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RT7271A ? ds7271a-00 february 2013 www.richtek.com 1 ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. 6a, 17v, 500khz csp synchronous step-down converter general description the RT7271A is a high efficiency, synchronous step-down dc/dc converter for applications operating from 4.5v to 17v and requiring up to 6a maximum load. the current mode architecture of RT7271A allows the transient response to be optimized. cycle-by-cycle current limit provides protection against shorted output and soft-start eliminates input current surge during start-up. fault conditions also include output under voltage protection, output over voltage protection, and thermal shutdown. the low current shutdown mode provides output disconnect, enabling easy power management in battery powered systems. features z z z z z 4.5v to 17v input voltage range z z z z z 6a output current z z z z z current mode control z z z z z 0.6v 1% voltage reference over temperature z z z z z latch off when short circuit z z z z z monotonic start-up in pre-biased output z z z z z 500khz switching frequency z z z z z low on-resistance 45m of high side mosfet 25m of low side mosfet z z z z z cycle-by-cycle current limit z z z z z power good monitor for uvp & ovp z z z z z input under voltage lockout z z z z z thermal shutdown z z z z z rohs compliant and halogen free applications z industrial and commercial low power systems z computer peripherals z lcd monitors and tvs z point of load regulation for high performance dsps, fpgas and asics z green electronics/appliances simplified application circuit pin configurations wl-csp-12b 1.65x1.95 (bsc) (top view) fb en agnd pgood comp boot vin gnd gnd lx lx lx a3 a1 a2 b3 b1 b2 c3 c1 c2 d3 d1 d2 vin pgood gnd boot fb lx l r1 r2 v out v in RT7271A en comp c c r c c p c boot c in c out pgood agnd v in 3.3v r pgood r3
RT7271A 2 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 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 functional pin description pin no. pin name pin function a1, a2, a3 lx switch node. connect this pin to the external inductor. b1 vin power input. connect two 10 f or larger ceramic capacitors to this pin. b2, b3 gnd power ground. c1 comp compensation node. connect external compensation elements to this pin to stabilize the control loop. c2 pgood power good indicator output. asserts low if output voltage is low due to otp, uvp, uvlo, ovp, en shutdown or during soft-start. c3 boot bootstrap supply for the high side mosfet. connect a capacitor between this pin and lx pin. d1 fb feedback voltage input. this pin receives the feedback voltage from a resistive divider connected across the output. d2 en enable control input. connecting this pin to ground forces the device into shutdown mode. pulling this pin over 1.4v enables the device. d3 agnd analog ground. package type wsc : wl-csp-12b 1.65x1.95 (bsc) RT7271A 17w 17 : product code w : date code
RT7271A 3 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. function block diagram operation the RT7271A is a constant frequency, current mode synchronous step-down converter. in normal operation, the high side n-mosfet is turned on when the driver control is set by the oscillator and is turned off when the current comparator resets the driver control. while the high side n-mosfet is turned off, the low side n-mosfet is turned on to conduct the inductor current until next cycle begins. error amplifier the error amplifier adjusts its output voltage by comparing the feedback signal (v fb ) with the internal 0.6v reference. when the load current increases, it causes a drop in the feedback voltage relative to the reference, the error amplifier's output voltage then rises to allow higher inductor current to match the load current. oscillator the internal oscillator runs at fixed frequency 500khz. internal regulator the regulator provides low voltage power to supply the internal control circuits and the bootstrap power for high side gate driver. enable the converter is turned on when the en pin is higher than 1.4v. when the en pin is lower than 1.15v, the converter will enter shutdown mode. soft-start (ss) an internal current source charges an