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SM8122A nippon precision circuits inc.? white led driver ic overview the SM8122A is a high ef?iency step-up dc/dc converter. due to high voltage cmos process realizing 25v output supply as maximum value, 2 to 6 lights of white led connected in series can be lighted. by con- necting in series, current variation among led is eliminated. current value sent to white led can be set by external resistors. in addition, brightness can also be adjusted by control to fb pin or ce pin. since the SM8122A has an over voltage protection circuit built-in, it dispenses with the existing external zd (zener diode). besides, the switching frequency of the SM8122A is higher (2.0mhz) than the existing product (sm8121a), so that it can respond to lower inductance value. features boost-up control using pwm 2 to 6 lights of white led (connected in series) lighted output current value can be set by external resis- tors (51 ? : 9.8ma, 33 ? : 15.2ma, 24 ? : 20.8ma) brightness adjustable by control to fb pin or ce pin current variation among led decreased by high precision high ef?ient drive by step-up model over voltage protection circuit built-in supply voltage range: 2.3 to 5.5v maximum output voltage: 25v quiescent current: 820 a (typ) standby current: 1.0 a (max) r on (switching mos-tr): 2 ? (typ) switching frequency: 2.0mhz (typ) output current detection accuracy: 2% package: sot23-6w (SM8122Ah) mson-6 (SM8122Ad) applications cellular phone pager digital still camera handy terminal pdas portable games white led drive lcd bias supply flash memory supply ordering information pinout (top view) sot23-6w mson-6 device package SM8122Ah sot23-6w SM8122Ad mson-6 sw vout fb 1 2 3 vdd ce 6 4 vss 5 6 4 3 1 ce sw vdd 2 vout 5 vss fb
SM8122A nippon precision circuits inc.? package dimensions (unit: mm) sot23-6w mson-6 2.9 ?0.2 1.9 ?0.2 1.8 ?0.2 2.8 ?0.2 (0.95) (0.95) 0.1 0.2 0 to 0.1 0 to 15 0.1min m 0.4 ? 0.05 + 0.1 0.15 ? 0.05 + 0.1 0.8 ?0.1 1.1 ?0.1 1 3 4 6 2.0 0.15 1.8 0.15 min1.45 min1.65 0.125 45 45 45 45 0.14 0.05 0.038 0.02 1.4 0.1 0.2 0.08 1.0 0.1 0.8 0.05 0.1 0.05 0.75 0.05 0.5 0.1 45 0.2 0.08 0.1 0.06 0.018 0.6 0.05 r0.075 0.8 0.1 r0.1 0.3 0.1 4 6 1 3 0.75 ? 0 + 0.1
SM8122A nippon precision circuits inc.? block diagram pin description number name i/o description sot23-6w mson-6 1 2 sw o coil switching 2 6 vout i output voltage detection 3 4 fb i feed back (output current detection) 43ceip 1 1. input with built-in pull-down resistor chip enable (high active) 5 5 vss gnd 6 1 vdd power supply ce sw vss vdd fb buff soft start osc pwm comp ramp generator vout ovp comp vref err amp
SM8122A nippon precision circuits inc.? specifications absolute maximum ratings electrical characteristics v dd = 3.6v, v ss = 0v, ta = 25 c unless otherwise noted parameter symbol rating unit supply voltage range v dd ? 0.3 to 6.5 v input voltage range v in v ss ?0.3 to v dd + 0.3 v sw output voltage range v sw ?.3 to 30 v sw input current i sw 500 ma power dissipation p d 250 (ta = 25 c) mw operating temperature range t opr ?0 to 85 c storage temperature range t stg ? 55 to 125 c parameter pin symbol condition rating unit min typ max supply voltage vdd v dd 2.3 3.6 5.5 v maximum output voltage sw v out 25 v standby current vdd i stb v ce = 0v 1.0 a quiescent current vdd i dd v fb = 1.0v 200 400 a v fb = 0v 820 1600 a sw-tr on resister sw r on i sw = 100ma, v dd = 3.6v 2.0 3.0 ? sw-tr leak current sw i leak v sw = v dd 1.0 a switching frequency sw f osc v fb = 0v 1.8 2.0 2.2 mhz maximum duty sw duty v fb = 0v 75 85 90 % input voltage ce v ih 2.0 v v il 0.6 v input current ce i ce v ce = 3.6v 5.0 10 a fb i fb v fb = 0.5v ?.0 1.0 a vout i vout v out = 25v 60 82 120 a soft-start time sw t ss1 switching stop time 10 20 70 s t ss2 maximum duty restriction time 500 s fb voltage fb v fb 0.49 0.50 0.51 v coil inductance sw l sw 4.7 10 h over voltage detection vout v ov 25 30.5 36 v over voltage detection release v ovr 23 28.5 v
