fn7370 rev 1.00 page 1 of 8 december 21, 2005 fn7370 rev 1.00 december 21, 2005 ISL97631 white led boost regulator with integrated schottky diode datasheet the ISL97631 represents a high efficiency, constant frequency pwm regulator wit h integrated boost diode and fet. designed for use in white led driving applications, the ISL97631 features efficienc ies up to 85%. it operates at 1.35mhz switching frequency and operates from an input voltage of between 2.7v and 5.5v. the maximum output voltage of 27v enables the isl9 7631 to drive up to 6 leds in series. it is also possible to use the ISL97631 to drive leds in series/parallel combination for applications requiring up to 15 leds. available in the 6 ld tsot package, the ISL97631 offers high efficiency, constant frequency operation. it is specified for operation over the -40c t o +85c ambient temperature range. pinout ISL97631 (6 ld tsot) top view features ? up to 6 leds in series ? 27v maximum output ? 2.7v to 5.5v input ? up to 85% efficient ? 1.35mhz constant frequency ? enable/pwm dimming control ? pb-free plus anneal available (rohs compliant) applications ? led backlighting ? cell phones ?pdas ? handheld games ?mp3 players ?gps ? other handheld devices 1 2 3 6 4 vout gnd lx vin enab 5fb ordering information part number part marking pcs. tape & reel package (tape and reel) pkg. dwg. # ISL97631ihtz-t7 (see note) 631z 3,000 7 6 ld tsot (pb-free) mdp0049 ISL97631ihtz-t7a (see note) 631z 270 7 6 ld tsot (pb-free) mdp0049 note: intersil pb-free plus anneal products employ special pb-fr ee material sets; molding compounds/die attach materials and 10 0% matte tin plate termination finish, which are rohs compliant and compatible wit h both snpb and pb-free solderi ng operations. intersil pb-free products are msl classified at pb-free peak reflow temperatures that meet or exc eed the pb-free requirements of ipc/jedec j std-020. not recommended for new designs recommended replacement part isl97634
ISL97631 fn7370 rev 1.00 page 2 of 8 december 21, 2005 absolute maximum ratings (t a = 25c) thermal information input voltage (v in ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3v to +6v lx, vout voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3v to +27v fb voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3v to +6v enab voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3v to +6v operating temperature . . . . . . . . . . . . . . . . . . . . . . .-40c to +85c thermal resistance (typical, note 1) ? ja (c/w) 6 ld tsot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 maximum junction temperat ure (plastic package . . . . . . . . . 150c maximum storage temperature range . . . . . . . . . .-65c to +1 50c maximum lead temperature (soldering, 10s). . . . . . . . . . . . +300c (tsot - lead tips only) caution: stresses above those listed in ?absolute maximum ratings? may cause permanent damage to the device. this is a stress o nly rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. important note: all parameters having min/ma x specifications are guaranteed over the specified temperature range. all parameter s are based on pulsed tests, therefore: t j = t c = t a note: 1. ? ja is measured with the component mounted on a high effective the rmal conductivity test board in free air. see tech brief tb379 for details. electrical specifications v in = 3v, v enab = 3v, t a = -40c to 85c unless otherwise specified. parameter description condition min typ max unit v in-min minimum operating voltage v out = 16v, i led = 20ma 2.7 v v in-max maximum operating voltage v out = 25v, i led = 20ma 5.5 v v fb feedback voltage 80 100 120 mv i fb fb pin bias current 100 na i in supply current enab = 3v, output not switching 0.6 1.0 ma enab = 0v 1 a f osc switching frequency 0.8 1.35 1.8 mhz d max maximum duty cycle t a = 25c 85 90 % 82 90 % i lim switch current limit t a = 25c 280 350 ma 250 350 ma r ds(on) switch on resistance i lx = 100ma 750 m ? i leak(lx) switch leakage current v lx = 27v, vout = 27v 0.01 1 a i leak(vout) diode leakage current v out = 27v 0.01 1 a v diode lx-v out diode forward voltage i diode = 100ma, t a = 25c 0.75 0.9 v v enab-hi enab voltage high 2.5 v v enab-lo enab voltage low 0.6 v i enab enab pin bias current 1a ? i led / ? v in line regulation v in = 2.7v to 5v 0.2 %/v eff efficiency i led = 20ma, 3 leds 85 %
ISL97631 fn7370 rev 1.00 page 3 of 8 december 21, 2005 typical application figure 1. typical application circuit figure 2. efficiency vs led current (v in = 4v) ISL97631 c1 l1 22h c2 0.22f r set 4.75 ? 2.7v~5.5v off/on leds 1f v in lx enab fb gnd v in v out 50 55 60 65 70 75 80 85 90 0 5 10 15 20 25 30 i led (ma) efficiency (%) 22h, v in = 4v typical performance curves figure 3. quiescent current vs v in (enab = hi) figure 4. load regulation (v in = 4v) figure 5. line regulation figure 6. switching frequency vs tempera ture 0 100 200 300 400 500 600 700 024 iq (a) 6 v in (v) 18.228 18.230 18.232 18.234 18.236 18.238 18.240 18.242 18.244 0 5 10 15 20 v out (v) i o (ma) 18.23 18.24 18.25 18.26 18.27 18.28 18.29 18.30 18.31 18.32 18.33 2.5 3.5 4.5 5.5 v in (v) i o (ma) 1.27 1.28 1.29 1.30 1.31 1.32 1.33 1.34 1.35 1.36 1.37 -60 -40 -20 0 20 40 60 80 100 temperature (c) frequency (mhz)
