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ht7938 high current and performance white led driver selection table part no. package marking ht7938 sot23-6 7938 rev 1.70 1 june 25, 2016 features efficiency up to 85% at v in =3.6v, 9leds, i led =20ma 1.2mhz fixed switching frequency low standby current: 0.1 a (typ.) at v en =0v matches led current tiny inductor and capacitors up to 10 strings white-led (led v f(max.) =3.5v) tiny 6-lead sot23-6 package built in ovp, ocp, otp, uvlo protection applications cellular phones pdas dscs gps handheld devices white led display backlighting general description ht7938 is high efficiency boost converter with constant current output to provide backlight in handheld devices. series connection of leds provides constant identical led currents resulting in uniform brightness. the con - tinuous led current is set with fb pin regulated voltage across an external sense resistor (r fb ) connected from that pin to ground. the built-in open load protection pre - vents the damage resulting from an open circuit condi - tion. low 200mv feedback voltage minimizes power loss in the current setting resistor for better efficiency. ht7938 switches at rates up to 1.2mhz to allow the use of extremely small inductor and filter capacitor.
block diagram pin assignment pin description pin name i/o description en i shutdown & dimming control input. don t allow this pin to float. gnd signal ground. fb i feedback pin. reference voltage. the ht7938 feedback voltage is 200mv. connect the sense resistor from fb to gnd to set the led current. calculate resistor value according to r mv i fb led = 200 . sw i switching pin. internal power mosfet drain. connected to inductor and diode. vin i input supply pin. the input supply pin for the ic. connect vin to a supply voltage between 2.6v~5.5v. ovp o over voltage protection pin which is connected to the output. ht7938 rev 1.70 2 june 25, 2016
uvlo=2.1v
absolute maximum ratings input voltage...........................................................6.0v sw voltage..............................................................46v fb voltage ..............................................................6.0v en ..........................................................................6.0v ovp voltage ............................................................46v operating temperature range ............. 40 cto+85 c storage temperature range .............. 55 c to +150c maximum junction temperature........................+150 c note: these are stress ratings only. stresses exceeding the range specified under absolute maximum ratings may cause substantial damage to the device. functional operation of this device at other conditions beyond those listed in the specification is not implied and prolonged exposure to extreme conditions may affect device reliability. electrical characteristics v in =3.6v, l=22 h, c in =1 f, c o =1 f, i led =20ma, ta=25 c symbol parameter test conditions min. typ. max. unit v in input voltage 2.6 5.5 v uvlo under voltage lockout 1.8 2.1 2.4 v i in supply current switching, v fb =gnd 1.0 2.5 ma v en =0v 0.1 1.0 a error amplifier v fb feedback voltage 190 200 210 mv power switch f osc switching frequency measurement at sw pin 0.8 1.2 1.6 mhz dc maximum duty cycle 92 95 % r ds(on) sw on resistance 0.7 i sw(off) switch leakage current 0.1 1.0 a en pin v ih en voltage high v in =2.6v~5.5v 2.0 v v il en voltage low v in =2.6v~5.5v 0.8 v ovp and ocp v ovp ovp threshold no load 36 39 42 v i ocp n-channel mosfet current limit 1000 ma thermal shutdown t shut thermal shutdown threshold 150 c thermal shutdown hysteresis 25 c ht7938 rev 1.70 3 june 25, 2016
ht7938 rev 1.70 4 june 25, 2016 function description vin under-voltage lockout uvlo the device contains an input under voltage lockout (uvlo) circuit. the purpose of the uvlo circuit is to en - sure that the input voltage is high enough for reliable op - eration. when the input voltage falls below the under voltage threshold, the internal fet switch is turned off. if the input voltage rises by the under voltage lockout hys - teresis, the device will restart. the uvlo threshold is set below the minimum input voltage of 2.6v to avoid any transient vin drops under the uvlo threshold and causing the converter to turn off. current limit protection the device has a cycle-by-cycle current limit to protect the internal power mosfet. if the inductor current reaches the current limit threshold, the mosfet will be turned off. it is import to note that this current limit will not protect the output from excessive current during an out - put short circuit. if an output short circuit has occurred, excessive current can damage both the inductor and di - ode. over-voltage protection