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RT8511B ? ds8511b-05 february 2015 www.richtek.com 1 ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. 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. marking information pin configurations (top view) wdfn-8l 2x2 43v asynchronous boost wled driver general description the RT8511B is an led driver ic that can support up to 10 wled in series. it is composed of a current mode boost converter integrated with a 43v/2.2a power switch running at a fixed 500khz frequency and covering a wide vin range from 2.7v to 24v. the white led current is set with an external resistor, and the feedback voltage is regulated to 200mv (typ.). during operation, the led current can be controlled by the pwm input signal in which the duty cycle determines the feedback reference voltage. for brightness dimming, the RT8511B is able to maintain steady control of the led current. therefore, no audible noises are generated on the output capacitor. the RT8511B also has programmable over voltage pin to prevent the output from exceeding absolute maximum ratings during open led conditions. the RT8511B is available in wdfn-8l 2x2 package. features ? ? ? ? ? wide input voltage range : 2.7v to 24v ? ? ? ? ? high output voltage : up to 43v ? ? ? ? ? direct pwm dimming control and frequency from 100hz to 8khz ? ? ? ? ? internal soft-start and compensation ? ? ? ? ? 200mv reference voltage ? ? ? ? ? pwm dimming with internal filter ? ? ? ? ? programmable over voltage protection ? ? ? ? ? over temperature protection ? ? ? ? ? current limit protection ? ? ? ? ? thin 8-lead wdfn package ? ? ? ? ? rohs compliant and halogen free applications ? umpc and notebook computer backlight ? gps, portable dvd backlight ovp fb gnd en pwm vin lx dimc 7 6 5 1 2 3 4 8 gnd 9 0f : product code w : date code 0fw package type qw : wdfn-8l 2x2 (w-type) lead plating system g : green (halogen free and pb free) RT8511B
RT8511B 2 ds8511b-05 february 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. pin no. pin name pin function 1 ovp over voltage protection for boost converter. the detecting threshold is 1.2v. 2 fb feedback. connect a resistor between this pin and gnd to set the led current. 3 dimc pwm filter. filter the pwm signal to a dc voltage. 4 gnd ground. 5 lx switch node for boost converter. 6 vin power supply input. 7 pwm dimming control input. 8 en chip enable (active high) for boost converter. 9 (exposed pad) gnd the exposed pad must be soldered to a large pcb and connected to agnd for maximum power dissipation. functional pin description typical application circuit figure 1. typical application circuit of normal operation figure 2. typical application circuit of low voltage operation RT8511B l fb vin gnd ovp lx v out : : : : : : : : : : : : wleds v in 4.2v to 24v en d c out r2 r1 c in chip enable 1 4, 9 (exposed pad) 2 5 6 8 1f x 2 3.3m 100k 10h 1f x 2 dimc 3 1f pwm 7 pwm 100hz to 8khz c dimc r set 3.3 RT8511B l fb vin gnd ovp lx v out : : : : : : : : : : : : wleds v led 2.7v to 24v en d c out r2 r1 c in chip enable 1 4, 9 (exposed pad) 2 5 6 8 1f x 2 3.3m 100k 10h 1f dimc 3 1f pwm 7 pwm 100hz to 8khz c dimc r set 3.3 c led 1f x 2 v in 2.7v to 4.2v
RT8511B 3 ds8511b-05 february 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. function block diagram r q s q osc pwm controller + - ocp otp + - 1.2v + - lx ovp dimc pwm d/a dimming fb gnd en vin
RT8511B 4 ds8511b-05 february 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. parameter symbol test conditions min typ max unit vin quiescent current i q v fb = 1.5v, no switching -- 725 -- ? a i q_sw v fb = 0v, switching -- -- 2.2 ma vin shutdown current i shdn v in = 4.5v, v en = 0v -- 1 4 ? a control input en, pwm threshold voltage logic-high v ih v in = 2.7v to 24v 1.6 -- -- v logic-low v il v in = 2.7v to 24v -- -- 0.8 en sink current i ih v en = 3v 1 -- 10 ? a shutdown delay t shdn en high to low 52 64 80 ms pwm dimming frequency 0.1 -- 8 khz boost converter switching frequency f osc v in = 2.7v to 24v 0.4 0.5 0.6 mhz lx on resistance (n-mosfet) r ds(on) v in > 5v -- 0.4 0.6 ? minimum on time -- 60 -- ns maximum duty cycle d max v fb = 0v, switching -- 92 -- % (v in = 4.5v, t a = 25 c, unless otherwise specified) electrical characteristics recommended operating conditions (note 4) ? supply input voltage, v in ------------------------------------------------------------------------------------------------ 2.7v to 24v ? junction temperature range -------------------------------------------------------------------------------------------- ? 