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1 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary high efficiency pwm buck led driver controller general description the RT8458 is a pwm controller with an integrated high side floating gate driver. it is used for step down converters by well controlling the external mosfet and regulating a constant output current. the output duty cycle of the RT8458 can be up to 100% for wider input voltage application, such as e27 and par30 off-line led lighting products. the RT8458 also features a 48khz fixed frequency oscillator, an internal 178mv precision reference, and a pwm comparator with latching logic. the accurate output led current is achieved by an averaging current feedback loop and the led current dimming can be easily controlled via the actl pin. the RT8458 also has multiple features to protect the controller from fault conditions, including under voltage lockout (uvlo), over current protection circuit (ocp) and over voltage protection (ovp). additionally, to ensure the system reliability, the RT8458 is built with the thermal protection function. the RT8458 is housed in a tsot-23-6 package. thus, the components in the whole led driver system can be made very compact. applications z e27, par30, offline led lights pin configurations (top view) tsot-23-6 ordering information note : richtek green 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 low cost and efficient buck converter solution z z z z z universal input voltage range with off-line topology z z z z z programmable constant led current z z z z z dimmable led current by actl z z z z z output led string open protection z z z z z output led string short protection z z z z z output led string over current protection z z z z z built-in thermal protection z z z z z tiny tsot-23-6 package z z z z z rohs compliant and halogen free vcc gnd gate sense vc actl 4 23 5 6 marking information 01= : product code dnn : date code package type j6 : tsot-23-6 RT8458 operating temperature range g : green (halogen free with comme- rcial standard) 01=dnn
2 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary typical application circuit functional pin description pin no. pin name pin function 1 vcc power supply pin of the chip. for good bypass, a ceramic capacitor near the vcc pin is required. 2 gnd ground of the chip. 3 gate gate driver for external mosfet switch. 4 actl analog dimming control. the typical effective dimming range is between 0.2v to 1.3v. 5 vc pwm loop compensation node. 6 sense led current sense input pin. typical sensing threshold is 178mv . figure 1. for typical application figure 2. for high pf application actl gnd sense vc gate vcc 2 5 1 4 6 3 RT8458 1m 10 optional optional led+ led- 1m 1m 10f 3.3k 3.3nf 4.7f optional actl gnd sense vc gate vcc 2 5 1 4 6 3 RT8458 optional optional led+ led- 330nf 1.5m 1.5m 1m 1m r4 10k c5 1.2nf c6 3.3nf 4.7f/50v c7 r s optional 1m 1m
3 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary function block diagram chip enable osc + - + - control circuit + - dimming 35v vcc vc ovp 12v 200k gate actl sense gnd r r s + + - 16v/7v
4 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary absolute maximum ratings (note 1) z supply input voltage, v cc ------------------------------------------------------------------------------------------------- 40v z gate v oltage ---------------------------------------------------------------------------------------------------------------- 14v z actl voltage ----------------------------------------------------------------------------------------------------------------- 8v z vc v oltage -------------------------------------------------------------------------------------------------------------------- 6v z sense v oltage -------------------------------------------------------------------------------------------------------------- ? 0.3v to 6v z power dissipation, p d @ t a = 25 c tsot-23-6 --------------------------------------------------------------------------------------------------------------------- 0.392w z package thermal resistance (note 2) tsot-23-6 , ja --------------------------------------------------------------------------------------------------------------- 255 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 input voltage, v cc ------------------------------------------------------------------------------------------------- 7v to 35v z junction temperature range ---------------------------------------------------------------------------------------------- ? 