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| IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a , 09/01 / 2011 1 1.0mhz boost converter with 35v internal nmos description the IS31LT3505 is a constant current step-up converter with internal nmos. the step-up converter topology allows series connection of the white leds so the led currents are identical for uniform brightness as well as constant output voltage to drive other devices. the output current of each channel can be set by an external resistor and dimming the brightness of leds with the pwm signal or dc voltage. the IS31LT3505 operates with a switching frequency up to 1 mhz. a low 0.3v feedback voltage minimizes power loss in the current setting resistor for better efficiency. with ovp circuit, the chip and the system can be safe even if the load is not connected. IS31LT3505 is available in msop-10. it operates from 6v to 30v over the temperature range of -40c to +85c. features ? 6v to 30v supply voltage ? high efficiency: 90 % typical ? pwm dimming control ? fast 1.0mhz switching frequency ? internal high power 35v nmos ? internal soft start ? adjustable led open protection ? over-temperature protection ? msop-10 applications ? tv monitor backlighting ? pda, handheld computer ? gps receiver typical application circuit ovp fb lx IS31LT3505 l 1 10 h r set 6 2 10 7 vp r 1 r 2 10 f c out 22 f c 3 1 f vdd gnd c 1 1 f 4 9 1, 3, 5 en c 2 1 f 300 100k d 1 (schottky,ss26) 10nf c in 22 f 6.0v~30v figure 1 application circuit (constant current to drive white leds) ? ? ? ? ? ? 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 october 2011
IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a , 09/01 / 2011 2 pin configuration package pin configuration (top view) msop-10 pin description pin name i/o description 1 pgnd - power ground. 3,5 gnd - ground. 2 vp i/o internal 5v regulator. a power supply for the internal nmos gate driver and the internal control circuitry. 4 en i enable control input. do not let this pin floating. 6 ovp i over-voltage protection of output. 7 fb i feedback voltage of output. 8 nc - no connection, must floating. 9 vdd - supply voltage. 10 lx o the drain of the internal nmos. thermal pad - connect to ground. ordering information industrial range: -40c to +85c order part no. package qty/reel IS31LT3505-sls2-tr msop-10, lead-free 2500 1 2 3 4 7 8 9 10 agnd vdd en fb nc lx vp pgnd 5 agnd 6ovp IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 3 absolute maximum ratings parameter value supply voltage, v dd -0.3v ~ +6.0v voltage at lx pin -0.3v ~ +40v all other pins -0.3v ~ +6.0v operate temperature range -40c ~ +85c storage temperature range -65c ~ +150c junction temperature range -40c ~ +150c lead temperature (soldering, 10s) 260c r ? ja 60c/w esd hbm 4kv 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 t a = -40c ~ +85c, v dd = 12v (unless otherwise noted), typical values are at t a = 25c. symbol parameter condition min. typ. max. unit v dd supply voltage 6 30 v uvlo undervoltage threshold vp falling 2.9 v ? uvlo undervoltage threshold hysteresis 100 mv i dd supply current continuous switching 2 ma no switching 1.1 i sd shutdown current v en = 0v 15 a vp internal regulator 6v IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a , 09/01 / 2011 5 670 680 690 700 710 720 730 25 26 27 28 29 30 iout(ma) vout(v) vin=24vdc,r set =0.42 ? ,l=10uh 50 60 70 80 90 100 25 26 27 28 29 30 efficiency(%) vout(v) vin=24vdc,r set =0.42 ? ,l=10uh 250 260 270 280 290 300 310 320 6 101418222630 reference ? voltage(mv) vin(v) ? ? ? ? ? ? ? ? ? ? ???????????????????????????????????? figure 8 iout vs. vout ??????????????????????????????????? ???????????????????????????? ???????????? figure 9 efficiency vs. vout ? ? ? ? ? ? ? ? ? ? ? figure 10 v fb voltage vs. vin ? ? ? ? ? ? ? ? ? ? ? figure 11 soft-start waveform vin=12vdc, ? r set =0.88 ? , ? l=10uh, ? vout=28v IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a , 09/01 / 2011 6 figure 12 operation waveform figure 13 ovp waveform vin=12vdc, ? r set =0.88 ? , ? l=10uh, ? vout=28v IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a, 09/01/2011 7 input and output capacitor the