SM8122A nippon precision circuits inc.? operation overview the SM8122A basic structure is a step-up dc/dc converter. the booster control employs pulse width modu- lation (pwm) which controls the pulse duty cycle (85% max.) at constant frequency (2.0mhz typ.). the led current is set by a current-setting resistor r1 connected between pins fb (with stable voltage of 0.5v typ.) and vss. when the switching transistor sw-tr is on, energy is stored in the inductor l. when sw-tr is rapidly switched off, the energy stored in the inductor generates a voltage across the terminals of the inductor. the induced voltage, after being added to the input voltage, turns on the schottky barrier diode sbd and the stored energy is transferred to the output capacitor. this sequence of events continues repeatedly, boosting the output voltage. the SM8122A features a built-in soft-start function. the soft-start time is approximately 500 s from after the chip enable input ce rising edge. during this interval, the maximum duty is restricted. r1 sbd l 4.7 h c in 4.7 f v in 2.3 to 5.5v c out 1.0 f enable disable led ce sw vss vdd fb buff soft start osc pwm comp ramp generator vout ovp comp vref err amp
SM8122A nippon precision circuits inc.? ovp (over voltage protection) SM8122A is always monitoring the vout terminal voltage in order to protect itself from the stress of v out over voltage. if SM8122A detects the v out over voltage, it immediately stop the switching of the inductor drive transistor. after the vout terminal voltage decreases below the release voltage, SM8122A restarts switching the inductor drive transistor. the over voltage is set as approximately 30.5v, the release voltage is approximately 28.5v. selecting the current-setting resistor (r1) the SM8122A control stabilizes the voltage on pin fb (0.5v typ.). hence, the current-setting resistor r1 con- nected between fb and vss sets the led current i led , where the resistance r1 is given by the following equation. r1 = 0.5 / i led 30.5v 28.5v v out sw tr = off sw tr switching sw tr = off over voltage detection release over voltage detection over voltage detection release over voltage detection i led =0.5/r1 fb v fb =0.5v r1=0.5/ i led
SM8122A nippon precision circuits inc.? selecting the inductor (l) the inductor dc resistance affects the power ef?iency, therefore a low dc resistance inductor is recom- mended. note also that the peak inductor current i peak should not exceed the inductor maximum current rating. in pulsed current mode control, the peak inductor current i peak is given by the following equation. i peak = (v in t on ) / l for example, if the input voltage v in is 3.6v, the inductance l is 4.7 h, and the sw-tr on time t on is 2mhz 85% = 0.425 s, then the peak inductor current i peak is (3.6 0.425 10 -6 ) / (4.7 10 -6 ) = 0.326a = 326ma. selecting the capacitors (c in , c out ) the recommended capacitances for use with the SM8122A are 4.7 f ceramic input capacitor c in and 1.0 f ceramic output capacitor c out . the capacitor esr ratings affect the ripple voltage, therefore capacitors with low esr rating are recommended. the input capacitor should be mounted close to the SM8122A ic. note that the capacitor voltage ratings should be selected to provide suf?ient margin for the applied input and output voltages. for example, if a lithium-ion battery (2.5 to 4.5v) is connected to the input and 3 white leds connected in series at the output draw 20ma, then the maximum input voltage is 4.5v and the maximum output voltage is (4.0v 3 leds) + 0.5v = 12.5v. therefore, the input capacitor should have a voltage rating of 6v, and the output capacitor should have a voltage rating of 16v. selecting the recti?r schottky barrier diode (sbd) the recti?r schottky barrier diode forward-direction voltage drop affects the power ef?iency, therefore a schottky barrier diode with low forward-direction voltage drop is recommended. note that the diode should be selected to provide suf?ient margin for the rated current and reverse-direction withstand voltage. board layout notes the following precautions should be followed for stable device operation. the inductor l and schottky barrier diode sbd should be connected close to the pin sw using thick, short circuit wiring. the input capacitor c in should be mounted close to the ic. the ic supply voltage v dd wiring and inductor supply wiring should be isolated, reducing any common impedances. the ground wiring should be connected at a single point, reducing any common impedances. sw fb l c in v in sbd c out r1 vdd ce vss led vout
