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RT8477A ? ds8477a-02 april 2015 www.richtek.com 1 copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. ? high voltage high multiple-topology current led driver general description the RT8477A is a current mode pwm controller designed to drive an external mosfet for high current led applications with wide input voltage (4.5v to 50v) and output voltage (up to 50v) ranges. with internal 380khz operating frequency, the size of the external pwm inductor and input/output capacitors can be minimized. high efficiency is achieved by a 100mv current sensing control. led dimming control can be done by analog. the RT8477A is now available in the sop-8 (exposed pad) package. features ? ? ? ? ? support multiple-topologies (buck / boost / buck- boost) ? ? ? ? ? high voltage : v in up to 50v, v out up to 50v ? ? ? ? ? 380khz fixed switching frequency ? ? ? ? ? analog or pwm control signal for led dimming ? ? ? ? ? internal soft-start to avoid inrush current ? ? ? ? ? under-voltage lockout ? ? ? ? ? thermal shutdown applications ? desk lights and room lighting ? industrial display backlight simplified application circuit ordering information note : richtek products are : ? rohs compliant and compatible with the current require- ments of ipc/jedec j-std-020. ? suitable for use in snpb or pb-free soldering processes. pin configurations (top view) sop-8 (exposed pad) marking information RT8477A package type sp : sop-8 (exposed pad-option 1) lead plating system g : green (halogen free and pb free) vcc isp isn vc creg drv ctl sens gnd 2 3 4 5 6 7 8 9 RT8477A gspymdnn RT8477Agsp : product number ymdnn : date code drv sens vcc RT8477A gnd ctl r1 v in c1 isn ... leds d1 c4 l1 r2 m1 analog dimming creg c2 r3 c3 vc r4 c5 isp c6 r5
RT8477A 2 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. function block diagram pin no. pin name pin function 1 vcc supply voltage input. for good bypass, a low esr capacitor is required. 2 isp positive input current sense. 3 isn negative input current sense. voltage threshold between isp and isn is 100mv. 4 vc vc compensation node for current loop. 5 ctl analog dimming control input. effective programming range is 0.33v to 2v. 6 sens current sense input for led current. connect the current sense resistor between external n-mosfet switch and the ground. 7 drv external mosfet switch gate driver output. 8 creg regulator output. placed 1 ? f capacitor to stabilize the 5v regulator output. 9 (exposed pad) gnd ground. the exposed pad must be soldered to a large pcb and connected to gnd for maximum power dissipation. functional pin description r r s osc + - 4.5v + - + - + - gm isn ctl vcc gnd creg drv 5v ldo sens soft-start vc isp
RT8477A 3 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. operation the RT8477A is a current mode pwm controller designed to drive an external mosfet for high current led applications. this device uses a fixed frequency, current- mode control scheme to provide excellent line and load regulation. the control loop has a current sense amplifier which senses the voltage between the isp and isn pins. a pwm comparator then turns off the external power switch when the sensed power switch current exceeds the internal compensated voltag e. the power switch will not be reset by the oscillator clock in each cycle. if the comparator does not turn off the switch in a cycle, the power switch will be on for more than a full switching period until the comparator is tripped. in this manner, the programmed voltage across the sense resistor is regulated by the control loop. the current through the sense resistor is set by the programmed voltage and the sense resistance. the voltage across the sense resistor can be programmed by the analog or digital signal at the ctl pin with good dimming linearity. the max sense threshold of 100mv can be obtained with ctl pin voltage greater than 2v (max dimming point). the sense threshold is intentionally forced to zero by an internal comparator when the ctl pin voltage is less than around 0.33v (min dimming point). because of that, the actual sense threshold right before cut off may vary from part to part over process variation. the RT8477A provides protection functions which include over-temperature, and switch current limit to prevent abnormal situations.