internal capacitor to build a soft-start ramp voltage. the fb voltage will track the internal ramp voltage during soft-start interval. the typical soft-start time is 1.5ms. uv comparator if the feedback voltage (v fb ) is lower than 0.3v, the uv comparator will go high to turn off the high side mosfet. the output under voltage protection is designed to operate in latch mode. when the uv condition is removed, the controller can be reset by en pin or vin pin. thermal shutdown the over temperature protection function will shut down the switching operation when the junction temperature exceeds 160 c. once the junction temperature cools down by the hysteresis, the converter will automatically resume switching. + - ov comparator oscillator protection control shutdown comparator boot en gnd lx fb comp + - ea 0.6v agnd + - 0.72v + driver control hs switch current comparator ls switch current comparator boot uvlo internal regulator uvlo current sense current sense slope compensation + - uv comparator 0.3v 1.4v soft-start pgood vin
RT7271A 4 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. electrical characteristics recommended operating conditions (note 4) z supply input voltage, vin ---------------------------------------------------------------------------------- 4.5v to 17v z junction temperature range ------------------------------------------------------------------------------- ? 40 c to 125 c z ambient temperature range ------------------------------------------------------------------------------- ? 40 c to 85 c (v in = 12v, t a = 25 c, unless otherwise specified) absolute maximum ratings (note 1) z supply input v oltage, vin ---------------------------------------------------------------------------------- ? 0.3v to 20v z lx pin switch voltage, v lx --------------------------------------------------------------------------------- ? 0.3v to (vin + 0.3v) z boot voltage, v boot ----------------------------------------------------------------------------------------- (v lx ? 0.3v) to (v lx + 6v) z en voltage, v en ---------------------------------------------------------------------------------------------- ? 0.3v to 3.6v z other pins ------------------------------------------------------------------------------------------------------ ? 0.3v to 6v z power dissipation, p d @ t a = 25 c wl-csp-12b 1.65x1.95 (bsc) ------------------------------------------------------------ --------- ------ 1.88w z package thermal resistance (note 2) wl-csp-12b 1.65x1.95 (bsc), ja --------------------------------------------------------------------- 53 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 model) --------------------------------------------------------------------------------- 2kv parameter symbol test conditions min typ max unit shutdown current i shdn v en = 0v -- 4 15 a vin uvlo threshold v in_th v in rising -- 4 4.5 v vin uvlo hysteresis v in_hys -- 250 -- mv enable threshold v enr rising -- 1.4 1.5 v enable threshold v enf falling 1.15 1.25 -- v quiescent current i q v fb = 0.61v -- 0.9 1.2 ma feedback reference voltage v ref 0.594 0.6 0.606 v high-side r ds(on)_h -- 45 -- m switch on-resistance low-side r ds(on)_l -- 25 -- m error amplifier transconductance gm ? 2 < i comp < 2 ,v comp = 1v -- 1600 -- a/v error amp source/sink v comp = 1v, 100mv input overdrive -- 110 -- a comp to current sense transconductance -- 16 -- a/v high side switch peak current limit i lim_h 8 12 -- a minimum on-time t on(min) -- 100 -- ns
RT7271A 5 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. parameter symbol test conditions min typ max unit thermal shutdown t sd -- 160 -- c thermal shutdown hysteresis t sd_hys -- 10 -- c switching frequency f sw 425 500 575 khz ovp threshold -- 120 -- % under voltage threshold v uvp -- 50 -- % soft-start time -- 1.5 -- ms power good threshold rising -- 90 -- % power good threshold falling -- 85 -- % power good output high leakage current v fb = v ref , v pgood = 5.5v -- 30 -- na power good output low i pgood = 2ma -- -- 0.3 v 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. note 3. devices are esd sensitive. handling precaution is recommended. note 4. the device is not guaranteed to function outside its operating conditions.
