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ht7937 built-in ovp white led step-up converter selection table part no. package marking ht7937 sot23-6 7937 rev 1.50 1 june 25, 2016 features integrated open-circuit protection over voltage protection up to 84% efficiency at v in =3v, 3leds, i led = 20ma 1.2mhz fixed switching frequency low standby current: 0.1ma (typ.) (v shdn =0v) matches led current small value inductor and capacitors small outline sot23-6 package applications cellular phones pdas dscs handheld devices white led display backlighting general description the ht7937 is high efficiency boost converter with a constant current output to provide back light functions in handheld devices. applications with series connected leds ensure constant and identical led currents re - sulting in uniform brightness. a continuous led output current is setup using the fb pin regulated voltage across an external sense resistor, r fb connected be - tween the fb pin and ground. the integrated open load protection circuitry prevents damage resulting from an open circuit condition. the low 95mv feedback voltage minimises power losses in the current setting resistor which improves efficiency. the ht7937 has a high switching frequency of up to 1.2mhz which permits the use of lower value extremely small outline inductor and filter capacitor. the ht7937 is supplied in a space-saving, 6-lead sot23-6 package type.
block diagram pin assignment pin description pin no. pin name description 1 sw switching pin 2 gnd ground pin 3 fb output voltage sense node. connect the cathode of the led to this pin. a resistor from this pin to ground sets the led current. internally compares to 95mv (typ.). 4 shdn shutdown device pin. connect to 1.5v or higher to enable device (on), 0.3v or lower to disable device (off). 5 ovp for over voltage protection connect to the output. 6 vin input voltage pin. 2.5v to 5.5v for internal circuitry. ht7937 rev 1.50 2 june 25, 2016 s w g n d t o p v i e w v i n s h d n f b 1 2 3 4 6 5 o v p s o t 2 3 - 6 7 9 3 7 ? o v p f b 1 . 2 m h z o s c i l l a t o r c o n t r o l l o g i c g n d s w s h d n 2 8 v 9 5 m v v i n
absolute maximum ratings input voltage..............................................................6v sw voltage..............................................................36v fb voltage .................................................................6v shdn ........................................................................6v ovp voltage ............................................................36v operating temperature range ............... -40c to 85c maximum junction temperature..........................125 c note: these are stress ratings only. stresses exceeding the range specified under 2 absolute maximum ratings 2 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 shdn =3v; v in =3v; ta=25 c (unless otherwise specified)(note) symbol parameter test conditions min. typ. max. unit v in input voltage ? 2.5 3 5.5 v i in supply current switching ? 1.0 1.25 ma non-switching ? 50 100 ma v shdn =0v ? 0.1 1 ma v fb feedback voltage 3 leds 85 95 105 mv f osc switching frequency measurement at sw pin 0.8 1.2 1.6 mhz d c maximum duty cycle 85 90 ? % r ds(on) sw on resistance ? ? 1.4 5 w i sw(off) switch leakage current ? ? 0.1 1 ma v ih shdn voltage high ? 1.5 ? ? v v il shdn voltage low ? ? ? 0.3 v v ovp ovp threshold no load 23 28 33 v note: specifications are production tested at ta=25 degree. specifications over -40 to 85 degree operating temperature range are assured by design, characterization. ht7937 rev 1.50 3 june 25, 2016
ht7937 rev 1.50 4 june 25, 2016 functional description shutdown the shutdown pin, shdn, must not be allowed to float. when the shdn pin voltage is taken below 0.3v, the internal mosfet, voltage reference, error ampli - fier, comparators and biasing circuitry will all be switched off reducing the quiescent supply current to less than 1 m a. if the shdn pin has a value greater than 1.5v, then the device will be fully enabled and op - erational. this pin also can be used as a pwm signal from 100hz to 1khz to allow brightness control. over voltage protection - ovp with an open circuit output, such as when no leds are connected, the fb pin will be pulled down to ground via the sense resistor r fb . as the device will now react by trying to increase the output voltage by generating a maximum duty cycle signal, this may cause the sw pin to exceed its maximum rated volt - age, which may damage the