output capacitor is decided by the output voltage ripple. a low esr electric capacitor (22uf or larger) and a 1f/50v ceramic capacitor in parallel will provide sufficient output capacitance for most applications. the input capacitor is used to reduce the input voltage ripple and noise. a low esr electric capacitor (22uf or larger) and a 1f/50v ceramic capacitor in parallel as output capacitor is recommended. place the input and output capacitors close to the IS31LT3505 to reduce the ripple. inductor inductor value involves trade-offs in performance. larger inductors reduce inductor ripple current and larger inductors also bring in unwanted parasitic resistor that degrade the performance. select an inductor with a rating current over input average current and the saturation current over the internal nmos current limit. a 10h inductor with saturation current over 2a is sufficient for the most applications. diode to achieve high efficiency, a schottky diode must be used. ensure that the diode's average and peak current rating exceed the output led current and inductor peak current. the diode's reverse breakdown voltage must exceed the over voltage protection voltage (v ovp ). therefore, a ss26 schottky diode is sufficient for the most applications. soft-start the function of soft-start is made for suppressing the inrush current to an acceptable value at startup. the IS31LT3505 provides a built-in soft-start function by clamping the input current and increasing step-by-step so that the output voltage will rise gradually in the soft- start period. led current control the IS31LT3505 regulates the led current by setting the external resistor connecting to feedback and ground. the internal feedback reference voltage is 0.3v(typ.). the led current can be set from the formula (1) easily. i led = v fb /r set (1) in order to have an accurate led current, precision resistors are preferred (1% is recommended). dimming control IS31LT3505 can modulate the brightness of leds by controlling the dc voltage or the pwm duty cycle (figure 14,15). note: the dc voltage (pwm duty cycle) is inversely proportional to the led current. that is when dc voltage is maximum (the pwm signal is 100% duty cycle), the output current is minimum, ideally zero, and when dc voltage is minimum (the pwm signal is 0% duty cycle), the output current is maximum. the output led voltage will decrease when the output current becomes lower. therefore, it must be ensure that the output voltage always higher than the input voltage during the dimming. dc voltage control figure 14 shows that the intensity of the leds can be adjusted by the dc voltage. as the dc voltage increases, the current pass through r3 increasingly and the voltage drop on r3 increases, i.e. the led current decreases. the led current can be calculated by the formula (2). the internal feedback voltage v fb is 0.3v (typ.). set fb dc fb led r r v v r v i 4 3 ) ( ? ? ? ? (2) when the dc voltage is from 0v to 5v, the value of r3 should be 10k ? . refer to figure 14. pwm signal control a filtered pwm signal acts as the dc voltage to regulate the output current. the recommended application circuit is shown as figure 15. in this circuit, the output ripple depends on the frequency of pwm signal. for smaller output voltage ripple, the recommended frequency of 5v pwm signal should be above 2khz. to the fixed frequency of pwm signal and change the duty cycle of pwm signal can get different output current. the led current can be calculated by the formula (3). the internal feedback voltage v fb is 0.3v (typ.). set fb pwm fb led r r r v duty v r v i 5 4 3 ) ( ? ? ? ? ? ? (3) when it?s the 5v pwm signal, the value of r3 should be 10k ? . refer to figure 15. setting the output voltage when IS31LT3505 drives other devices (figure 16) with the constant voltage, the output voltage is set through the formula (4). the internal feedback voltage v fb is 0.3v (typ.). v out =v fb (r 3 +r set )/r set (4) setting the over voltage protection the open string protection is achieved through the over voltage protection (ovp). in some cases, if the output voltage reaches the programmed ovp voltage (v ovp ), the protection will