RT7271A 6 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. typical application circuit gnd boot fb lx l r1 r2 v out RT7271A comp c c r c c p c boot c out agnd b1 c2 d2 b2, b3 d3 c3 a1, a2, a3 d1 c1 vin pgood v in en c in pgood v in 3.3v r pgood r3 table 1. recommended component selection v out (v) r1 (k ) r2 (k ) r c (k ) c c (nf) c p (pf) l ( h) c out ( f) 5 73.2 10 8.2 6.8 150 3.3 66 3.3 45.3 10 6.8 3.9 150 3.3 66 2.5 31.6 10 7.5 4.7 150 1.5 66 1.8 20 10 6.2 3.9 150 1.5 66 1.5 15 10 5.6 3.9 150 1.5 66 1.05 7.5 10 3 3.3 150 1.0 66
RT7271A 7 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. typical operating characteristics efficiency vs. load current 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0123456 load current (a) efficiency (%) 100 0 90 0 80 0 70 0 60 0 50 0 40 0 30 0 20 0 10 0 0 0 v out = 3.3v v in = 7.4v v in = 12v v in = 17v efficiency vs. load current 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0123456 load current (a) efficiency (%) 100 0 90 0 80 0 70 0 60 0 50 0 40 0 30 0 20 0 10 0 0 0 v out = 1.8v v in = 7.4v v in = 12v v in = 17v output voltage vs. load current 1.78 1.79 1.79 1.80 1.80 1.81 1.81 1.82 1.82 0123456 load current (a) output voltage (v) v out = 1.8v v in = 17v v in = 12v v in = 7.4v efficiency vs. load current 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0123456 load current (a) efficiency (%) 100 0 90 0 80 0 70 0 60 0 50 0 40 0 30 0 20 0 10 0 0 0 v out = 1.05v v in = 7.4v v in = 12v v in = 17v current limit vs. input voltage 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 4 6 8 101214161820 input voltage (v) current limit (a) v out = 1.05v current limit vs. temperature 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 -50 -25 0 25 50 75 100 125 temperature (c) current limit (a) v out = 1.05v v in = 17v v in = 12v v in = 7.4v
RT7271A 8 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. time (2 s/div) output voltage ripple v in = 12v, v out = 1.8v, i out = 3a v lx (10v/div) i l (2a/div) v out (10mv/div) time (2 s/div) output voltage ripple v in = 12v, v out = 1.8v, i out = 6a v in = 12v, v out = 1.8v, i out = 1a to 4a time (200 s/div) load transient response i l (2a/div) v out (50mv/div) time (200 s/div) load transient response time (1ms/div) over voltage protection v out (1v/div) v lx (5v/div) v in = 12v, v out = 1.8v, i out = 1a time (5 s/div) under voltage protection v in = 12v, v out = 1.8v v in = 12v, v out = 1.8v, i out = 3a to 6a i l (2a/div) v out (50mv/div) v lx (10v/div) i l (2a/div) v out (10mv/div) v out (1v/div) v lx (5v/div)
RT7271A 9 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. time (2ms/div) power off from vin v in = 12v, v out = 1.8v, i out = 6a time (1ms/div) power on from en v in = 12v, v out = 1.8v, i out = 6a v in = 12v, v out = 1.8v time (50 s/div) over current protection i l (5a/div) v out (2v/div) v in = 12v, v out = 1.8v, i out = 6a time (2ms/div) power on from vin time (20 s/div) power off from en v in = 12v, v out = 1.8v, i out = 6a v lx (10v/div) v out (1v/div) v in (10v/div) i l (3a/div) v out (1v/div) v en (3v/div) i l (3a/div) v out (1v/div) v in (10v/div) i l (3a/div) v out (1v/div) v en (3v/div) i l (3a/div)
RT7271A 10 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. application information the RT7271A is a single-phase buck converter. it provides single feedback loop, current mode control with fast transient response. an internal 0.6v reference allows the output voltage to be precisely regulated for low output voltage applications. a fixed switching frequency (500khz) oscillator and internal compensation are integrated to minimize external component count. protection features include over current protection, under voltage protection, over voltage protection and over temperature protection. output voltage setting connect a resistive voltage divider at the fb between v out and gnd to adjust the output voltage. the output voltage is set according to the following equation : chip enable and disable the en pin allows for power sequencing between the controller bias voltage and another voltage rail. the RT7271A remains in shutdown if the en pin is lower than 1.25v. when the en pin rises above the v en threshold, the RT7271A begins a new initialization and soft-start cycle. internal soft-start the RT7271A provides an internal soft-start function to prevent large inrush current and output voltage overshoot when the converter