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1 ds8577-00 november 2011 www.richtek.com RT8577 high voltage 4-ch led driver general description the RT8577 is an 4-ch led driver capable of delivering 200ma for each channel. the RT8577 is a current mode boost converter with an adjustable switching frequency via the rt pin from 100khz to 1mhz and a wide vin range from 9v to 28v. the pwm output voltage loop selects and regulates the led pin with the highest voltage string to 0.6v, hence allowing voltage mismatches between led strings. the RT8577 automatically detects and disconnects any unconnected and/or broken strings during operation from pwm loop to prevent v out from over voltage. the 1.5% matched led currents on all channels are simply programmed with a resistor. a very high contrast ratio true digital pwm dimming can be achieved by driving the pwm pin with a pwm signal. when an abnormal situation (open/short/thermal) occurs, a status signal will be sent to the system to shut down the ic. 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. features z z z z z wide input supply voltage range : 9v to 28v z z z z z adjustable boost controller switching frequency from 100khz to 1mhz z z z z z programmable channel current z z z z z channel current matching : 1.5% z z z z z external dimming control z z z z z boost mosfet over current protection z z z z z automatic led open/short protection to avoid output over voltage z z z z z vcc under voltage lockout z z z z z adjustable over voltage protection z z z z z under voltage protection z z z z z thermal shutdown protection z z z z z abnormal status indicator for open/short/thermal condition z z z z z rohs compliant and halogen free applications z lcd tv, monitor display backlight z led driver application z general purpose constant current source pin configurations (top view) wqfn-20l 5x5 marking information RT8577zqw : product number ymdnn : date code package type qw : wqfn-20l 5x5 (w-type) RT8577 lead plating system z : eco (ecological element with halogen free and pb free) RT8577 zqw ymdnn 21 gnd led2 ovp/uvp gnd led1 creg sen pgnd drv riset en 15 14 13 12 1 2 3 4 11 5 status rt vc ss led4 gnd gnd vcc pwm led3 7 8910 19 18 17 16 6 20
2 ds8577-00 november 2011 www.richtek.com RT8577 typical application circuit figure 1. general application figure 2. external p-mosfet isolation application led1 led4 v out 3, 17, 18, 21 (exposed pad) gnd 2 19 vc 9 560 0.22f r c c c pgnd 13 11 6 external/internal pwm en pwm : : : : : : : : r ovp2 r ovp1 c out 4.7f x 6 r rt rt 8 9.1k riset 5 r iset c reg 15 creg c ss 10 ss 0.1f 1f 47f c in c f r f1 r sense r drv 5 1f c vcc v in 9v to 28v 14 12 16 drv sen vcc m sw ovp/uvp 4 chip enable RT8577 10h l1 d1 status r status 100k 7 flt r f2 1k 1k 100 0 1nf v in 100m 56k 3.3m 82k 45v : : : : led2 1 3, 17, 18, 21 (exposed pad) gnd vc 9 560 0.22f r c c c pgnd 13 11 6 external/internal pwm en pwm r rt rt 8 9.1k riset 5 r iset c reg 15 creg c ss 10 ss 0.1f 1f 47f c in c f r f1 r sense r drv 5 1f c vcc v in 9v to 28v 14 12 16 drv sen vcc m sw ovp/uvp 1 chip enable RT8577 10h l1 d1 status r status 100k 7 flt r f2 1k 1k 100 0 1nf v in flt 100m 56k r ovp2 r ovp1 3.3m 82k led1 led4 v out 2 19 : : : : : : : : c out 4.7f x 6 45v : : : : led2 1
3 ds8577-00 november 2011 www.richtek.com RT8577 functional pin description pin no. pin name pin function 1 led2 channel 2 led current sink, leave this pin unconnected if it is not used. 2 led1 channel 1 led current sink, leave this pin unconnected if it is not used. 3, 17, 18, 21 (exposed pad) gnd ground. the exposed pad must be soldered to a large pcb and connected to gnd for maximum power dissipation. 4 ovp/uvp over voltage and under voltage protection. pwm boost converter turns off when v ovp or v uvp goes higher than 1.2v or lower than 0.6v, respectively. 5 riset led current set pin. a resistor or a current from dac on this pin programs the full led current. 6 pwm dimming control input. 7 status boost converter operation status output. 8 rt switching frequency set. connect a resistor between rt and gnd to set the boost converter switching frequency. 9 vc pwm boost converter loop compensation node. 10 ss soft-start pin. place a capacitor of at least 10nf from this pin to gnd to set the soft-start time period. 11 en chip enable. when en is pulled low, the chip will be shut down. 12 sen current sense input. during normal operation, this pin senses the voltage across the external inductor current sensing resistor for peak current mode control and also to limit the inductor current during every switching cycle. 13 pgnd boost converter power ground. 14 drv boost converter power switch gate output. this pin drives the external power n-mosfet device. 15 creg 1 f capacitor should be placed on this pin to stabilize the 5v output of the internal regulator. this regulator is for chip internal use only. 16 vcc power supply of the chip. for good bypass, a low esr capacitor close to the pin is required. 19, 20 led4, led3 channel 4 and 3 led current sinks. leave the pins unconnected if not used.