ISL97631 fn7370 rev 1.00 page 4 of 8 december 21, 2005 block diagram detailed description the ISL97631 uses a consta nt frequency, c urrent mode control scheme to provide excellent line and load regulation. it can drive up to 6 leds in series or 15 leds in parallel/series configuration, with efficiencies of up 85%. the ISL97631 operates from an input voltage of 2.7v to 5.5v and can boost up to 27v. steady-state operation the ISL97631 operates with constant fre quency pwm. the switching frequency is around 1.35mhz. depending on the input voltage, inductance, number of leds and the led current, the converter operates in either continuous conduction mode or discontinuous conduction mode. both are normal. the forward current of the led is set using the r set resistor. in steady state mode, this curr ent is given by the equation: shut-down when taken low the enab pin places the ISL97631 into power down mode. when in power down, the supply current is reduced to less than 1a. dimming control pwm dimming the enab pin also doubles as a brightness control. there are two different possible dimm ing control meth ods. the first dimming method is controlled th rough the duty- cycle of the enab input pwm waveform, which can operate at frequencies up to 1khz. for frequencies greater than 1khz, see analog dimming. the leds operate at either zero or full current. this is the pwm dimming control method. the relationship between the av erage led current and the duty- cycle (d) of the enab pins waveform is as follows: figure 7. ISL97631 block diagram fet driver pwm logic controller current sense gm amplifier 1.2mhz oscillator an ramp generator bangap reference generator 95mv gm amp compensation pwm comparator vin enable lx gnd fb ISL97631 vout - fet driver pwm logic controller current sense gm amplifier 1.2mhz oscillator and ramp generator bandgap reference generator 95mv gm amp compensation pwm comparator vin enable lx gnd fb ISL97631 vout - pin descriptions pin number pin name description 1 vout output pin. connect to the anode of the top led and the output filter capacitor . 2 gnd ground pin. connect to local ground. 3 lx switching pin. connect to inductor. 4 enab enable pin. connect to enable signal to turn-on or off the device. 5 fb feedback pin. connect to the cathode of bottom led and the sense resistor. 6 vin input supply pin. connect to the input supply voltage, the inductor and the input supply decoupling capacitor. i led v fb r set --------------- = (eq. 1) average i led v fb r set --------------- d ? = (eq. 2)
ISL97631 fn7370 rev 1.00 page 5 of 8 december 21, 2005 the magnitude of the pwm signal should be higher than the minimum enab voltage high. t he bench pwm dimming test results are shown in figure 8. in the test, two pwm frequencies 400hz and 1khz are chosen to compare the linear dimming range. it is clear that there is a wider linear dimming range for the lower pwm frequency than for the higher one, due to the self discharge of the output capacitor through the leds during the low enab periods. to achieve a better linearity with high frequencies an nmos fet can be placed between the fb pin and the led stack, with its gate driven by the same signal as enab. this acts to prevent self discharge of the output capacitor during the off periods. in the pwm dimming test, the output capacitor is 0.22f. analog dimming the second dimming method applies a variable dc voltage (v dim ) at fb pin (see figure 9) to adjust the led current. as the dc dimming signal voltage increases above v fb , the voltages drop on r 1 and r 2 increase and the voltage drop on r set decreases. thus, the led current decreases. the dc dimming signal v oltage can be a va riable dc voltage or a dc voltage generated by filtering a high frequency pwm control signal. as brightness is directly proportional to led currents, v dim may be calculated for any desired relative brightness (f) using equation 4. where f = i led (dimmed)/i led (undimmed). these equations are valid for values of r1 and r2 such that both r1>>rset and r2>>rset. the analog dimming circuit can be tailored to a desired relativ e brightness for different v dim ranges using equation 5. where v dim_max is the maximum v dim voltage and f min is the minimum relative brightness (i.e., the brightness with v dim_max applied). i.e., v dim_max = 5v, f min = 10% (i.e., 0.1), r 2 = 189k i.e., v dim_max = 1v, f min = 10% (i.e., 0.1), r 2 = 35k open-voltage protection in some applications, it is possible that the output is opened, e.g. when the leds are disconnected from the circuit or the leds fail. in this case the feedback voltage wil l be zero. the ISL97631 will then switch to a high duty cycle resulting in a high output voltage, which may cause the lx pin voltage to exceed its maximum 27v rating. to implement overvoltage protection, a z ener diode dz and a resistor r 1 can be used at the output and fb pin to limit the voltage on the lx pin as shown in figure 10. it is clear that as the zener is turned on, due to the overvoltage, the zener diodes current wi ll set up a voltage on r 1 and r set and this voltage is applied on fb pin as the feedback node. this feedback will prevent the output from reaching the overvoltage condition. in the overvoltage protection circuit d esign, the zener voltage should be larger than the maximum forward voltage of the led string. figure 8. pwm dimming linear range (for 400hz and 1khz pwm frequencies condition, c out = 0.22f) 0 2 4 6 8 10 12 14 16 18 20 020406080100 duty-cycle (%) i o (ma) 400hz 1khz i led v fb r 1 r 2 + ?? v dim r 1 ? C ? r 2 r ? set --------------------------------------------------------------- ---------- - = (eq. 3) v dim r 2 r 1 ------ - v fb 1 r 1 r 2 ------ - f C + ?? ?? ?? ?? = (eq. 4) figure 9. analog dimming control application circuit ISL97631 c1 v in lx enab fb gnd l1 22h c2 0.22f r set 4.75 ? off/on leds 1f lx v in 2.7v~5.5v r1 r2 v dim v out 3.3k r 2 v dimmax v fb C ?? r 1 ? ?? v fb 1f min C ?? ? ?? --------------------------------------------------------------- ---- = (eq. 5)