be triggered. to make sure the chip functions properly, the ovp setting resistor divider must be set with a proper value. the ovp voltage should be 3v higher than normal operation output voltage and the maximum should not exceed 35v. ovp pin should be connectted to a 10nf ceramic IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a , 09/01 / 2011 8 capacitor to gnd to avoid unexpected noise coupling into this pin and affecting the ovp function. the ovp threshold is calculated through the formula (5). v ovp = 0.9v(r 1 +r 2 )/r 2 (5) setting other components there is a r, c between power supply positive terminal to vdd pin. a 300 ? resistor for r and 1f ceramic capacitor for c are recommended. (note: when the input voltage is lower than 8v, the recommended value of r is 50 ? ) the vp pin, output of the internal regulator, must be connected to a 10f bypass capacitor. if the en pin is not used to enable and disable the IS31LT3505, it should be connected to power supply positive through a 100k ? resistor. the enable pin needs to be terminated and should not be left floating. pcb layout consideration as for all switching power supplies, especially those providing high current and using high switching frequencies, layout is an important design step. if layout is not carefully done, the regulator could show instability as well as emi problems. ? wide traces should be used for connection of the high current loop. ? when laying out the signal ground (pin 5), it is recommended to use the traces separate from power ground (pin1) traces and connect them together at the input capacitor negative terminal or the large ground plane that will avoid the signal ground shift. both of signal and power ground should be as wide as possible. other components ground must be connected to signal ground. especially the r set ground to signal ground (pin 5) connection should be as short as possible to have an accurate led current . ? the capacitor c vdd and c vp should be placed as close as possible to vdd and vp pin for good filtering. ? lx pin is a fast switching node. the inductor and diode should be placed as close as possible to the switch pin and the connection between this pin to the inductor and the schottky diode should be kept as short and wide as possible. avoid other traces crossing and routing too long in parallel with this node to minimum the noise coupling into these traces. ? the feedback network (fb, ovp) should be as short as possible and routed away from the inductor, the schottky diode and lx pin. the feedback pin and feedback network should be shielded with a ground plane or trace to minimize noise coupling into this circuit. ? the thermal pad on the back of package must be soldered to the large ground plane for ideal power dissipation. ovp fb lx IS31LT3505 l 1 10 h r set 6 2 10 7 vp r 1 r 2 r 3 10k r 4 156k dc control 0v - 5v 10 f c out 22 f c 3 1 f vdd gnd c 1 1 f 6.0v~30v 4 9 1, 3, 5 en c 2 1 f 300 100k c in 22f d 1 (schottky,ss26) 10 nf figure 14 application circuit (constant current to drive white leds with dc dimming) IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a , 09/01 / 2011 9 ovp fb lx IS31LT3505 l 1 10 h r se t 6 2 10 7 vp r 1 r 2 r 3 10k r 4 56k r 5 100k 1 f pwm signal 0v 5v micro controller 10 f c 3 1 f c out 22 f vdd gnd c 1 1 f 6.0v~30v 4 9 1, 3, 5 en c 2 1 f 30 0 100k c in 22f d 1 (schottky ,ss26) 10nf figure 15 application circuit (constant current to drive white leds with pwm dimming) ovp fb lx vdd gnd IS31LT3505 10 f l 1 10h c 1 1 f r set 6.0v~30v 4 9 1, 3, 5 6 2 10 7 en vp c 2 1 f c in 22f c 3 1 f r 1 r 2 r 3 d 1 (schottky,ss26) v out = v fb (r 3 +r set )/r set load 300 ? 100k ? c out 22 f 10 nf figure 16 application circuit (constant voltage to drive other devices) IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a , 09/01 / 2011 10 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 17 classification profile IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a , 09/01 / 2011 11 tape and reel information msop-10 IS31LT3505 integrated silicon solution, inc. ? www.issi.com rev. a , 09/01 / 2011 12 package information msop-10 |
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