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features ? on board power mosfet ? drives up to 8 series white leds ? up to 87% efficiency ? over 645 khz fixed switching frequency ? open load shutdown ? low 100mv feedback voltage ? soft start/pwm dimming ? uvlo, thermal shutdown ? internal current limit ? available in 6l-sot26 and qfn-8 packages applications ? cell phones ? handheld computers and pdas. ? digital still cameras. ? small lcd displays description the PS1518 is a step up converter designed for driving up to 8 series white leds from 5v input. the PS1518 uses current mode, fixed frequency architecture to regulate the led current, which is measured through an external current sense resistor. its low 100mv feedback voltage reduces power loss and improves efficiency. the ov pin monitors the output voltage and turns off the converter if an over-voltage condition is present due to an open circuit condition. the PS1518 includes under-voltage lockout, current limiting and thermal overload protection preventing damage in the event of an output overload. the PS1518 is available in small 6-pin tsot23. typical application circuit high efficiency, fixed frequency wled driver PS1518 july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 1
PS1518 pin configuration pin designator name pin type function sw 1 switch connect inductor between sw and in. gnd 2 ground ground pin fb 3 feedback adjustable feedback input, connect to resistor voltage divider. en 4 enable input. en=high: normal operat ion. (supports both ttl and cmos logic). ovp 5 over voltage input. measures the output voltage for over voltage protection. in 6 battery input. boost regulator input. july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 2
PS1518 block diagram july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 3
1. absolute maximum ratings symbol parameter value unit v sw dc voltage at pin 1 -0.5 to +30 v v en enable input voltage at pin 2 -0.3 to +6.0 v p d continuous power dissipation internally limited w t stg storage temperature range -65 to +150 r ja thermal resistance, junction-to-air 235 /w t j,max operating junction temperature -40 to +125 t l lead temperature (soldering, 5sec) 260 PS1518 esd esd capability, hbm model 2.0 kv recommended operating conditions symbol parameter value unit v in dc supply voltage at pin 1 +2.5 to 6 v v en enable input voltage at pin 2 0 to v in v -40 to +125 t j operating junction temperature july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 4
electrical characterstics: (all specifications are at t a = +25c. v in = 5.0v, unless otherwise specified.) symbol parameter test condition min typ max units v in operating input voltage 2.5 6 v uvlo under voltage lock-out vin going lo 2.35 2.4 v uvlo hysteresis 20 mv v out-max maximum output voltage 25 v i q supply current (quiescent) no switching (vin =5v, v fb =125mv) 150 260 a supply current switching fb=0 540 750 a i sh supply current (shut- down) v en =0 0.1 1 a f osc operation frequency 645 khz d max maximum duty cycle 80 90 % v fb feedback voltage 90 100 110 mv feedback input bias current v fb =125mv 2 na r ds(on) mosfet on resistance vin=2.5, isw= 800ma 0.65 i lim current limit 690 ma turn on 1 v v en enable threshold turn off 0.4 v i en enable input bias current v en = 5v 1 a thermal shut-down 160 PS1518 0 c july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 5
PS1518 typical operating characterstics ( all specifications are at t a = +25c, v in = 5.0v, unless otherwise specified.) july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 6
PS1518 typical operating characterstics (continued) july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 7
PS1518 typical operating characterstics (continued) july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 8
PS1518 typical operating characterstics (continued) july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 9
operation description the PS1518 is a high efficiency, fixed frequency, peak current mode boost regulator. it has the architecture to regulate the voltage at the feedback pin, so that a regulated fixed current is achieved to drive white leds. the power mos is turned on through the control circuitry, at the start of each oscillator cycle and thus the charging phase is initiated. the error amplifier, consisting of a voltage comparator and current sense amplifier, is basically, a pwm comparator. the voltage comparator amplifies the difference between the reference and feed back voltage. when the output of the current sense amplifier, reaches the output of the voltage comparator, the power mos is turned off and thus the charging phase is terminated. to prevent sub-harmonic oscillations at duty cycles greater than 50 percent, a stabilizing nonlinear ramp is added to the current sense signal. in this way the peak current level keeps the output in regulation. the PS1518 has internal soft start mechanism, to limit the inrush current at startup and to limit the amount of overshoot on the output, also. an internal blanking time is provided during start up, to prevent the start of switching before all the circuitry become ready for operation. the current limit is increased by around a fourth every 15s giving a total soft start time of 70s. setting the led current : p s 1 5 1 8 the