4 ds8577-00 november 2011 www.richtek.com RT8577 function block diagram r r s + - + - + - uvlo 1.2v + - 1.2v shutdown status vc en ovp/uvp vcc osc drv v out regulation unit 6a rt sen ovp uvp otp 5v ldo creg ss + - 0.6v led short uvp + - + - 5v led1 led4 gnd riset pgnd + - pwm
5 ds8577-00 november 2011 www.richtek.com RT8577 electrical characteristics (v cc = 12v, t a = 25 c, unless otherwise specified) absolute maximum ratings (note 1) z supply voltage, v cc , status ------------------------------------------------------------------------------------------ 33v z led1 to led4 --------------------------------------------------------------------------------------------------------------- 50v z pwm, en, drv, sen, ss, vc, rt , creg, ovp/uvp, riset ------------------------------------------------- 5.5v z power dissipation, p d @ t a = 25 c wqfn-20l 5x5 ------------------------------------------------------------------------------------------------------------- 2.778w z package thermal resistance (note 2) wqfn-20l 5x5, ja -------------------------------------------------------------------------------------------------------- 36 c/w wqfn-20l 5x5, jc ------------------------------------------------------------------------------------------------------- 6 c/w z junction temperature ------------------------------------------------------------------------------------------------------ 150 c z lead temperature (soldering, 10 sec.) -------------------------------------------------------------------------------- 260 c z storage temperature range --------------------------------------------------------------------------------------------- ? 65 c to 150 c z esd susceptibility (note 3) hbm (human body mode) ----------------------------------------------------------------------------------------------- 2kv mm (ma chine mode) ------------------------------------------------------------------------------------------------------- 200v recommended operating conditions (note 4) z supply voltage, v cc ------------------------------------------------------------------------------------------------------- 9v to 28v z led1 to led4 --------------------------------------------------------------------------------------------------------------- 45v z i led1 to i led4 ----------------------------------------------------------------------------------------------------------------- 10ma to 200ma z junction temperature range --------------------------------------------------------------------------------------------- ? 40 c to 125 c z ambient temperature range --------------------------------------------------------------------------------------------- ? 40 c to 85 c parameter symbol test conditions min typ max unit supply voltage supply current i vcc switching off -- 5 -- ma shutdown current i shdn v en < 0.7v -- -- 10 a vdd ldo output v creg -- 5 -- v vdd ldo capability i creg 30 -- -- ma vcc rising -- 6.7 8 vcc uvlo threshold v uvlo hysteresis -- 1.4 -- v logic-high v enh 1.5 -- -- en threshold voltage logic-low v enl -- -- 0.8 v led current programming led current accuracy r iset = 9.1k , v pw m > 1.2v 114 120 126 ma led current matching r iset = 9.1k , v pw m > 1.2v ledx led_ave led_ave ii 100% i ? -- 1.5 3 % led1 to led4 regulation voltage i led = 200ma -- 0.6 -- v v led threshold no connection -- 0.1 -- v riset pin voltage -- 1.2 -- v to be continued
6 ds8577-00 november 2011 www.richtek.com RT8577 parameter symbol test conditions min typ max unit dimming logic-high v pwmh 1.2 -- -- pwm threshold voltage logic-low v pwml -- -- 0.35 v pwm boost controller r rt = 24k -- 1 -- mhz switching frequency f sw r rt = open -- 100 -- khz minimum on time t on -- 100 -- ns maximum duty cycle dmax 80 -- -- % sen current sense limit input current limit -- 0.5 -- v gate driver source -- 2.5 -- a gate driver sink -- 3 -- a ovp , uvp, scp, otp and soft-start ovp threshold v ovp -- 1.2 -- v uvp threshold v uvp -- 0.6 -- v scp threshold v scp led1 to led4 -- 4.3 -- v soft-start current i ss v ss < 2.5v -- 6 -- a thermal shutdown temperature t sd lockout temperature point -- 150 -- c thermal shutdown hysteresis t sd resume temperature point -- 20 -- c status low voltage v status open drain at 10ma -- -- 0.5 v note 1. stresses listed as the above ? absolute maximum ratings ? may cause permanent damage to the device. these are for stress ratings. 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 for extended periods may remain possibility to affect device reliability. note 2. ja is measured in natural convection at t a = 25 c on a high-effective thermal conductivity four-layer test board of jedec 51-7 thermal measurement standard. the measurement case position of jc is on the exposed pad of the package. note 3. devices are esd sensitive. handling precaution is recommended. note 4. the device is not guaranteed to function outside its operating conditions.
