calculated by: f off rms diode v i p ? ? _ ? ? ? ? ? ? ? ? ? ? ? ? 12 2 2 2 _ l in out in off rms i i v v i (17) where v f is the forward voltage of the schottky diode. step 8: the IS31LT3554 integrates a soft start and control loop compensation in comp pin. the soft start feature allows the boost c onverter to gradually reach its initial steady state outpu t voltage, thereby reducing startup stresses and current surges. the startup time is controlled by an internal 130 a (i cs ) current source and the external compensation circuit composed of c c and r c . when powering on, after the v in uvlo threshold is satisfied, the internal 130 a current source charges the external capacitor c c . the comp pin voltage will ramp up slowly and limit the inrush current during startup. the soft start time is determined by the equation (18): cs c comp ss i c v t ? ? (18) where v comp is the voltage of comp pin and i cs is the internal source current. typically, a value of r c =1k ? and c c =0.22f is sufficient for operating at 500khz. low-esr surface-mount ceramic capacitors (mlccs) are recommended. for most applications, ceramic capacitors with x7r temperature characteristics are preferred. these capacitors have tight capacitance tolerance (as good as 10%) and hold their value over temperature (x7r: 15% over -55c to 125c). step 9: the IS31LT3554 includes an internal low dropout linear regulator with the output pin vcc. this pin is used to power the internal pwm controller, control logic and mosfet driver. the regulator generates a 5v supply when v in 5.5v. when v in is less than 5.5v, connect vcc directly to vin. the vcc pin should be decoupled with a 2.2 f ceramic capacitor placed as close to the pin as possible. this capacitor keeps vcc voltage steady when the system operates at a high frequency. x7r type ceramic capacitors should be used for decoupling due to their good thermal stability.
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 12 rev. 00a, 08/01/2014 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 2 classification profile
IS31LT3554 integrated silicon solution, inc. ? www.issi.com 13 rev. 00a, 08/01/2014 package information etssop-16 note: all dimensions in mm unless otherwise stated.

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