40 c to 125 c ? ambient temperature range -------------------------------------------------------------------------------------------- ? 40 c to 85 c absolute maximum ratings (note 1) ? vin, en, pwm, dimc to gnd ------------------------------------------------------------------------------------------ ? 0.3v to 26.5v ? fb, ovp to gnd ---------------------------------------------------------------------------------------------------------- ? 0.3v to 48v ? lx to gnd ------------------------------------------------------------------------------------------------------------------ ? 0.3v to 48v < 500ns ---------------------------------------------------------------------------------------------------------------------- ? 1v to 48v ? power dissipation, p d @ t a = 25 c wdf n-8l 2x2 -------------------------------------------------------------------------------------------------------------- 0.8 33w ? package thermal resistance (note 2) wdfn-8l 2x2, ja --------------------------------------------------------------------------------------------------------- 120 c/w wdfn-8l 2x2, jc --------------------------------------------------------------------------------------------------------- 8.2 c/w ? lead temperature (soldering, 10 se c.) ------------------------------------------------------------------------------- 260 c ? junction temperature ----------------------------------------------------------------------------------------------------- 150 c ? storage temperature range -------------------------------------------------------------------------------------------- ? 65 c to 150 c ? esd susceptibility (note 3) hbm (human body model) ---------------------------------------------------------------------------------------------- 2kv mm (ma chine model) ----------------------------------------------------------------------------------------------------- 200v
RT8511B 5 ds8511b-05 february 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. parameter symbol test conditions min typ max unit led current minimum pwm dimming duty cycle d min dimming freq. = 100hz to 8khz 1 -- -- % feedback voltage v fb 195 200 205 mv fault protection lx current limit i lim 1.66 2.2 2.74 a over voltage protection threshold v ovp 1.14 1.2 1.26 v thermal shutdown temperature t sd -- 160 -- c thermal shutdown hysteresis ? t sd -- 30 -- c 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. 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 3. devices are esd sensitive. handling precaution is recommended. note 4. the device is not guaranteed to function outside its operating conditions.
RT8511B 6 ds8511b-05 february 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. led current vs. pwm duty cycle 0 10 20 30 40 50 60 0 102030405060708090100 pwm duty cycle (%) led current (ma) pwm = 100hz pwm = 2khz pwm = 8khz typical operating characteristics fb reference voltage vs. input voltage 198.0 198.3 198.6 198.9 199.2 199.5 4 8 12 16 20 24 input voltage (v) fb reference voltage (mv) frequency vs. input voltage 350 400 450 500 550 600 4 8 12 16 20 24 input voltage (v) frequency (khz) 1 efficiency vs. input voltage 60 65 70 75 80 85 90 95 100 4 7 9 121417192224 input voltage (v) efficiency (%) v out = 29.5v fb reference voltage vs. temperature 190 192 194 196 198 200 -20 5 30 55 80 105 temperature (c) fb reference voltage (mv) v in = 4.5v current limit vs. input voltage 1.0 1.4 1.8 2.2 2.6 3.0 2.5 5.25 8 10.75 13.5 16.25 19 21.75 24.5 input voltage (v) current limit (a)