7 ds8577-00 november 2011 www.richtek.com RT8577 led current vs. input voltage 60 80 100 120 140 160 8 10121416182022242628 input voltage (v) led current (ma) typical operating characteristics efficiency vs. input voltage 70 75 80 85 90 95 100 8 10121416182022242628 input voltage (v) efficiency (%) 96leds, r iset = 9.1k led current vs. pwm duty cycle 0 20 40 60 80 100 120 0 102030405060708090100 pwm duty cycle (%) led current (ma) v in = 12v, 96leds, r iset = 9.1k pwm = 200hz pwm = 1khz pwm = 10khz 96leds, r iset = 9.1k led1 led2 led3 led4 power on from vin time (5ms/div) v in (5v/div) drv (5v/div) i in (1a/div) v in = 12v, c ss = 0.1 f, 96leds, r iset = 18.2k power on from en time (5ms/div) v en (2v/div) drv (5v/div) i in (1a/div) v in = 12v, c ss = 0.1 f, 96leds, r iset = 18.2k power on from pwm time (5ms/div) pwm (2v/div) drv (5v/div) i in (1a/div) v in = 12v, c ss = 0.1 f, 96leds, r iset = 18.2k
8 ds8577-00 november 2011 www.richtek.com RT8577 application information the RT8577 is an 4-ch driver controller that delivers well matched led current to each channel of led strings. the external n-mosfet current source will accommodate the power dissipation difference among channels resulting from the forward voltage difference between the led strings. with high speed current source n-mosfet drivers, the RT8577 features highly accurate current matching, while also providing very fast turn-on and turn-off times. this allows a very narrow minimum on or off pulse. the RT8577 integrates adjustable switching frequency and soft-start and provides circuitry for over temperature, over voltage, under voltage and current limit protection. soft-start the RT8577 employs a soft-start feature to limit the inrush current. the soft-start circuit prevents excessive inrush current and input voltage droop. the soft-start time is determined by a capacitor, c ss , connected between ss and gnd and charged with a 6 a constant current as shown in the following equation. t ss (max) = c ss x 4.8 x 10 5 (s) the value of capacitor c ss is user-defined to satisfy the designer' requirement. compensation the regulator loop can be compensated by adjusting the external components connected to the vc pin. the vc pin is the output of the internal error amplifier. the compensation capacitor will adjust the integrator zero to maintain stability and the resistor value will adjust the frequency integrator gain for fast transient response. typical values of the compensation components are r c = 560 , c c = 0.22 f. led connection the RT8577 equips 4-ch led drivers and each channel supports up to 15 leds. the led strings are connected from the output of the boost converter to pin ledx (x = 1 to 4) respectively. if one of the led channel is not in use, the led pin should be opened directly. setting and regulation of led current the led current can be calculated by the following equation : ? led iset 1092 i r where r iset is the resistor between the riset pin and gnd. this setting is the reference for the led current at pin ledx and represents the sensed led current for each string. the dc/dc converter regulates the led current according to the setting. over voltage and under voltage protection the RT8577 integrates over voltage protection (ovp) and under voltage protection (uvp). when the voltage at the ovp/uvp pin rises above the threshold voltage of approximately 1.2v or falls below the threshold voltage of approximately 0.6v, the internal switch will be turned off and status pin will be pulled high. the internal switch will be turned on again once the voltage at the ovp/uvp pin returns to normal range. the output voltage can be clamped at a certain voltage level and can be calculated by the following