RT8477A 4 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. electrical characteristics parameter symbol test conditions min typ max unit overall regulator output voltage v creg i creg = 20ma 4.5 5 5.5 v supply current i vcc v ctl = 3v -- -- 3 ma vin under voltage lockout threshold v uvlo v in rising -- 4.25 4.5 v v in falling -- 4.2 -- current sense amplifier input threshold (v isp ? v isn ) 4.5 ? common mode ? 20v 95 100 105 mv input current i isp v isp = 24v -- 150 -- ? a i isn v isn = 24v -- 50 -- output current i vc 2.4v > vc > 0.3v -- ? 10 -- ? a vc threshold for ctl switch off -- 0.4 -- v led dimming input current of ctl pin i ctl 0.2v ? v ctl ? 1.2v -- 1 2 ? a led current off threshold at ctl v ctl_off -- 0.33 0.4 v led current on threshold at ctl v ctl_on -- 2 2.5 v recommended operating conditions (note 5) ? supply input voltage, vcc ------------------------------------------------------------------------------------ 4.5v to 50v ? junction temperature range ---------------------------------------------------------------------------------- ? 40 c to 125 c ? ambient temperature range ---------------------------------------------------------------------------------- ? 40 c to 85 c absolute maximum ratings (note 1) ? supply input v oltage, vcc ------------------------------------------------------------------------------------ ? 0.3v to 60v ? isp, isn ------------------------------------------------------------------------------------------------------------ ? 0.3v to 60v ? sens, drv, creg, vc pin voltage ----------------------------------------------------------------------- ? 0.3v to 5.5v ? ctl pin voltage ------------------------------------------------------------------------------------------------- ? 0.3v to 20v (note 2) ? power dissipation, p d @ t a = 25 c sop-8 (exposed pad) ----------------------------------------------------------------------------------------- 3.26w ? package thermal resistance (note 3) sop-8 (exposed pad), ja ------------------------------------------------------------------------------------ 30.6 c/w sop-8 (exposed pad), jc ----------------------------------------------------------------------------------- 3.4 c/w ? junction temperature ------------------------------------------------------------------------------------------- 150 c ? lead temperature (soldering, 10 sec.) --------------------------------------------------------------------- 260 c ? storage temperature range ---------------------------------------------------------------------------------- ? 65 c to 150 c ? esd susceptibility (note 4) hbm (human body model) ------------------------------------------------------------------------------------ 2kv (v cc = 12v, t a = 25 c, unless otherwise specified)
RT8477A 5 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. note 1. stresses beyond those listed ? absolute maximum ratings ? may cause permanent damage to the device. these are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions may affect device reliability. note 2. if connected with a 20k serial resistor, ctl can go up to 40v. note 3. ja is measured at t a = 25 c on a high effective thermal conductivity four-layer test board per jedec 51-7. jc is measured at the exposed pad of the package. note 4. devices are esd sensitive. handling precaution is recommended. note 5. the device is not guaranteed to function outside its operating conditions. note 6. when the natural maximum duty cycle of the switching frequency is reached, the switching cycle will be skipped (not reset) as the operating condition requires to effectively stretch and achieve higher on cycle than the natural maximum duty cycle set by the switching frequency. note 7. guaranteed by design, not subjected to production test. parameter symbol test conditions min typ max unit pwm converter switch frequency f sw 330 380 430 khz maximum duty cycle d max (note 6) -- -- 100 % minimum on-time -- 200 -- ns gate high voltage v ga te_h i gate = 20ma 4.5 5 5.5 v gate driver source 1 2.5 -- a gate driver sink 1 3.5 -- a soft-start time (note 7) -- 2 -- ms sense current limit threshold i sens_lim 100 150 -- mv over-temperature protection thermal shutdown temperature t sd -- 150 -- ? c thermal shutdown hysteresis ? t sd -- 20 -- ? c
RT8477A 6 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. typical application circuit buck configuration boost configuration drv sens vcc RT8477A gnd 7 6 9 1 ctl 5 r1 v in c1 10f 4.5v to 50v 0.1 isn 3 ... leds d1 c4 1f l1 22h r2 0.03 m1 analog dimming creg 8 c2 1f r3 51 c3 1nf vc 4 r4 10k c5 3.3nf isp 2 c6 1f r5 10 (short option) r6 (short option) r7 note : v isp , v isn < 50v drv vcc RT8477A gnd 7 9 1 ctl 5 r1 v in c1 10f 0.1 isn 3 ... d1 c4 1f analog dimming creg 8 c2 1f vc 4 r5 10k c6 3.3nf isp 2 c5 1f r4 10 sens 6 l1 22h r2 m1 r3 51 c3 1nf 0.05 51r v led 50v (max) v z (v z > v led ) 20r note : 1. v in , v isp , v isn < 50v 2. vled : the voltage across the led string 3. vz : zener diode breakdown voltage
