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is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 1 1.0mhz boost converter with 38v internal mosfet switch may 2013 general description is31bl3508a is a boost converter featuring an integrated mosfet switch designed for driving series strings of led or other device. input operating voltage range is 2.7v~5.5v. the device regulates the led load current is set using an external, low value sensing resistor. the device adjusts the pwm duty cycle of the switch to maintain the voltage at the fb pin at a value of 0.3v. the is31bl3508a operates at a constant switching frequency of 1.0mhz to allow for small component size. is31bl3508a features an over voltage shutdown pin which prevents the output voltage from exceeding 38v (typ.) in the case of an open circuit condition. the device is offered in a tsot-23-6 and sot-23-6 package and a temperature rating from -40c ~ +85c. features ? supply voltage: 2.7v~5.5v ? built-in soft start ? drive series strings of 10 leds or 3 8 leds ? pwm or dc voltage dimming ? frequency of pwm is 500hz ~ 500khz ? 1.0mhz high speed switching frequency ? 0.3v feedback voltage ? integrated high power mosfet ? over voltage protection ? -40c ~ +85c working temperature range ? tsot-23-6 and sot-23-6 package applications ? small lcd backlights ? notebook pc ? gps typical application circuit figure 1 constant current white led driver using pwm at the en pin to control intensity note 1: c out must be rated for 50v. smaller values of output capacitance will cause the led current ripple to increase. note 2: the resistor r est should be placed as close as possible to the fb pin to prevent the emi. note 3: d 1 must choose 60v and peak current is determined by the load. please refer to page 9 for detail information.
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 2 ? ? figure 2 constant current white led driver using pwm at the en pin to control intensity (4~8) 2 leds figure 3 constant current white led driver using pwm at the en pin to control intensity 3 8 leds figure 4 constant output voltage for general purpose use
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 3 figure 5 constant output current with dc dimming input figure 6 constant output current with pwm dimming control
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 4 pin configuration package pin configurations (top view) tsot-23-6 sot-23-6 1 2 34 5 6 fb out en vcc gnd lx pin description no. pin description 1 lx switch input, connect to the inductor. 2 gnd ground. 3 fb feedback input pin. 4 en device enable pin, active high. 5 out output voltage sense pin for over voltage protection. 6 vcc supply voltage input pin. copyright ? ? ? 2013 ? 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
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 5 ordering information industrial range: -40c to +85c order part no. package qty/reel is31bl3508a-ttls2-tr IS31BL3508A-STLS2-TR tsot-23-6, lead-free sot-23-6, lead-free 3000
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 6 absolute maximum ratings supply voltage, v cc - 0.3v ~ +6.0v lx pin voltage - 0.3v ~ +40v voltage at any input pin - 0.3v ~ +6.0v maximum junction temperature, t jmax 150c storage temperature range, t stg - 65c ~ +150c operating temperature range, t a - 40c ~ +85c note: stresses beyond those listed under ?absolute maximum ratings? may cause permanent damage to the device. these are stress rating s only and functional operation of the device at these or any other condition beyond those indicated in the operational sections of th e specifications is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. electrical characteristics test conditions t a = 25c, v cc = 3.6v, unless otherwise noted. symbol parameter condition min typ max unit v cc supply voltage 2.7 5.5 v i cc quiescent current switching 1.1 1.3 ma i sd shutdown current v en = 0v 2 2.5 a v en_on en input logic high v en rising 1.4 v v en_off en input logic low v en falling 0.4 v f osc operating frequency 1 mhz d max maximum duty cycle 91.5 92.7 95.5 % r ds_on lx switch on resistance 0.7 ? i sw_lk ls switch leakage current v sw = 38v 1 a i sw_limit switch over current limit duty = 90% 1.2 1.6 1.8 a v ovp out over voltage protection 36 38 40 v v fb feedback voltage 285 300 315 mv t st over temperature protection (note 1) 160 c note 1: guaranteed by design.