equations : where r ovp1 and r ovp2 are the resistors in the resistive voltage divider connected to the ovp/uvp pin. if at least one string is in normal operation, the controller will automatically ignore the open strings and continue to regulate the current for the strings in normal operation. suggested value for r ovp2 is up to 3m to prevent loading effect. led short circuit protection the RT8577 integrates led short circuit protection (scp). if one of the led1 to led4 pin voltages exceeds a threshold of approximately 4.3v during normal operation, the status pin will be pulled high for a fault signal. status after the ic is enable. status will output logic high if led short/ovp/uvp/otp conditions exist. status will be reset after v in or en is re-applied. ?? ?? ?? ovp2 out(ovp) ovp ovp1 r v = v 1 + r ?? ?? ?? ovp2 out(uvp) uvp ovp1 r v = v 1 + r
9 ds8577-00 november 2011 www.richtek.com RT8577 ? 9 sw rt 21.6 10 f100k + r sense 0.5v current limit (a) r ? current limit protection the RT8577 can sense the r sense voltage between the sen pin and gnd to achieve over current protection. the boost converter senses the inductor current during the on period. the duty cycle depends on the current signal and internal slope compensation compared with the error signal. the external switch will be turned off when the current signal is larger than the internal slope compensation. in the off period, the inductor current will decrease until the internal switch is turned on by the oscillator. the current limit value can be calculated by the following equation : brightness control the RT8577 features a digital dimming control scheme. a very high contrast ratio true digital pwm dimming is achieved by driving the pwm pin with a pwm signal. the recommended pwm frequency is 200hz to 10khz, but the led current cannot be 100% proportional to duty cycle, especially for high frequency and low duty ratio. over temperature protection the RT8577 has over temperature protection function to prevent the ic from overheating due to excessive power dissipation. the ic will shut down and the status pin will be pulled high when junction temperature exceeds 150 c. main converter starts switching after junction temperature cools down by approximately 20 c. inductor selection the value of the inductance, l, can be approximated by the following equation, where the transition is from discontinuous conduction mode (dcm) to continuous conduction mode (ccm) : the duty cycle can be calculated as the following equation : where v out is the maximum output voltage, v in is the minimum input voltage, f is the operating frequency, and i out is the sum of current from all led strings. the boost converter operates in dcm over the entire input voltage range when the inductor value is less than this value, l. with an inductance greater than l, the converter operates in ccm at the minimum input voltage and may be discontinuous at higher voltages. the inductor must be selected with a saturated current rating that is greater than the peak current as provided by the following equation : where is the efficiency of the power converter. diode selection schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. power dissipation, reverse voltage rating, and pulsating peak current are important parameters for consideration when making a schottky diode selection. make sure that the diode's peak current rating exceeds i peak and reverse voltage rating exceeds the maximum output voltage. capacitor selection the input capacitor reduces current spikes from the input supply and minimizes noise injection to the converter. for general applications, six 4.7 f ceramic capacitors are 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. it is recommended to choose a ceramic capacitor based on the output voltage ripple requirements. the minimum value of the output capacitor, c out , can be calculated by the following equation : () ? 2 out out d1d v l = 2fi ? out in out vv d = v + out out in peak in vi vdt i = v2l setting the switching frequency the RT8577 switching frequency is programmable from 100khz to 1mhz by adjusting the oscillator resistor, r rt . the switching frequency can be calculated by the following equation : ? out out out id c = vf where v out is the peak-to-peak ripple voltage at the output.