RT8511B 7 ds8511b-05 february 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. application information the RT8511B is a current mode boost converter which operates at a fixed frequency of 500khz. it is capable of driving up to 10 white leds in series and integrates functions such as soft-start, compensation, and internal analog dimming control. the protection block also provides over-voltage, over-temperature, and current- limit protection features. led current setting the loop structure of the boost converter keeps the fb pin voltage equal to the reference voltage v fb . therefore, by connecting the resistor, r set between the fb pin and gnd, the led current will be determined by the current through r set . the led current can be calculated by the following equation : fb led set v i = r brightness control for the brightness dimming control of the RT8511B, the ic provides typically 200mv reference voltage when the pwm pin is constantly pulled high. however, the pwm pin allows a pwm signal to adjust the reference voltage by changing the pwm duty cycle to achieve led brightness dimming control. the relationship between the duty cycle and the fb voltage can be calculated according to the following equation : v fb = 200mv x duty where 200mv is the typical internal reference voltage and duty is the duty cycle of the pwm signal. as shown in figure 3, the duty cycle of the pwm signal is used to modify the internal 200mv reference voltage. with an on-chip output clamping amplifier and a serial resistor, the pwm dimming signal is easily low-pass filtered to an analog dimming signal with one external capacitor, c dimc , for noise-free pwm dimming. dimming frequency can be sufficiently adjusted from 100hz to 8khz. however, the led current cannot be 100% proportional to the duty cycle. referring to table 1, the minimum dimming duty can be as low as 1% for the frequency range from 100hz to 8khz. it should be noted that the accuracy of 1% duty is not guaranteed. figure 3. block diagram of programmable fb voltage table 1. minimum duty for dimming frequency dimming frequency minimum duty cycle 100hz to 8khz 1% because the voltage of dimc and fb is small to 2mv and easily affected by lx switching noise. + - dimc fb to controller 200mv r c dimc 1f pwm ea the fb pin voltage will be decreased by lower pwm duty ratio . that will achieve led current diming function for different brightness. but led current is more accurate when higher pwm duty. the table 2. shows typical variation value comparison between different pwm duty and condition is v in = 3.7v, led array = 6s2p, r set = 5 . table 2. led current variation vs pwm duty pwm duty (%) variation (%) pwm duty (%) variation (%) 1 60 8 7 2 25 9 6 3 17 10 5 4 13 20 4 5 10 50 3 6 9 100 2.5 7 8 it also should be noted that when the input voltage is too close to the output voltage [(v out ? v in ) < 6v] , excessive audible noise may occur. additionally, for accurate brightness dimming control, the input voltage should be kept lower than the leds' turn on voltage. when operating in the light load, excessive output ripple may occur.
RT8511B 8 ds8511b-05 february 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. mode2 mode3 figure 4. power on sequence mode1 mode2 mode3 figure 5. power off sequence v in v out soft-start en v in v out en pwm soft-start v in v out en pwm v in v out en pwm v in v out en pwm shutdown delay mode1 v in v out soft-start en pwm soft-start the RT8511B provides a built-in soft-start function to limit the inrush current, while allowing for an increased pwm frequency for dimming. current limiting protection the RT8511B can limit the peak current to achieve over current protection. the ic senses the inductor current through the lx pin in the charging period. when the value exceeds the current limiting threshold, the internal n- mosfet will be turned off. in the off period, the inductor current will descend. the internal mosfet is turned on by the oscillator during the beginning of the next cycle. power sequence in order to assure that the normal soft-start function is in place for suppressing the inrush current, the input voltage and enable voltage should be ready before pwm pulls high. figure 4 and figure 5 show the power on and power off sequences.