SM8122A nippon precision circuits inc.? brightness adjustment brightness adjustment using fb pin the led brightness can be adjusted using an input dc control voltage connected through resistor r3 to the fb pin. alternatively, the brightness can be controlled by a pwm signal by adding a low-pass ?ter comprising resistor r4 and capacitor c1. the pwm signal frequency range is determined by the low-pass ?ter coef? cients. for example, the recommended values for resistor r4 (50k ? ) and capacitor c1 (0.1 f) provide a pwm signal frequency range of 1khz to 1mhz. brightness adjustment using fb pin (dc voltage input) when the brightness is controlled by dc voltage (v dc ) connected to resistor r3, the led current (i led ) is given by equation 1. if the values r1 = 30 ? , r2 = 20k ? , r3 = 100k ? , v fb = 0.5v, and v dc = 0v are inserted in equation 1, the led current i led = 20ma, as shown in equation 2. if the values r1 = 30 ? , r2 = 20k ? , r3 = 100k ? , v fb = 0.5v, and v dc = 3v are inserted in equation 1, the led current i led = 0ma, as shown in equation 3. taking the above diagram as an example, inserting the values r1 = 30 ? , r2 = 20k ? , r3 = 100k ? , v fb = 0.5v, and v dc = 0 to 3v into equation 1 gives the maximum led current i led of 20ma when v dc = 0v (equation 2) and the minimum led current i led of 0ma when v dc = 3v (equation 3). brightness adjustment circuit using fb pin (dc voltage input) sw vout fb sm8122 r1 30 ? sbd dc voltage 0 to 3v r3 100k ? r2 20k ? vdd ce c out 1.0 f l 4.7 h c in 4.7 f v in 3.6v led vss dc voltage vs. led current 0 5 10 15 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 dc voltage [v] led current [ma] r1 r2 ( v dc ? v fb ) r3 i led = v fb ? ... (1) 20,000 ( 0 ? 0.5) 100,000 30 30 i led === 0.5 ? 0.6 20ma ... (2) 20,000 ( 3 ? 0.5) 100,000 30 30 i led === 0.5 ? 0 0ma ... (3)
SM8122A nippon precision circuits inc.? brightness adjustment using fb pin (pwm signal input) when the brightness is controlled by pwm signal (v pwm duty), the led current (i led ) is given by equa- tion 4. if the values r1 = 30 ? , r2 = 20k ? , r3 = 50k ? , r4 = 50k ? , v fb = 0.5v, v pwm = 3v, and duty = 0% are inserted in equation 4, the led current i led = 20ma, as shown in equation 5. if the values r1 = 30 ? , r2 = 20k ? , r3 = 50k ? , r4 = 50k ? , v fb = 0.5v, v pwm = 3v, and duty = 100% are inserted in equation 4, the led current i led = 0ma, as shown in equation 6. taking the above diagram as an example, inserting the values r1 = 30 ? , r2 = 20k ? , r3 = 50k ? , r4 = 50k ? , v fb = 0.5v, v pwm = 3v, and duty = 0 to 100% into equation 4 gives the maximum led current i led of 20ma when duty = 0% (equation 5) and the minimum led current i led of 0ma when duty = 100% (equa- tion 6). brightness adjustment circuit using fb pin (pwm signal input) sw vout fb sm8122 r1 30 ? sbd r3 50k ? r2 20k ? vdd ce c out 1.0 f cin 4.7 f v in 3.6v led r4 50k ? c1 0.1 f pwm signal duty [ % ] v pwm [v] vss l 4.7 h pwm signal vs. led current 0 5 10 15 20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 v pwm duty [v] led current [ma] r2 ( v pwm duty ? v fb ) r3 +r4 r1 i led = v fb ? ... (4) 30 30 i led === 20,000 ( 3 0 ? 0.5) 50,000 + 50,000 0.5 ? 0.6 20ma ... (5) 30 30 i led === 20,000 ( 3 1 ? 0.5) 50,000 + 50,000 0.5 ? 0 0ma ... (6)
SM8122A nippon precision circuits inc.?0 brightness adjustment using ce pin the led average current can be adjusted by controlling the duty of a pwm signal input on the ce pin. when ce goes from low to high, the soft start function operates (with 500 s constant soft start time) and, there- fore, the led average current ratio for a given pwm signal duty falls with increasing pwm signal frequency. taking this into consideration, the recommended pwm control signal has a frequency range of 100 to 400hz with duty cycle range of 10 to 90%. when adjusting the brightness using the ce pin, a ripple voltage synchronized to the pwm signal is generated across the output capacitor c out . the amplitude of the ripple