10 ds8577-00 november 2011 www.richtek.com RT8577 figure 3. derating curve for RT8577 packages layout considerations careful pcb layout is very important for designing switching power converter circuits. the following layout guidelines should be strictly followed for best performance of the RT8577. ` the power components l1, d1, c in , c out must be placed as close as possible to the ic to reduce current loop. the pcb trace between power components must be as short and wide as possible. ` the compensation circuit should be kept away from the power loops and shielded with a ground trace to prevent any noise coupling. place the compensation components, r c and c c , as close as possible t o pin 9. ` the exposed pad of the chip should be connected to ground plane for thermal consideration. 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 of RT8577, the maximum junction temperature is 125 c and t a is the ambient temperature. the junction to ambient thermal resistance, ja , is layout dependent. for wqfn- 20l 5x5 packages, the thermal resistance, ja , is 36 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 / ( 36 c/ w) = 2.778w for wqfn-20l 5x5 package the maximum power dissipation depends on the operating ambient temperature for fixed t j(max) and thermal resistance, ja . for RT8577 package, the derating curve in figure 3 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. 0.00 0.40 0.80 1.20 1.60 2.00 2.40 2.80 3.20 0255075100125 ambient temperature (c) maximum power dissipation (w) 1 four-layer pcb
11 ds8577-00 november 2011 www.richtek.com RT8577 pgnd d1 l1 + agnd v in c in r c c c c ss r sense c out 21 gnd led2 ovp/uvp gnd led1 creg sen pgnd drv riset en 15 14 13 12 1 2 3 4 11 5 status rt vc ss led4 gnd gnd vcc pwm led3 78910 19 18 17 16 6 20 r rt c f m sw r1 v out v in r f1 r f2 r drv c vcc pgnd agnd locate the c vcc as close to vcc as possible. place the power components as close as possible. the traces should be wide and short especially for the high current loop. the exposed pad of the chip should be connected to ground plane for thermal consideration. separate power ground (pgnd) and analog ground (agnd). connect agnd and pgnd islands at a single end. make sure there are no other connections between these separate ground planes. the pgnd should be wide and short enough to connect ground plane. the compensation circuit and r iset resistor should be kept away from the power loops and should be shielded with a ground trace to prevent any noise coupling. r iset figure 4. pcb layout guide
12 ds8577-00 november 2011 www.richtek.com RT8577 richtek technology corporation headquarter 5f, no. 20, taiyuen street, chupei city hsinchu, taiwan, r.o.c. tel: (8863)5526789 fax: (8863)5526611 information that is provided by richtek technology corporation is believed to be accurate and reliable. richtek reserves the ri ght to make any change in circuit design, specification or other related things if necessary without notice at any time. no third party intellectual property inf ringement of the applications should be guaranteed by users when integrating richtek products into any application. no legal responsibility for any said applications i s assumed by richtek. richtek technology corporation taipei office (marketing) 5f, no. 95, minchiuan road, hsintien city taipei county, taiwan, r.o.c. tel: (8862)86672399 fax: (8862)86672377 email: marketing@richtek.com outline dimension dimensions in millimeters dimensions in inches symbol min max min max a 0.700 0.800 0.028 0.031 a1 0.000 0.050 0.000 0.002 a3 0.175 0.250 0.007 0.010 b 0.250 0.350 0.010 0.014 d 4.900 5.100 0.193 0.201 d2 3.100 3.200 0.122 0.126 e 4.900 5.100 0.193 0.201 e2 3.100 3.200 0.122 0.126 e 0.650 0.026 l 0.550 0.650 0.022 0.026 w-type 20l qfn 5x5 package

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