led current can be set, according to the requirement, by feedback resistor, r1. the current through the leds is selected by the following equation: i led =100mv/r1 current limit : the PS1518 includes a current limiter. it monitors the peak current through the inductor and controls gate of the power device. enable input : the PS1518 features an active-high enable input (en) pin that allows on/off control of the regulator. the PS1518 bias current reduces to less than microampere when it is shutdown. the output remains at a schottky forward voltage lower than the sw pin, if a schottky diode is connected between sw and ov pin, at shut. the enable input is ttl/cmos compatible. under voltage lockout: when the input supply goes too low (below 2.35v) the PS1518 produces an internal uvlo (under voltage lockout) signal that shuts down the chip. this mechanism protects the chip from producing false logic due to low input supply. thermal overload protection: thermal-overload protection limits total power dissipation in the PS1518. when the junction temperature exceeds t j = +160 c, the thermal sensor signals the shutdown logic and turning off most of the internal circuitry. the thermal sensor turns internal circuitry on again after the ic?s junction temperature drops by 20 c. the regulator then starts functioning in the required mode based on the supply voltage. thermal-overload protection is designed to protect the PS1518 in the event of a fault condition. for continual operation, do not exceed the absolute maximum junction temperature rating of tj = +150 c. open load protection: open load protection in PS1518, protects the chip from destruction, due to excessive high voltage. when, in any case, one or more leds in the led string fails, the feedback pin is pulled down to zero. as a result, the chip runs at maximum duty cycle, boosting the output voltage higher and higher. the open load protection mechanism, checks this condition, if ov pin is tied to the top of the led string. if the output voltage exceeds the ov threshold (28v), the switching stops, allowing the output to discharge. the open load protection mechanism includes a hysteretic comparator. the switching is enabled again, when the output voltage has fallen to a certain level. because the chip doesn?t turn off fully, no power recycling is necessary, when this condition takes place. but it?s highly recommended, not to use the chip in that mode of operation for longer span of time. july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 10
PS1518 dimming control: led is a current driven device. hence, current through the led needs to be controlled to have dimming control. different ways are there to control dimming for PS1518, in the normal mode of operation. in the first way, the feedback voltage is controlled using an external voltage source. as shown in the figure 8, current starts flowing down r1, r2 and r3, as the external voltage increases. the loop will continue to regulate the feedback voltage to 110mv, and as a result, the current through the leds has to decrease as the same amount of that being injected from the external source. with the external voltage from 0 to 2v, the resistor values shown for r2 and r3 can control led current from 0 to 20ma. dimming can also be achieved using logic signal to en pin. as shown in figure 9, the pwm signal can be applied to the en pin of PS1518. the leds will switch between full load to completely shut down state. the average current through the leds will be proportional to the duty cycle of pwm. the pwm signal in figure 9 should be of 1 khz or less, because of the presence of the soft-start mechanism. july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 11 figure-b: pwm dimming control using logic signal PS1518 figure 9: pwm dimming control using logic signal figure-a : dimming control using a dc voltage figure-c: dimming control using a filtered pwm signal PS1518 fi g ure 8: dimmin g control usin g a dc volta g e PS1518 figure 10: dimming control using a filtered pwm signal.
PS1518 disclaimers life support power ic?s products are not designed to be used as co mponents in devices intended to support or sustain human life. the use of power ic?s products in components in tended for surgical implants into the body or other applications, in which failure of power ic?s products co uld create a situation where personal death or injury may occur, is not authorized without the express wri tten approval of power ic?s chief executive officer. power ic will not be held liable for any damages or claims resulting from the use of its products in medical applications. military power ic?s products are not designed for use in milit ary applications. the use of power ic?s products in military applications is not authorized without the ex press written approval of power ic?s chief executive officer. power ic will not be held liable for any damages or claims resulting from the use of its products in military applications. right to make changes power ic reserves the right to change this document and/or this product without notice. customers are advised to consult their power ic sale s representative before ordering. july 2007 www.picsemi.com ? 2007 power ic. all rights reserved. 12

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