1 GM19371 v2.01 GM19371 boost white led driver with ovp www.gammamicro.com.tw application cellular phones pdas, handheld computers digital cameras mp3 player gps receivers mr16 lighting typical application circuits description the GM19371 is a step up dc/dc convert specifically designed to drive white led;s with a constant current. the device can drive up to 27 leds from a 5v supply. additional feature includes output voltage limiting when leds are disconnected. GM19371 is available in sot-26 package. features ? inherently matched led current ? drives up to 126leds (9s14p) from a 12v supply ? 36v rugged bipolar switch ? fast 1.2mhz switching frequency ? over voltage protection, 30v c1 4.7f r set 20ma x 14 14 strings in parallel 9 led in series GM19371 1 sw gnd 2 4 ctrl ovp 5 6 v in fb 3 vi n 6v to 18v c2 4.7f 10h d1 dim figure 1 GM19371 typical application
2 GM19371 v201 GM19371 boost white led driver with ovp marking information and pin configurations (top view) sot26 pin descriptions pin number pin name pin function 1 sw this is the collector of the internal npn power switch. connect to inductor and diode. minimize the metal trace area connected to this pin to reduce emi. 2 gnd ground 3 fb reference voltage is 95mv. connect leds and a resistor at this pin. led current is determined by the resistance and ctrl voltage. r set resistor value is calculated as r set = 95mv/i led 4 ctrl this pin could be used as either shut down or dimming functions: when v ctrl > 1.8v, generates full scale led current v ctrl < 0.4v, chip is off 0.4v v ctrl 1.8v, pwm duty cycle controls the led current 5 ovp over voltage protection, 30v 6 v in the input supply pin, bypass this pin with a capacitor as close to the device as possible ordering information ? green products ordering number package shipping GM19371-st26rg sot-26 3,000 units/tape and reel 70yww v in 6 3 fb ctrl 4 2 gnd 1 sw ovp 5 70: device code, green product y: year ww: week code
3 GM19371 v2.01 GM19371 boost white led driver with ovp absolute maximum ratings (note 1) parameter symbol ratings units input voltage v in 20 v sw voltage v sw 36 v fb voltage v fb 10 v /shdn voltage v ctrl 10 v junction temperature t j -40 to 125 c storage temperature t stg - 65 to 150 c lead temperature (soldering, 5 sec) 260 c .note 1: stresses greater than those listed under maximu m ratings may cause permanent damage to the device. recommended operating conditions rating symbol value unit maximum supply voltage v in 2.5 to 18 v output voltage v out 28 v operating temperature range t opr 0 to 70 c electrical characteristics (v in = 5v, v /shdn = 5v, t a = 25 ? c, unless otherwise noted) parameter symbol condition min typ max unit operating voltage range v in 2.5 18 v feedback voltage v fb i load = 180ma 80 95 107 mv fb pin bias current i fb v in = v out + 1v to 16v 10 45 100 na supply current i cc 2.1 3.0 ma standby current i stby v /shdn = 0v 0.1 1.0 a switching frequency f osc 1.1 1.3 1.6 mhz maximum duty cycle d max 85 90 % switch current limit i cl 650 ma switch v cesat v sat i sw = 250ma 350 mv switch leakage current i leak v sw = 5v 0.01 5 a ctrl voltage high v th 1.5 v ctrl voltage low v tl 0.4 v ctrl pin bias current i ctrl 65 a ovp threshold 29 v
4 GM19371 v201 GM19371 boost white led driver with ovp v in c c fb gnd ramp generator driver r comparator a1 95mv q1 sw a2 1.2mhz oscillator over voltage protection v ref 1.25v r c s q q2 0.3 ovp /shdn figure 2 GM19371 block diagram
5 GM19371 v2.01 GM19371 boost white led driver with ovp typica; application circuits figure 3 GM19371, typical application r set 20ma x 14 14 strings in parallel 6 - 9 led in series GM19371 1 sw gnd 2 4 ctrl ovp 5 6 v in fb 3 v in 5v to 12v c out 4.7f l d1 off on c in 4.7f figure 4. GM19371 ? application for 1w led x 6 GM19371 1 sw gnd 2 4 ctrl ovp 5 6 v in fb 3 v in 12v to 15v l1 68 h d1 ss14 c in 4.7f c out 4.7f r set 0.22 350ma dimming control GM19371 1 sw gnd 2 4 ctrl ovp 5 6 v in fb 3 v in 12v ac l1 d1 c in c out r set 5 led (1w) in series figure 5. GM19371 ? application for 1mr16 with 1w led in series
6 GM19371 v201 GM19371 boost white led driver with ovp application information general operation the GM19371 uses a constant frequency, current m ode control scheme to provide excellent line and load regulations. see fig 2 for GM19371operation. at the st art of each oscillator cycle, the rs latch is set, which turns on the power switch q1. a voltage proporti onal to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive te rminal of the pwm comparator a2. when this voltage exceeds the level at the negative input of a2, the rs latch is reset turning off the power switch. the level at the negative input of a2 is set by the error amp lifier a1, and is simply an amplified version of the difference between the feedback voltage and the refere nce voltage of 95mv. in this manner, the error amplifier sets the correct peak current level to keep th e output in regulation. if t he error amplifier?s output increases, more current is delivered to the outpu t; if it decreases, less current is delivered. over voltage protection and open circuit protection in case of output open circuit, while the led strings are disconnected from the circuit or the leds fail open, the feedback voltage, vfb, will be zero. the GM19371 will then switch at a high duty cycle resulting in a high output voltage, which may cause the sw pin voltage to exceed its maximum voltage rating. in order to prevent this condition occur, the gm19317 builds a latch-mode open-circuit protection circuit. based on the result from the sensing pin, o vp pin, the chip can detect the output open condition when the ovp voltage is higher than 30v. the chip will shutdown until vi n reset inductor selection a 10uh inductor is recommended for most GM19371 applications. although small size and high efficiency are major concerns, the inductor should have low co re losses at 1.2mhz and low dcr (copper wire resistance). capacitor selection the small size of ceramic capacitors makes them ideal for GM19371 applications. x5r and x7r types are recommended because they retain their capacitanc e over wider voltage and temperature ranges than other types such as y5v or z5u. a 4.7uf input ca pacitor and a 4.7uf output capacitor are sufficient for most GM19371 applications. diode selection schottky diodes, with their low forward voltage drop a nd fast reverse recovery, are the ideal choices for GM19371 applications. the forward voltage drop of a scho ttky diode represents the conduction losses in the diode while the diode capacitance (ct or cd) repres ents the switching losses. for diode selection, both forward voltage drop and diode capacitance need to be considered. schottky diodes with higher current ratings usually have lower forward voltage dr op and larger diode capacitance, which can cause significant switching losses at the 1.2mhz switch ing frequency of the GM19371. a schottky diode rated at 1000ma is sufficient for most GM19371 applications. soft start and current limit the internal soft start circuit minimizes the inrush current during turning on GM19371. the typical switch current is limited to about 650ma by the chip.