voltage is determined by the number of leds and their forward-bias voltage drop characteristics. if a ceramic capacitor is used for the output capacitor c out , an audible noise may be generated due to the ceramic capacitor s piezoelectric effect. the audible noise level depends on the ceramic capacitor (capacitance, bias dependency, withstand voltage etc.), leds (number, forward-bias voltage drop etc.), and mounting board (thickness, mounting conditions etc.), and thus should be veri ed under actual conditions. brightness adjustment circuit using ce pin vout sm8122 r1 25 ? sbd pwm signal vdd sw ce fb c out 1.0 f v in 3.6v c in 4.7 f led vss l 4.7 h pwm signal duty vs. led average current 10 20 0.0 5.0 10.0 15.0 20.0 0 30405060708090100 pwm signal duty [%] average led current [ma] 400 [hz] 100 [hz] 1000 [hz] 1400 [hz] alternatively, a tantalum capacitor or lm capacitor with low piezoelectric effect can be used as the output capacitor c out to minimize the noise level, or the brightness can be adjusted using the fb pin as described earlier. the audible noise generated when using the ce pin is not an inherent phenomena of the SM8122A device, but of the brightness adjustment method employed. output voltage with leds on output voltage with leds off ce input signal and output ripple voltage 20ma c out 3.5v 3.5v 3.5v 0.5v 11.0v 0ma c out 2.7v 2.7v 2.7v 0v 8.1v
SM8122A nippon precision circuits inc.?1 current switching using external transistors if only a few brightness steps are required, the led current can be adjusted by switching the led current set- ting resistance using external transistors (tr). sw vout fb sm8122 r1 250 ? sbd select signal 1 tr1 r2 100 ? vdd ce c out 1.0 f c in 4.7 f v in 3.6v led select signal 2 tr2 r3 40 ? vss l 4.7 h select signal 2 select signal 1 i led low low 2ma low high 2 + 5 = 7ma high low 2 + 12.5 = 14.5ma high high 2 + 5 + 12.5 = 19.5ma
SM8122A nippon precision circuits inc.?2 recommend pattern sot23-6w mson-6 footprint pattern 0.7 1.0 0.95 2.4 0.95 footprint pattern 2.4 0.8 0.5 1.0 0.5 0.5 0.5 0.225 0.25 1.4 2.0 45 ? metalmask pattern 2.3 0.8 0.4 1.0 0.4 0.5 0.5 0.225 0.25 1.4 1.9 0.6 45
SM8122A nippon precision circuits inc.?3 nc0323ae 2005.05 please pay your attention to the following points at time of using the products shown in this document. the products shown in this document (hereinafter ?roducts? are not intended to be used for the apparatus that exerts harmful in?ence on human lives due to the defects, failure or malfunction of the products. customers are requested to obtain prior written agreeme nt for such use from nippon precision circuits inc. (hereinafter ?pc?. customers shall be solely responsible for, and indemnify and hold npc free and harmless from, any and all claims, damages, losses, expenses or lawsuits, due to such use without such agreement. npc reserves the right to change the speci?ations of the products in order to improve the characteristic or reliability thereof. npc makes no claim or warranty that the contents described in this document dose not infringe any intellectual property right or other similar right owned by third parties. therefore, npc shall not be responsible for such problems, even if the use is in accordance with the descriptions prov ided in this document. any descriptions including applications, circuits, and the parameters of the products in this document are for refere nce to use the products, and shall not be guaranteed free from defect, inapplicability to the design for the mass-production products without further testing or modi?ation. customers are requested not to export or re-export, directly or indirectly, the products to any country or any entity not in compliance with or in violation of the national export administration laws, treaties, orders and regulations. customers are req uested appropriately take steps to obtain required permissions or approvals from appropriate government agencies. nippon precision circuits inc. 15-6, nihombashi-kabutocho, chuo-ku, tokyo 103-0026, japan telephone: +81-3-6667-6601 facsimile: +81-3-6667-6611 http://www.npc.co.jp/ email: sales @ npc.co.jp

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