ovp the device provides an over-voltage protection func - tion. if the fb pin is shorted to ground or an led is dis- connected from the circuit, the fb pin voltage will fall to zero and the internal power mosfet will switch with its full duty cycle. this may cause the output voltage to ex- ceed its maximum voltage rating, possibly damaging the ic and external components. internal over-voltage pro- tection circuitry turns off the power mosfet and shuts down the ic as soon as the output voltage exceeds the v ovp threshold. as a result, the output voltage falls to the level of the input supply voltage. the device remains in shutdown mode until the power is recycled. over-temperature protection otp a thermal shutdown is implemented to prevent dam - ages due to excessive heat and power dissipation. typically the thermal shutdown threshold is 150 c. when the thermal shutdown is triggered the device stops switching until the temperature falls below typi - cally 125 c. then the device starts switching again. application information inductor selection the selection of the inductor affects steady state op - eration as well as transient behavior and loop stability. there are three important electrical parameters which need to be considered when choosing an inductor: the value of inductor, dcr (copper wire resistance) and the saturation current. choose an inductor that can handle the necessary peak current without saturating, and ensure that the in - ductor has a low dcr to minimise power losses. a 10 h/22 h inductor should be a good choice for most ht7938 applications. however, a more exact induc - tance value can be calculated. a good rule for choosing an inductor value is to allow the peak-to-peak ripple current to be approximately 30~50% of the maximum input current. calculate the required inductance value using the following equation: in the equation above, i out(max) is the maximum load current,
i l is the peak-to-peak inductor ripple current, is the converter efficiency, f sw is the switching fre - quency and i l(peak) is the peak inductor current. output capacitor selection the output capacitor determines the steady state out- put voltage ripple. the voltage ripple is related to the capacitor s capacitance and its esr (equivalent se- ries resistance). a ceramic capacitor with a low esr value will provide the lowest voltage ripple and are therefore recommended. due to its low esr, the ca- pacitance value can be calculated by the equation: in the equation above, v ripple =peak to peak output rip - ple, f sw is the switching frequency. a1 f~10 f ceramic capacitor is suitable for most ap - plication. input capacitor selection an input capacitor is required to supply the ripple cur - rent to the inductor, while limiting noise at the input source. a low esr ceramic capacitors is required to keep the noise at the ic to a minimum. a1 f~10 f ceramic capacitor is suitable for most ap - plication. this capacitor must be connected very close to the vin pin and inductor, with short traces for good noise performance. ! " # ! $%# $" " & ' ( ! ) " & ' ( ! " ' * ! " & ' ( ! " & ' ( ! + , - . / !"
ht7938 rev 1.70 5 june 25, 2016 schottky diode selection the output rectifier diode conducts during the internal mosfet is turn off. the average and peak current rating must be greater than the maximum output cur - rent and peak inductor current. the reverse break - down voltage must be greater than the maximum output voltage. it is recommended to use a schottky diode with low forward voltage to minimize the power dissipation and therefore to maximize the efficiency of the converter. a 1n5819 type diode is recommended for ht7938 applications. led current selection the led current is controlled by the current sense feedback resistor r fb , the current sense feedback ref - erence voltage is 200mv. in order to have accurate led currents, precision resistors are the preferred type with a 1% tolerance. the led current can be cal - culated from the following formula. where i led is the total output led current, v fb =feed - back voltage, r fb =current sense resistor. digital and analog dimming control the led illumination level can be controlled using both digital and analog methods. the digital method uses a pwm signal applied to the en pin. this is shown in figure 9. the average led current increases proportionally with the pwm signal duty cycle. a 0% duty cycle corresponds to zero led current. a 100% duty cycle corresponds to full led current. the pwm signal frequency should be set be- low 1khz due to the delay time of device startup. there are two methods of analog led brightness con - trol. the first method uses a dc voltage to control the feedback voltage. if the dc voltage range is from 0v to 3.3v, the