ISL97631 fn7370 rev 1.00 page 6 of 8 december 21, 2005 components selection the input capacitance is typically 0.22f. the output capacitor should be in the range of 0.22f to 1f. x5r or x7r type of ceramic capacitors of the appropriate voltage rating are recommended. the output capacitor value affects pwm dimming performance. lower outpu t capacitor values increase the range of pwm dimmin g. however, the ripple voltage will be greater for lower values. when choosing an inductor, make sure the average and peak current ratings are adequate by using the following formulas (80% efficiency assumed): where: ? ? i l is the peak-to-peak inductor current ripple in amps ? l is the inductance in h. ?f osc is the switching frequency, typically 1.35mhz the ISL97631 supports a wide range of inductance values (22h~82h). for lower inductor values or lighter loads, the boost inductor current may become discontinuous. for high boost inductor values, the boost inductor current will be in continuous mode. the demo board efficiency bench test results are shown in figure 11. the input voltage is 4v and curves are shown for 2, 3, 4, 5 and 6 leds (boost inductor l = 22h). white led connections one leg of leds connected in series will ensure brightness uniformity. the 27v maximum output voltage specification enables up to 6 leds to be placed in series. in order to drive more leds, s eries/parallel co nnections are used. a current mirror circuit (as shown in figure 12) should b e used to balance led currents. parallel strings of leds may draw significantly different currents due to manufacturing and temperature differences. for correct operation of the current mirror circuit, the total voltage between fb pin and the top of the primary led string must be equal to or greater than the other strings. to ensure this, a small value resistor may be inserted between fb pin and the primary led string. figure 10. led driver with overvoltage protection circuit ISL97631 c1 l1 22h c2 0.22f r set 4.75 ? 2.7v~5.5v off/on leds 1f v in lx enab fb gnd v in r1 dz v out i lavg i led v out ? 0.8 v in ? --------------------------------- = (eq. 6) i lpk i lavg 1 2 -- - i l ? ? + = (eq. 7) i l ? v in v out v in C ?? ? lv out f osc ?? -------------------------------------------------- - = (eq. 8) 50% 55% 60% 65% 70% 75% 80% 85% 90% 0 5 10 15 20 25 30 i led (ma) efficiency (%) figure 11. efficiency curve with 2, 3, 4, 5 and 6 leds load 22h, v in = 4v 3led 4led 5led 2led 6led figure 12. leds in series/parallel with mirror current balance el7630 c1 vdd lx enab fb gnd l1 d1 c2 r set v in 2.7v~5.5v off/on leds ISL97631 vin lx enab fb gnd l1 d1 vout
ISL97631 fn7370 rev 1.00 page 7 of 8 december 21, 2005 pcb layout considerations the layout is very important for the converter to function properly. r set must be located as close as possible to the fb and gnd pins. longer traces to t he leds are ok . similarly, the supply decoupling cap and the ou tput filter cap should be as close as possible to the v in and v out pins. the heat of the ic is mainly dissipated through the gnd pin. maximizing the copper area connected to this pin is preferable. in addition, a solid ground plane is always helpful for the emi performance. the demo board is a good example of layout based on the principle. please refer to the ISL97631 application brief for t he layout.
fn7370 rev 1.00 page 8 of 8 december 21, 2005 ISL97631 intersil products are manufactured, assembled and tested utilizing iso9001 quality systems as noted in the quality certifications found at www.intersil.com/en/suppor t/qualandreliability.html intersil products are sold by description on ly. intersil may modify the circuit design an d/or specifications of products at any time without notice, provided that such modification does not, in intersil's sole judgment, affect the form, fit or function of the product. accordingly, the reader is cautioned to verify that datasheets are current before placing orders. information fu rnished by intersil is believed to be accu rate and reliable. however, no responsib ility is assumed by intersil or its subsidiaries for its use; nor for any infrin gements of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of intersil or its subsidiaries. for information regarding intersil corporation and its products, see www.intersil.com for additional products, see www.intersil.com/en/products.html ? copyright intersil americas llc 2005. all rights reserved. all trademarks and registered trademarks are the property of their respective owners. package outline
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