internal n-mos switch - ing transistor. the ovp function is designed to prevent damage to the internal nmos switching transistor. when the out - put voltage rises above the ovp threshold voltage, typically 28v, the converter will clamp the output volt - age to this level. when the output voltage returns to a value below the ovp threshold, it will automatically re - sume normal switching operation. dimming control there are three methods to control the leds bright- ness as listed below: a pwm signal on the shdn pin a pwm signal is applied to the shdn pin as shown in figure 1. the magnitude of the pwm signal should be higher than the enable voltage of the shdn pin, the leds operate with either zero or full current. the average led current is proportional to the duty cycle of the applied pwm signal with a duty cycle increase re - sulting in higher leds brightness. typical pwm fre - quencies should be between 100hz and 1khz. a dc signal on the fb pin this method of dimming control uses a dc voltage circuit as shown in figure 2. the led brightness is directly proportional to the led current which is given by the following equation: where v fb = feedback voltage is 95mv v dc = dc voltage r1 and r2 >> r fb a filtered pwm signal on the fb pin for frequencies greater than 1khz, dimming can be implemented by using the circuit shown in figure 3. the pwm control circuitry is connected to the fb pin. reducing the duty cycle on the pwm signal re- sults in increased leds brightness levels. the led brightness is directly proportional to the led cur- rent which is given by the following equation: where v fb = feedback voltage is 95mv v pwm = pwm high level voltage d = pwm duty cycle r1 and r2 >> r fb pwm frequency >> h t 7 9 3 7 v i n s h d n g n d s w o v p f b 1 0 m h l c 1 1 m f 1 n 5 8 1 9 d 1 c 2 1 m f r f b 4 . 7 w v i n p w m f = 1 0 0 h z ~ 1 k h z u p t o 6 w l e d s figure 1. dimming control with pwm signal h t 7 9 3 7 v i n s h d n g n d s w o v p f b 1 0 m h l c 1 1 m f 1 n 5 8 1 9 d 1 c 2 1 m f r f b 4 . 7 w v i n 1 k w 5 1 k w r 1 r 2 v d c u p t o 6 w l e d s v f b figure 2. dimming control using a dc voltage h t 7 9 3 7 v i n s h d n g n d s w o v p f b 1 0 m h l c 1 1 m f 1 n 5 8 1 9 d 1 c 2 1 m f r f b 4 . 7 w v i n 1 k w 5 1 k w r 1 r 2 v p w m r 3 5 . 1 k w c 3 0 . 1 m f u p t o 6 w l e d s v f b 0 v figure 3. dimming control using a filter pwm signal
ht7937 rev 1.50 5 june 25, 2016 component selection setting the led current the step-up converter regulates the led current by regulating the voltage across the current sense resis - tor, r fb . to ensure the generation of accurate led currents, it is recommended that a precision resistor is used for r fb . the voltage across the sense resistor is regulated to the internal reference voltage of 95mv . the led current is calculated using the following equation: i v r led f b fb = where v fb = feedback voltage is 95mv inductor selection the selected inductor must have a saturation current greater than the maximum peak current of the step-up converter. a recommended value of inductor for 3 to 6 white led applications is 4.7 m h to 22m h. for good ef - ficiency the inductor should have low core loss and low dcr (dc resistance). the peak inductor current is calculated using the following equation: where v o = output voltage v i = input voltage i o = output current h = ef ficiency l = inductance f s = switching frequency is 1.2mhz diode selection the diode must have a rating greater than the output voltage and output current. in switching applications both forward voltage drop and diode capacitance are important considerations. a schottky diode is used due to its low forward voltage drop and fast switching speeds which improves effi - ciency. the schottky diode has average current of i o , and a peak current which is the same as the inductor s peak current and a voltage rating at least 1.5 times the output voltage. a recommended schottky diode type is the 1n5819. capacitor selection a low esr (equivalent series resistance) capacitor should be used for the input/output capacitors to mini - mize ripples. both x5r and x7r types are suitable due to their wider voltage and temperature ranges. in - put and output ceramic capacitors of 1 m f are recom - mended for ht7937 applications. the output ripple voltage is calculated as the following equation: where i l(peak) = peak inductor current c o(esr) = output capacitors equivalent series resistance i o = output current v o = output voltage v i = input voltage c o = output capacitance f s = switching frequency is 1.2mhz layout considerations c ircuit 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 minimise 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, vout and gnd pins. the schottky diode, d1, and inductor, l1, must all be placed close to the sw pin. feedback resistor, r1, 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 locations is shown below. top layer bottom layer