7 GM19371 v2.01 GM19371 boost white led driver with ovp GM19371 r set 6.3 ? v dc r2 90k r1 5k GM19371 r set 6.3 ? pwm r2 90k r1 5k 100k 0.1 f led current control the led current is controlled by t he feedback resistor (r1, in fig 1). the feedback reference is 95mv. the led current is 95mv/r1. the formula and table 1 for r1 selection are shown below. r1=95mv/i led table 1 r set resistor value selection i led r1 ( ? ) 5 19.1 10 9.53 15 6.34 20 4.75 60 1.56 350 0.22 dimming control 1. using a pwm signal to ctrl pin with the pwm signal applied to the ctrl pin, t he GM19371 is turned on or off by the pwm signal. the led operate at either zero or full current. t he average led current incr eases proportionally with the duty cycle of the pwm signal. a 0% duty cycle will turn off the GM19371 and corresponds to zero led current. a 100% duty cycle corresponds to full cu rrent. the typical frequency range of the pwm signal is 100hz to 1khz. the magnitude of the pwm signal should be higher than the minimum ctrl voltage high. 2. using a dc voltage for some applications, the preferred method of brightness control is a variable dc voltage to adjust the led current. the dimming control using a dc voltage is shown in below figure 6. as the dc voltage increases, the voltage drop on r1 increases and the voltage drop on rset decreases. thus, the led current decreases. the selection of r1 and r2 should make the current from the variable dc source much smaller than the led current and much larger than the fb pin bias current. for vdc range from 0v to 2v, the selection of resistors in below figure gives dimming control of led current from 0ma to 15ma. 3. using a dc voltage for the filtered pwm signal can be considered as an adjustable dc voltage. it can be used to replace the variable dc voltage sour ce in dimming control. the circuit is show in figure 7 figure 6 figure 7
8 GM19371 v201 GM19371 boost white led driver with ovp GM19371 r set 6.3 ? r inc logic signal 4. using a logic signal for applications that need to adjust the led current in discrete steps, a logic signal can be used as shown in figure 8. r set sets the minimum led current (when the nmos is off). r inc sets how much the led current increases when the nmos is turned on. the selection of r set and r inc follows the formula and table in ?led current control? section. board layout consideration as with all switching regulators, careful attenti on must be paid to the pcb board layout and component placement. to maximize efficiency, the rising and falling times of the switch are made as short as possible. to prevent electromagnetic interference (emi) problems , proper layout of the high frequency switching path is essential. the voltage signal of the sw pin has sh arp rising and falling edges. minimize the length and area of all traces connected to the sw pin and alwa ys use a ground plane under the switching regulator to minimize inter-plane coupling. in addition, the ground connection for the feedback resistor r set should be tied directly to the gnd pin and not shared with any other component, ensuring a clean, noise-free connection. figure 8
9 GM19371 v2.01 GM19371 boost white led driver with ovp package outline dimensions ? sot 26 (2.820 - 3.020) 0.111 - 0.119 0.104 - 0.116 (2.650 - 2.950) 0.037 bsc (0.950) (1.800 - 2.000) 0.071 - 0.079 0.059 - 0.067 (1.500 - 1.700) 0.041 - 0.049 (1.050 - 1.250) 0.041 - 0.045 (1.050 - 1.150) 0.000 - 0.004 (0.000 - 0.100) 0.004 - 0.008 (0.100 - 0.200) 0.012 - 0.024 (0.300 - 0.600) 0 - 8 inch (mm)
10 GM19371 v201 GM19371 boost white led driver with ovp ordering number g m 19371 st26 r g apm gamma micro circuit type package type shipping type st26: sot 26 r: t ape & reel blank:pb-free g:green note: pb-free products: ? rohs compliant and compatible with the cu rrent require-ments of ipc/jedec j-std-020. ? suitable for use in snpb or pb-free soldering processes with 100% matte tin (sn) plating. green products: ? lead-free (rohs compliant) ? halogen free(br or cl does not exceed 900ppm by weight in homogeneous material and total of br and cl does not exceed 1500ppm by weight)
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