RT8477A 7 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. buck-boost configuration drv vcc RT8477A gnd 7 9 1 ctl 5 r1 v in c1 10f 0.1 isp 2 ... d1 c4 1f analog dimming creg 8 c2 1f vc 4 isn 3 sens 6 l1 22h r2 m1 r3 51 c3 1nf 0.05 51r 20r note : 1. v isp , v isn < 50v 2. vled : the voltage across the led string 3. vz : zener diode breakdown voltage v z (v z > v led ) v led r5 10k c6 3.3nf
RT8477A 8 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. typical operating characteristics supply current vs. v cc 1.0 1.1 1.2 1.3 1.4 1.5 0 5 10 15 20 25 30 35 40 45 50 v cc (v) supply current (ma ) frequency vs. vcc 360 370 380 390 400 410 0 5 10 15 20 25 30 35 40 45 50 vcc (v) frequency (khz) 1 efficiency vs. input voltage 80 82 84 86 88 90 92 94 96 98 100 5 1525354555 input voltage (v) efficiency (%) v out = 21v v out = 18v v out = 15v v out = 12v v out = 6v v out = 9v buck, led current = 2a, l = 22 h led current vs. v ctl 0 50 100 150 200 250 300 350 400 450 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3 v ctl (v) led current (ma) led current = 300ma, led = 6pcs power on from vin time (2.5ms/div) i out (2a/div) v in (20v/div) v in = 30v, i out = 2a, led = 42pcs, l = 47 h v out (10v/div) isp - isn threshold vs. temperature 70 80 90 100 110 120 -40 -10 20 50 80 110 140 temperature (c) isp - isn threshold (mv) v cc = 24v
RT8477A 9 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. power off from vin time (25ms/div) i out (2a/div) v in (20v/div) v in = 30v, i out = 2a, led = 42pcs, l = 47 h v out (10v/div)
RT8477A 10 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. application information the RT8477A is a current mode pwm controller designed to drive an external mosfet for high current led applications. this device uses a fixed frequency, current mode control scheme to provide excellent line and load regulation. the control loop has a current sense amplifier which senses the voltage between the isp and isn pins. the power switch will not be reset by the oscillator clock in each cycle. if the comparator does not turn off the switch in a cycle, the power switch will be on for more than a full switching period until the comparator is tripped. in this manner, the programmed voltage across the sense resistor is regulated by the control loop. frequency compensation the RT8477A has an external compensation pin, allowing the loop response to be optimized for specific applications. an external resistor in series with a capacitor is connected from the vc pin to gnd to provide a pole and a zero for proper loop compensation. the typical value for the RT8477A is 10k and 3.3nf. led current setting the led current can be calculated by the following equation : ? led(max) v(isp isn) i= r1 where v(isp ? isn) is the voltage between isp and isn (100mv typ. if ctl dimming is not applied) and the r1 is the resister between isp and isn. sense resistor selection the resistor, r2, between the source of the external n- mosfet and gnd should be selected to provide adequate switch current to drive the application without exceeding the current limit threshold set by the sense pin sense threshold of RT8477A. the sense resistor value can be calculated according to the formula below : ocp current limlit threshold minimum value r2 i ? where i ocp is about 1.33 to 1.5 times of inductor peak current i peak . the placement of r2 should be close to the source of the n-mosfet and the ic gnd of the RT8477A. the sense pin input to rt8477 should be a kelvin sense connection to the positive terminal of r2. over-temperature protection the RT8477A has over-temperature protection (otp) function to prevent the excessive power dissipation from overheating. the otp function will shut down switching operation when the die junction temperature exceeds 150 c. the chip will automatically start to switch again when the die junction temperature cools off. inductor selection the converter operates in discontinuous conduction mode when the inductance value is less than the value l bcm . with an inductance greater than l bcm , the converter operates in continuous conduction mode (ccm). the inductance l bcm is determined by the following equations. for buck application : ? ?? ?? ?? ?? ?? out in out bcm out in vvv l 2i f v for boost application : ? ?? ?? ?? ?? ?? in out in bcm out out vvv l 2i f v where v out = output voltage. v in = input voltage. f = operating frequency. i out = led current. choose an inductance based on the operating frequency, input voltage and output voltage to provide a current mode ramp signal during the mosfet on period for pwm control loop regulation. the inductance also determines the inductor ripple current. operating the converter in ccm is recommended, which will have the smaller inductor ripple current and hence the less conduction losses from all converter components. for buck-boost application : in out bcm out in out vv l 2i f v + v ?? ?? ?? ?? ??