selection of resistors control the led cur - rent from 20ma to 0ma. the other way is to use a fil - tered pwm signal, as shown in figure 11. the filtered pwm signal application acts in the same way as the dc voltage dimming control. layout considerations circuit board layout is a very important consideration for switching regulators if they are to function properly. poor circuit layout may result in related noise problems. in order to minimize emi and switching noise, please fol - low the guidelines below: all tracks should be as wide as possible. the input and output capacitors, c1 and c2, should be placed close to the vin, vo and gnd pins. the schottky diode, d1, and inductor, l, must be placed close to the sw pin. feedback resistor, rfb, must be placed close to the fb and gnd pins. a full ground plane is always helpful for better emi performance. a recommended pcb layout with component loca - tions is shown below. f b r f b r f b v l e d i 2 0 0 m v == top layer bottom layer
typical performance characteristics ht7938 rev 1.70 6 june 25, 2016 figure 2. efficiency vs. input voltage (boost mode v out =10v) figure 6. led current vs. pwm duty cycle (9s1p) figure 4. reference voltage vs. input voltage (10s1p) figure 7. switching frequency vs. input voltage figure 1. efficiency vs. input voltage figure 3. efficiency vs. input voltage (boost mode v out =33v) figure 5. led current vs. pwm duty cycle (10s1p) figure 8. supply current vs. input voltage (v fb =gnd)
application circuits figure 9. application circuits for driving 10s1p wleds figure 10. application circuit for dimming control using a dc voltage figure 11. application circuit for dimming control using a filtered pwm signal ht7938 rev 1.70 7 june 25, 2016
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$ 4 5 4 $ 9 $ 9 / / 2 $ $ / 3 3 7 6 $ % :
$ 4 5 4 $ 9 & 6 7 8 . $ 9 $ : $ 9 / / 2 $ $
note: as the above application circuits are unable to show the full drive capabilities and series/parallel drive combi - nations of this device the following supplemental information is provided. for the general full voltage range situation: 1. vin variable voltage of 2.6v~5.5v 2. led forward voltage of 3.5v max. maximum led drive capability series parallel total 339 428 521 0 616 717 818 919 1 011 0 it is also important to note that for an input voltage supply which can be maintained at 5.0v or higher, the device possesses even higher drive capabilities which can drive a higher number of parallel connected leds. the fol - lowing table depicts the maximum led numbers possible for this higher input voltage range. 1. vin variable voltage of 5.0v~5.5v 2. led forward voltage of 3.5v max. maximum led drive capability series parallel total 392 7 483 2 542 0 642 4 732 1 821 6 921 8 1 022 0 ht7938 rev 1.70 8 june 25, 2016
package information ht7938 rev 1.60 9 october 1, 2013 note that the package information provided here is for consultation purposes only. as this information may be updated at regular intervals users are reminded to consult the holtek website for the latest version of the package/carton information . additional supplementary information with regard to packaging is listed below. click on the relevant section to be transferred to the relevant website page. ? package information (include outline dimensions, product tape and reel specifications) ? the operation instruction of packing materials ? carton information
6-pin sot23-6 outline dimensions symbol dimensions in inch min. nom. max. a 0.057 a1 0.006 a2 0.035 0.045 0.051 b 0.012 0.020 c 0.003 0.009 d 0.114 bsc e 0.063 bsc e 0.037 bsc e1 0.075 bsc h 0.110 bsc l1 0.024 bsc 08 symbol dimensions in mm min. nom. max. a 1.45 a1 0.15 a2 0.90 1.15 1.30 b 0.30 0.50 c 0.08 0.22 d 2.90 bsc e 1.60 bsc e 0.95 bsc e1 1.90 bsc h 2.80 bsc l1 0.60 bsc 08 ht7938 rev 1.60 10 october 1, 2013
ht7938 rev 1.70 11 june 25, 2016 copyright 2016 by holtek semiconductor inc. the information appearing in this data sheet is believed to be accurate at the time of publication. however, holtek assumes no responsibility arising from the use of the specifications described. the applications mentioned herein are used solely for the purpose of illustration and holtek makes no warranty or representation that such applications will be suitable without further modi - fication, nor recommends the use of its products for application that may present a risk to human life due to malfunction or otherwise. holtek s products are not authorized for use as critical com - ponents in life support devices or systems. holtek reserves the right to alter its products without prior notification. for the most up-to-date information, please visit our web site at http://www.holtek.com.tw.

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