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 7 typical performance characteristic power supply(v) efficiency(%) 0 10 20 30 40 50 60 70 80 90 100 2.5 3 3.5 4 4.5 5 8 wleds i led = 20ma 10 wleds i led = 20ma 38 wleds i led = 160ma figure 7 efficiency vs. v cc v cc = 3.6v i led = 20ma v cc 50mv/div v out 20mv/div time(1s/div) figure 9 ripple voltage duty cycle(%) output current(ma) 0 20406080100 0 5 10 15 20 25 en pin dimming v en = 2.8v v cc = 3.6v f = 20khz 8wleds figure 11 enable pin dimming power supply(v) reference v oltage(v) 2.5 3 3.5 4 4.5 5 5.5 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 8 wleds i led = 20ma figure 8 v fb vs. v cc v cc = 3.6v i led = 20ma f = 20khz v en 2v/div i out 10ma/div time(20s/div) figure 10 pwm dimming from en duty cycle(%) output current(ma) 0 5 10 15 20 25 020406080100 fb pin pwm dimming v pwm = 2.8v v cc = 3.6v f = 20khz 8wleds figure 12 fb pin pwm dimming
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 8 functional block diagram fb bias&ref current limite ovp band gap shutdown block slope compensation lx ramp generater osc r s q thermal vref out vcc en gnd
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 9 application information input and output capacitor a 2.2 f input capacitor is used to reduce input ripple and noise, where the input ripple amplitude is inversely proportional to the value of the input capacitance. the input and output capacitors should be placed as close to the device as possible, so as to reduce the effect of voltage ripple. the value of the output capacitor is dependent upon the output ripple. a 2.2 f or 10 f ceramic capacitor with a 50v rating is sufficient for the majority of applications. inductor value the choice of the inductor value will affect the performance of the device. large value inductors will reduce the current ripple in the inductor, thus reducing output voltage ripple. however large value inductors of the same current rating, typically also have larger dc resistance, thus reducing the system efficiency. for example in figure 1, the loads are 10 wleds and output current is 20ma. the maximum input current can reach 600ma. for this application a 10 h 700ma inductor is sufficient. diode the diode, d 1 , should use schottky diode to optimize the efficiency of system. the rated current and peak current of d 1 should be above output current and peak current of l 1 . and the recurrent peak reverse voltage of d 1 should be above ovp voltage. a 1a 60v schottky diode is recommended. led current setpoint when the is31bl3508a is used for led driving applications (figure 1), the led current flows through the external feedback resistor r set to regulate the led current. the internal feedback regulation point is 0.3v. the maximum led current may be calculated by the following equation (1). set fb max r v i / ? (1) for accurate led current, resistors should be 1% or better tolerance. led intensity control en pin dimming the en pin of the is31bl3508a can accept a pwm signal to implement led dimming (figure 1). led current may be computed using the following equation (2). pwm max led d i i ? ? (2) i max is computed using equation (1). recommended pwm frequency is in the range of 500hz ~ 500khz. so as to guarantee a reasonably good dimming effect, the minimum duty cycle should be about 10%. driving the en pin with a pwm signal can effectively adjust the led intensity. the pwm signal voltage levels must meet the en pin input voltage levels, v en_on and v en_off . is31bl3508a can also use a dc voltage or pwm signal to directly control the led current, and thus provide fine adjustment for the led intensity. fb pin dc voltage dimming the schematic of figure 5 show an example of using a dc input voltage at the fb pin to implement led dimming. when the dc input voltage increases, the current through r 2 and r 3 increases producing an offset voltage across r 2 , thereby causing the led current to be reduced. the led current may be computed using the following equation (3). v fb is 0.3v (typ). set fb dc fb led r r v v r v i 3 2 ) ( ? ? ? ? (3) when the dc voltage range is 0v~1.8v, a good value for r 2 is 75k ? and for r 3 is 360k ? . fb pin pwm dimming a pwm signal connected to the fb pin can also be used to directly control the led current (figure 6). a low pass filter converts the pwm duty cycle to a dc level, which is in turn summed together with the feedback voltage of rset to effectively reduce the led current proportionally. due to the filtering of the pwm signal, the frequency of the pwm signal can have an affect on the output voltage ripple. for a pwm signal input signal of 2.8v, the recommended frequency is greater than 2khz. using a fixed frequency pwm signal and adjusting the pwm duty cycle effectively adjusts the led intensity. led current may be computed using the following equation (4). v fb is 0.3v (typ). set fb pwm fb led r r r v duty v r v i 3 4 2 ) ( ? ? ? ? ? ? (4) for a pwm signal range of 0v~1.8v, a suitable value for r 2 is 75k ? , r 3 is 150k ? , r 4 is 200k ? . setting the output voltage the is31bl3508a can be configured to provide a constant output voltage (figure 4). the output voltage level can be computed using the following equation (5). the device internal v fb is 0.3v (typ). ? ? set set fb out r r r v v / 1 ? ? ? (5)
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 10 overvoltage threshold device open circuit protection is realized using the overvoltage protection function (ovp). if the output voltage surpasses the overvoltage threshold, the overvoltage protection circuit is activated and the device will stop working.
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 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 13 classification profile
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 12 package outline drawing tsot-23-6
is31bl3508a integrated silicon solution, inc. ? www.issi.com rev. a, 05/10/2013 13 sot-23-6 note: all dimensions in millimeters unless otherwise stated.

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