RT8477A 11 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. out l out 1 viesr + 8fc ?? ???? ?? ?? ?? schottky diode selection the schottky diode, with their low forward voltage drop and fast switching speed, is necessary for RT8477A applications. in addition, power dissipation, reverse voltage rating and pulsating peak current are important parameters of the schottky diode that must be considered. the diode's average current rating must exceed the average output current. the diode conducts current only when the power switch is turned off (typically less than 50% duty cycle). thermal considerations for continuous operation, do not exceed absolute maximum junction temperature. the maximum power dissipation depends on the thermal resistance of the ic package, pcb layout, rate of surrounding airflow, and difference between junction and ambient temperature. the maximum power dissipation can be calculated by the following formula : p d(max) = (t j(max) ? t a ) / ja where t j(max) is the maximum junction temperature, t a is the ambient temperature, and ja is the junction to ambient thermal resistance. for recommended operating condition specifications, the maximum junction temperature is 125 c. the junction to ambient thermal resistance, ja , is layout dependent. for sop-8 (exposed pad) package, the thermal resistance, ja , is 30.6 c/w on a standard jedec 51-7 four-layer thermal test board. the maximum power dissipation at t a = 25 c can be calculated by the following formula : p d(max) = (125 c ? 25 c) / (30.6 c/w) = 3.26w for sop-8 (exposed pad) package the maximum power dissipation depends on the operating ambient temperature for fixed t j(max) and thermal resistance, ja . the derating curve in figure 1 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. ? ?? ? ?? ?? ? ?? out in out out in vvv l = 20.3i f v for boost application : ? ?? ? ?? ?? ? ?? in out in out out vvv l = 20.3i f v the inductor must also be selected with a saturation current rating greater than the maximum inductor current during normal operation. the maximum inductor current can be calculated by the following equations. for buck application : ? ?? ? ?? ?? ?? out in out peak out in vvv i= i + 2lf v for boost application : ? ?? ?? ? ?? ??? ?? out out in out in peak in out vi v v v i= + v2lfv where is the efficiency of the power converter. capacitor selection the input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. for most RT8477A applications, a 4.7 f ceramic capacitor is sufficient. a value higher or lower may be used depending on the noise level from the input supply and the input current to the converter. in buck application, the output capacitor is typically ceramic and selection is mainly based on the output voltage ripple requirements. the output ripple, v out , is determined by the following equation : as a design example, to design the peak to peak inductor ripple to be 30% of the output current, the following equations can be used to estimate the size of the needed inductance : for buck application : for buck-boost application : ? ? in out out in out peak in in out v + v i vv i= + v2lfv + v ? ? ?? ? ?? ??? ?? for buck-boost application : in out out in out vv l = 20.3i f v + v ?? ? ?? ?? ? ??
RT8477A 12 ds8477a-02 april 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. figure 1. derating curve of maximum power dissipation 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 0 25 50 75 100 125 ambient temperature (c) maximum power dissipation (w) 1 four-layer pcb layout considerations pcb layout is very important when designing power switching converter circuits. some recommended layout guide lines are as follows : ? the power components m1, l1, d1 and c4 must be placed as close to each other as possible to reduce the ac current loop area. the pcb trace between power components must be as short and wide as possible due to large current flow through these traces during operation. ? place m1, l1 and d1 as close to each other as possible. the trace should be as short and wide as possible. ? the input capacitor c6 must be placed as close to vcc pin as possible. figure 2. pcb layout guide gnd ... isn locate input capacitor as close vcc as possible. c1 v in c2 r3 c3 r2 m1 l1 c4 r1 d1 place these components as close as possible. keep the isp and isn with the kelvin sense connection. c5 r4 locate the compensation components to vc pin as close as possible. vcc isp isn vc creg drv ctl sens gnd 2 3 4 5 6 7 8 9 isp c6 normal trace. r5 vin power trace to isp must be wide and short. power trace must be wide and short when compared to the normal trace.
RT8477A 13 ds8477a-02 april 2015 www.richtek.com richtek technology corporation 14f, no. 8, tai yuen 1 st street, chupei city hsinchu, taiwan, r.o.c. tel: (8863)5526789 richtek products are sold by description only. richtek reserves the right to change the circuitry and/or specifications without notice at any time. customers should obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. richtek cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a richtek product. information furnish ed by richtek is believed to be accurate and reliable. however, no responsibility is assumed by richtek or its subsidiaries for its use; nor for any infringeme nts 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 r ichtek or its subsidiaries. outline dimension a b j f h m c d i y x exposed thermal pad (bottom of package) 8-lead sop (exposed pad) plastic package dimensions in millimeters dimensions in inches symbol min max min max a 4.801 5.004 0.189 0.197 b 3.810 4.000 0.150 0.157 c 1.346 1.753 0.053 0.069 d 0.330 0.510 0.013 0.020 f 1.194 1.346 0.047 0.053 h 0.170 0.254 0.007 0.010 i 0.000 0.152 0.000 0.006 j 5.791 6.200 0.228 0.244 m 0.406 1.270 0.016 0.050 x 2.000 2.300 0.079 0.091 option 1 y 2.000 2.300 0.079 0.091 x 2.100 2.500 0.083 0.098 option 2 y 3.000 3.500 0.118 0.138

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