typical performance characteristics ht7937 rev 1.50 6 june 25, 2016 5 wleds efficiency vs. led current 60 65 70 75 80 85 90 0 5 10 15 20 led current (ma) efficiency(%) vin=3v vin=3.5v vin=4v l: 10uh, sr0602 10ml (abc electronics corp.) d1: 1n5819 c1, c2: 1uf vin=5v 5 wleds efficiency vs. led current 3 wleds efficiency vs. led current 60 65 70 75 80 85 90 0 5 10 15 20 led current (ma) efficiency(%) vin=3v vin=3.5v vin=4v vin=5v l: 10uh, sr0602 10ml (abc electronics corp.) d1: 1n5819 c1, c2: 1uf 3 wleds efficiency vs. led current 85 87 89 91 93 95 97 99 101 103 105 -45 -30 -15 0 15 30 45 60 75 90 temperature ( j ) reference voltage (mv) 3 wleds, vin=3v l: 10uh, sr0602 10ml(abc electronics corp.) d1: 1n5819 c1, c2: 1uf temperature vs reference voltage 85 86 87 88 89 90 91 92 93 94 95 -45 -30 -15 0 15 30 45 60 75 90 temperature ( j) switching duty cycle (%) 3 wleds, vin=3v l: 10uh, sr0602 10ml(abc electronics corp.) d1: 1n5819 c1, c2: 1uf temperature vs switching duty cycle 85 87 89 91 93 95 97 99 101 103 105 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 input voltage (v) reference voltage (mv) 3 wleds, vin=3v l: 10uh, sr0602 10ml(abc electronics corp.) d1: 1n5819 c1, c2: 1uf input voltage vs reference voltage 0.8 1.0 1.2 1.4 1.6 1.8 2.0 -45 -30 -15 0 15 30 45 60 75 90 temperature ( j) switching frequency (mhz) 3 wleds, vin=3v l: 10uh, sr0602 10ml(abc electronics corp.) d1: 1n5819 c1, c2: 1uf temperature vs switching frequency
ht7937 rev 1.50 7 june 25, 2016 start-up from shutdown waveform note: vin=3.6v, 3 wleds, l=10 mh, d= 1n5819, ci=co=1m f, iled=20ma start-up from shutdown waveform note: vin=3.6v, 5 wleds, l=10 mh, d=1n5819, ci=co=1m f, iled=20ma operation waveform note: vin=3.6v, 5 wleds, l=10 mh, d=1n5819, ci=co=1m f, iled=20ma 0% 20% 40% 60% 80% 100% 0% 20% 40% 60% 80% 100% shutdown pin pwm duty average iled / iled max 100hz 200hz 500hz 1khz vin=3.6v, l=10uh, ci=co=1uf, 3leds dimming control by shutdown pin operation waveform note: vin=3.6v, 3 wleds, l=10 mh, d=1n5819, ci=co=1m f, iled=20ma 60 65 70 75 80 85 90 95 2.5 3.0 3.5 4.0 4.5 5.0 vin(v) efficiency(%) 6s1p 3s2p efficiency vs. vin
ht7937 rev 1.50 8 june 25, 2016 3 wleds, vin=3.6v, f=200hz, duty=50% 3 wleds, vin=3.6v, f=500hz, duty=50% 3 wleds, vin=3.6v, f=1khz, duty=50%
application circuits 6 wleds application circuit dimming control with a pwm signal dimming control using a dc voltage dimming control using a filter pwm signal ht7937 rev 1.50 9 june 25, 2016 h t 7 9 3 7 v i n s h d n g n d s w o v p f b 1 0 m h l c 1 1 m f 1 n 5 8 1 9 d 1 v i n c 2 1 m f r f b 4 . 7 w u p t o 6 w l e d s 2 0 m a n o t e : " l " s r 0 6 0 2 1 0 0 m l b ( a b c t a i w a n e l e c t r o n i c s c o r p . ) h t 7 9 3 7 v i n s h d n g n d s w o v p f b 1 0 m h l c 1 1 m f 1 n 5 8 1 9 d 1 c 2 1 m f r f b 4 . 7 w v i n u p t o 6 w l e d s p w m h t 7 9 3 7 v i n s h d n g n d s w o v p f b 1 0 m h l c 1 1 m f 1 n 5 8 1 9 d 1 c 2 1 m f r f b 4 . 7 w v i n 1 k w 5 1 k w r 1 r 2 v d c 0 v ~ 5 v u p t o 6 w l e d s h t 7 9 3 7 v i n s h d n g n d s w o v p f b 1 0 m h l c 1 1 m f 1 n 5 8 1 9 d 1 c 2 1 m f r f b 4 . 7 w v i n 1 k w 5 1 k w r 1 r 2 p w m r 3 5 . 1 k w c 3 0 . 1 m f u p t o 6 w l e d s
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 3 2 6 4 2 8 5 1 5 6 1 6 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 3 3 9 4 2 8 5 1 5 6 1 6 ht7937 rev 1.50 10 june 25, 2016
package information ht7937 rev 1.50 11 june 25, 2016 note that the package information provided her e 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 ? q 0 ? 8 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 ? q 0 ? 8 ht7937 rev 1.50 12 june 25, 2016
ht7937 rev 1.50 13 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 publica - tion. however, holtek assumes no responsibility arising from the use of the specifications de - scribed. 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 fur - ther modification, 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 components 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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