RT8511B 9 ds8511b-05 february 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. out, ovp ovp r2 v = v1+ r1 ?? ? ?? ?? where r1 and r2 make up the voltage divider connected to the ovp pin. over temperature protection the RT8511B has an over temperature protection (otp) function to prevent overheating caused by excessive power dissipation from overheating the device. the otp will shut down switching operation if the junction temperature exceeds 160 c. the boost converter will start switching again when the junction temperature is cooled down by approximately 30 c. inductor selection the inductance depends on the maximum input current. as a general rule, the inductor ripple current range is 20% to 40% of the maximum input current. if 40% is selected as an example, the inductor ripple current can be calculated according to the following equation : out out in(max) (min) in(min) ripple in(max) vi i = v i = 0.4i ? ? ? ? where is the efficiency of the boost converter, i in(max) is the maximum input current, i out is the total current from all led strings, and i ripple is the inductor ripple current. the input peak current can be calculated by maximum input current plus half of inductor ripple current shown as following equation : i peak = 1.2 x i in(max) note that the saturated current of the inductor must be greater than i peak . the inductance can eventually be determined according to the following equation : over voltage protection the RT8511B equips over voltage protection (ovp) function. when the voltage at the ovp pin reaches a threshold of approximately 1.2v, the mosfet drive output will turn off. the mosfet drive output will turn on again once the voltage at the ovp pin drops below the threshold. thus, the output voltage can be clamped at a certain voltage level, as shown in the following equation : ?? ?? ) 2 in out in 2 out out osc v(v v l = 0.4 v i f ? ??? ??? where f osc is the switching frequency. for better efficiency, it is suggested to choose an inductor with small series resistance. diode selection the schottky diode is a good choice for an asynchronous boost converter due to its small forward voltage. however, when selecting a schottky diode, important parameters such as power dissipation, reverse voltage rating, and pulsating peak current must all be taken into consideration. a suitable schottky diode's reverse voltage rating must be greater than the maximum output voltage, and its average current rating must exceed the average output current. capacitor selection two 1 f ceramic input capacitors and two 1 f ceramic output capacitors are recommended for driving 10 wleds in series. for better voltage filtering, ceramic capacitors with low esr are recommended. note that the x5r and x7r types are suitable because of their wide voltage and temperature ranges. 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 wdfn-8l 2x2 package, the thermal resistance, ja , is 120 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 formulas :
RT8511B 10 ds8511b-05 february 2015 www.richtek.com ? copyright 2015 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. figure 7. pcb layout guide layout consideration for high frequency switching power supplies, the pcb layout is important to obtain good regulation, high efficiency and stability. the following descriptions are the suggestions for better pcb layout. ? input and output capacitors should be placed close to the ic and connected to the ground plane to reduce noise coupling. ? the gnd and exposed pad should be connected to a strong ground plane for heat sinking and noise protection. ? the components l, d, c in and c out must be placed as close as possible to reduce current loop. keep the main current traces as possible as short and wide. ? the lx node of the dc/dc converter experiences is with high frequency voltage swings. it should be kept in a small area. ? the component r set should be placed as close as possible to the ic and kept away from noisy devices. figure 6. derating curve of maximum power dissipation p d(max) = (125 c ? 25 c) / (120 c/w) = 0.833w for wdfn-8l 2x2 package the maximum power dissipation depends on operating ambient temperature for fixed t j(max) and thermal resistance, ja . the derating curves in figure 6 allow the designer to see the effect of rising ambient temperature on the maximum power dissipation. 0.0 0.2 0.4 0.6 0.8 1.0 0 25 50 75 100 125 ambient temperature (c) maximum power dissipation (w) 1 four-layer pcb ovp fb gnd en pwm vin lx dimc 7 6 5 1 2 3 4 8 gnd 9 c in should be placed as closed as possible to v i n pin for good filtering. the c out should be connected directly from the output schottky diode to ground rather than across the wleds. the inductor should be placed as close as possible to the switch pin to mi nimize the noise coupling into other circuits. lx node copper area should be minimized for reducing emi locate r set close to fb as possible : : : : : : : : : : : : wleds v in v out c out c in l d r1 r2 c dimc r set
RT8511B 11 ds8511b-05 february 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 w-type 8l dfn 2x2 package 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 0.200 0.300 0.008 0.012 d 1.950 2.050 0.077 0.081 d2 1.000 1.250 0.039 0.049 e 1.950 2.050 0.077 0.081 e2 0.400 0.650 0.016 0.026 e 0.500 0.020 l 0.300 0.400 0.012 0.016 1 1 2 2 note : the configuration of the pin #1 identifier is optional, but must be located within the zone indicated. det ail a pin #1 id and tie bar mark options d 1 e a3 a a1 d2 e2 l b e see detail a

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