40 c to 125 c electrical characteristics (v cc = 24v dc , c load = 1nf, r load = 2.2 in series, t a = 25 c, unless otherwise specified) to be continued parameter symbol test conditions min typ max unit input start-up voltage v st -- 16 -- v maximum startup current i st(max) -- 250 300 a input supply current i cc after start-up, v cc = 24v -- 1.65 5 ma input quiescent current i qc before start-up, v cc = 15v -- 0.1 -- a over voltage protection v ovp vcc pin -- 35 -- v current sense voltage v sense 169 178 187 mv switching frequency f sw -- 48 -- khz oscillator maximum duty cycle d max -- -- 100 % minimum turn-on time t on(min) 300 -- -- ns gate pin maximum voltage v gate -- 12.5 -- v i gate = ? 20ma -- 12.4 -- gate voltage high v gate_h i gate = ? 100 a -- 12.5 -- v i gate = 20ma -- 0.75 -- gate voltage low v gate_f i gate = 100 a -- 0.5 -- v gate drive rise and fall time 10nf load at gate -- 150 -- ns gate drive source and sink peak current 10nf load at gate -- 0.8 -- a
5 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary 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 low effective thermal conductivity test board of jedec 51-3 thermal measurement standard. note 3. devices are esd sensitive. handling precaution is recommended. note 4. the device is not guaranteed to function outside its operating conditions. parameter symbol test conditions min typ max unit led dimming analog dimming actl pin input current i actl -- 1 -- a led current off threshold at actl v actl_off -- -- 0.1 v vc threshold for pwm switch off v vc -- 1.25 -- v thermal protection thermal shutdown temperature t sd -- 150 -- c
6 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary led current vs. v actl 0 20 40 60 80 100 120 140 160 180 200 0 0.5 1 1.5 2 2.5 v actl (v) led current (ma) v in = 110vac, i out = 178ma, 10leds, l = 1mh typical operating characteristics efficiency vs. input voltage 70% 75% 80% 85% 90% 95% 100% 110 130 150 170 190 210 230 input voltage (v) efficiency (%) 10led 9led 8led 7led 6led 5led 4led 3led v in = 110v to 220vac, i out = 350ma efficiency vs. number of led 70% 75% 80% 85% 90% 95% 100% 345678910 number of led (pcs) efficiency (%) 110vac 180vac 220vac v in = 110v to 220vac, i out = 350ma switching frequency vs. supply voltage 35 39 43 47 51 55 0 5 10 15 20 25 30 35 supply voltage (v) switching frequency (khz) 1 switching frequency vs. temperature 40 42 44 46 48 50 -50 -25 0 25 50 75 100 125 temperature (c) switching frequency (khz) 1 led current vs. input voltage 160 165 170 175 180 185 190 70 100 130 160 190 220 250 280 input voltage (v) led current (ma) v in = 70v to 264vac, i out = 78ma, 10leds, l = 1mh
7 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary led current vs. output voltage 160 165 170 175 180 185 190 9 121518212427303336 output voltage (v) led current (ma) v in = 70v to 264vac i out = 178ma, 10 leds, l = 1mh input voltage and input current time (10ms/div) v in = 264vac, i out = 240ma 10 leds, l = 1mh i in (500ma/div) v out (100v/div) gate voltage and inductor current time (10 s/div) v in = 264vac, i out = 240ma, 10 leds, l = 1mh i l (200ma/div) v gate (5v/div) output current and output voltage time (25 s/div) v in = 264vac, i out = 240ma, 10 leds, l = 1mh i out (100ma/div) v out (20v/div) i out v out power on time (2.5ms/div) v in = 264vac, i out = 240ma, 10 leds, l = 1mh i out (200ma/div) v out (20v/div) power off time (2.5ms/div) v in = 264vac, i out = 240ma, 10 leds, l = 1mh i out (200ma/div) v out (20v/div)
8 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary application information the RT8458 is a high efficiency pwm buck led driver controller for high brightness led application. its high side floating gate driver is used to control the buck converter via an external mosfet and regulate the constant output current. the RT8458 can achieve high accuracy led output current via the average current feedback loop control. the internal sense voltage ( 178mv typ.) is used to set the average output current. the oscillator?s frequency is fixed at 48khz to get better switching performance. once the average current is set by the external resistor, r s , the output led current can be dimmed by varying the actl voltage. under voltage lockout (uvlo) the RT8458 includes a uvlo feature with 8v hysteresis. the gate terminal turns on when v in rises over 16v (typ.). the gate terminal turns off when v in falls below 8v (typ.) setting average output current the output current that flows through the led string is set by an external resistor, r s , which is connected between the gnd and sense terminal. the relationship between output current, i out , and r s is shown below : out s 0.17 i = r 8 (a) analog dimming control the actl terminal is driven by an external voltage, v actl , to adjust the output current to an average value set by r s . the voltage range for v actl to adjust the output current is from 0.2v to 1.3v. if v actl becomes larger than 1.3v, the output current value will just be determined by the external resistor, r s . ? actl outavg s v0.2 i = ( /r 0.17 ) 8v x 1.1 component selection for component selection, an example is shown below for a typical RT8458 application, where v in = 110vac/60hz, led output voltage = 78v, and output current = 200ma. the user can follow this procedure to design applications with wider ac voltage input and dc output voltage as well. start-up resistor start-up resistor should be chosen not to exceed the maximum start-up current. otherwise, the RT8458 may latch low and will never start. start-up current = 130v/r1 for 110vac regions, 260v/r1 for 220vac regions. the typical start-up current is 250 a. input diode bridge rectifier selection the current rating of the input bridge rectifier is dependent on the v out /v in transformation ratio. the voltage rating of the input bridge rectifier, v br , on the other hand, is only dependent on the input voltage. thus, the v br rating is calculated as below : br ac(max) v = 1.2 x ( 2 x v ) where v ac,max is the maximum input voltage (rms) and the parameter 1.2 is used for safety margin. for this example : br ac(max) v = 1.2 x ( 2 x v ) = (1.2 x 2 x 110) = 187v if the input source was universal, v br will reach 466v. in this case, a 600v, 0.5a bridge rectifier can be chosen. input capacitor selection the input capacitor supplies the peak current to the inductor and flattens the current ripple on the input. the low esr condition is required to avoid increasing power loss. the ceramic capacitor is recommended due to its excellent high frequency characteristic and low esr. for maximum stability over the entire operating temperature range, capacitors with better dielectric are suggested. the minimum capacitor is given by : out(max) out(max) in 2 a c(min) ac v x i c (2 x v ) x x f where f ac is the ac input source frequency and is the efficiency of whole system. for this particular example : in 2 78 x 0.2 c = 12 f (2 x 110 ) x 0.9 x 60 in addition, the voltage rating of the input filter capacitor, v cin , should be large enough to handle the input voltage. cin ac(max) v (1.2 x 2 x v ) = (1.2 x 2 x 110) = 187v
9 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary thus, a 22 f / 250v electrolytic capacitor can be chosen in this case. due to its large esr, the electrolytic capacitor is not suggested for high current ripple applications. inductor selection the inductor value and operating frequency determine the ripple current according to a specific input and output voltage. the ripple current, i l , increases with higher v in and decreases with higher inductance, as shown in equation below : ?? ?? = ? ?? ?? ?? ?? out out l in vv i x 1 f x l v to optimize the ripple current, the RT8458 operates the buck converter in bcm (boundary-condition mode). the largest ripple current will occur at the highest v in . to guarantee that the ripple current stays below the specified value, the inductor value should be chosen according to the following equation : ? ? out s out v x t x (1 d) l = 2 x i 78 x 20.83 s x (1 0.5) = = 2.03mh 2 x 0.2 where d is the duty cycle and t s is the switching period. forward diode selection when the power switch turns off, the path for the current is through the diode connected between the switch output and ground. this forward biased diode must have minimum voltage drop and recovery time. the reverse voltage rating of the diode should be greater than the maximum input voltage and the current rating should be greater than the maximum load current. in reality, the peak current through the diode is more than the maximum output current. this component current rating should be greater than 1.2 times the maximum load current and the diode reverse voltage rating should be greater than 1.2 times the maximum input voltage, assuming a 20% output current ripple. the peak voltage stress of diode is : dac(max) v = 1.2 x ( 2 x v ) = 1.2 x ( 2 x 110) = 187v the current rating of diode is : dout,pk i = 1.2 x i = 1.2 x 1.2 x 0.2 = 0.288a if the input source is universal (v in = 90v to 264v), v d will reach 466v. a 600v, 2a ultra-fast diode can be used in this example. mosfet selection the peak current through this mosfet will be over the maximum output current. this component current rating should be greater than 1.2 times the maximum load current and the reverse voltage rating of the mosfet should be greater than 1.2 times the maximum input voltage, assuming a 20% output current ripple. the peak voltage rating of the mosfet is : 22 qac(max) v = 1.2 x ( x v ) = 1.2 x ( x 110) = 187v the current rating of mosfet is : qout,pk i = 1.2 x i = 1.2 x 1.2 x 0.2 = 0.288a if the input source was universal (v in = 90v to 264v), v q will reach 466v. a 600v, 2a n-mosfet can be chosen for this example. output capacitor selection the selection of c out is determined by the required esr to minimize output voltage ripple. moreover, the amount of bulk capacitance is also a key for c out selection to ensure that the control loop is stable. loop stability can be checked by viewing the load transient response. the output voltage ripple, v out , is determined by : ?? ?? + ?? ?? out l osc out 1 v i esr 8f c where f osc is the switching frequency and i l is the inductor ripple current. the output voltage ripple will be the highest at the maximum input voltage since i l increases with input voltage. multiple capacitors placed in parallel may be needed to meet the esr and rms current handling requirement. dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all common selections and available in surface mount packages. tantalum capacitors have the highest capacitance density, but it is important to only use ones that pass the surge test for use in switching power
10 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary supplies. special polymer capacitors offer very low esr value, but with the trade-off of lower capacitance density. aluminum electrolytic capacitors have significantly higher esr, but still can be used in cost-sensitive applications for ripple current rating and long term reliability considerations. thermal protection a thermal protection feature is included to protect the RT8458 from excessive heat damage. when the junction temperature exceeds a threshold of 150 c, the thermal protection will turn off the gate terminal. 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 the RT8458, 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 tsot-23-6 packages, the thermal resistance, ja , is 255 c/w on a standard jedec 51-3 single-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) / (255 c/w) = 0.392w for tsot-23-6 package the maximum power dissipation depends on the operating ambient temperature for fixed t j(max) and thermal resistance, ja . for the RT8458 package, the derating curve in figure 3 allows the designer to see the effect of rising ambient temperature on the maximum power dissipation. figure 3. derating curve for RT8458 package figure 4. pcb layout guide analog gnd sense vc actl vcc gnd gate c1 c2 5 1 2 4 6 3 r g q1 r s l1 c s r1 c in v in led+ led- c out d f power gnd place the capacitor c1 as close as possible to the vcc. place the output capacitor c out as close as possible to led terminal. layout considerations for best performance of the RT8458, the following layout guidelines should be strictly followed. ` the hold up capacitor, c 1 , must be placed as close as possible to the vcc pin. ` the output capacitor, c out , must be placed as close as possible to the led terminal. ` the power gnd should be connected to a strong ground plane. ` r s should be connected between the gnd pin and sense pin. ` keep the main current traces as short and wide as possible. ` place l1, q1, r s , and d f as close to each other as possible. 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0 25 50 75 100 125 ambient temperature (c) maximum power dissipation (w) 1 single-layer pcb
11 ds8458-p03t00 february 2011 www.richtek.com RT8458 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. preliminary richtek technology corporation headquarter 5f, no. 20, taiyuen street, chupei city hsinchu, taiwan, r.o.c. tel: (8863)5526789 fax: (8863)5526611 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 tsot-23-6 surface mount package dimensions in millimeters dimensions in inches symbol min max min max a 0.700 1.000 0.028 0.039 a1 0.000 0.100 0.000 0.004 b 1.397 1.803 0.055 0.071 b 0.300 0.559 0.012 0.022 c 2.591 3.000 0.102 0.118 d 2.692 3.099 0.106 0.122 e 0.838 1.041 0.033 0.041 h 0.080 0.254 0.003 0.010 l 0.300 0.610 0.012 0.024 a a1 e b b d c h l
12 ds8458-p03t00 february 2011 www.richtek.com RT8458 preliminary datasheet revision history version date page no. item description p00 2010/7/29 first edition p01 2010/9/1 general description features typical application circuit functional pin description function block diagram absolute maximum ratings recommended operating conditions electrical characteristics mod ify p02 2010/11/29 general description typical application circuit electrical characteristics mod ify add typical operating characteristics and application information p03 2011/2/14 general description ordering information features marking information function pin description absolute maximum ratings electrical characteristics application information outline dimension mod ify changer sot-23-6 to tsot-23-6

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