starts up. the soft-start (ss) automatically begins once the chip is enabled. during soft- start, the internal soft-start capacitor becomes charged and generates a linear ramping up voltage across the capacitor. this voltage clamps the voltage at the fb pin, causing pwm pulse width to increase slowly and in turn fb1 out ref fb2 r v = v 1 + r ?? ?? ?? where v ref is 0.6v (typ.). figure 1. setting v out with a voltage divider reduce the output surge current. the internal 0.6v reference takes over the loop control once the internal ramping up voltage becomes higher than 0.6v. uvlo protection the RT7271A provides input under voltage lockout protection (uvlo). if the input voltage exceeds the uvlo rising threshold voltage (4v typ.), the converter resets and prepares the pwm for operation. if the input voltage falls below the uvlo falling threshold voltage during normal operation, the device will stop switching. the uvlo rising and falling threshold voltage has a hysteresis to prevent noise-caused reset. inductor selection the switching frequency (on-time) and operating point (% ripple or lir) determine the inductor value as shown below: ( ) out in out sw load(max) in vv v l = f lir i v ? where lir is the ratio of the peak-to-peak ripple current to the average inductor current. find a low loss inductor having the lowest possible dc resistance that fits in the allotted dimensions. ferrite cores are often the best choice, although powdered iron is inexpensive and can work well at 200khz. the core must be large enough and will not saturate at the peak inductor current (i peak ) : peak load(max) load(max) lir i = i + i 2 ?? ?? ?? the calculation above serves as a general reference. to further improve transient response, the output inductor can be further reduced. this relation should be considered along with the selection of the output capacitor. input capacitor selection high quality ceramic input decoupling capacitor, such as x5r or x7r, with values greater than 20 f are recommended for the input capacitor. the x5r and x7r ceramic capacitors are usually selected for power regulator capacitors because the dielectric material has less capacitance variation and more temperature stability. fb gnd v out r fb1 r fb2
RT7271A 11 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. out out in_rms load in in vv i = i 1 vv ?? ? ?? ?? the next step is selecting a proper capacitor for rms current rating. a good design uses more than one capacitor with low equivalent series resistance (esr) in parallel to form a capacitor bank. the input capacitance value determines the input ripple voltage of the regulator. the input voltage ripple can be approximately calculated using the following equation : out(max) out in in sw in iv v = cf v for example, if i out_max = 6a, c in = 22 f, f sw = 500khz, v in = 12v and v out = 1.05v, the input voltage ripple will be 47.7mv. output capacitor selection the output capacitor and the inductor form a low pass filter in the buck topology. in steady state condition, the ripple current flowing into/out of the capacitor results in ripple voltage. the output voltage ripple (v p-p ) can be calculated by the following equation : p_p load(max) out sw 1 v= liri esr + 8c f ?? ?? ?? for a given output voltage sag specification, the esr value can be determined. another parameter that has influence on the output voltage sag is the equivalent series inductance (esl). the rapid change in load current results in di/dt during transient. when load transient occurs, the output capacitor supplies the load current before the controller can respond. therefore, the esr will dominate the output voltage sag during load transient. the output voltage undershoot (v sag ) can be calculated by the following equation : sag load v = i esr ? therefore, the esl contributes to part of the voltage sag. using a capacitor with low esl can obtain better transient performance. generally, using several capacitors connected in parallel can have better transient performance than using a single capacitor for the same total esr. unlike the electrolytic capacitor, the ceramic capacitor has relatively low esr and can reduce the voltage deviation during load transient. however, the ceramic capacitor can only provide low capacitance value. therefore, use a mixed combination of electrolytic capacitor and ceramic capacitor to obtain better transient performance. power good output (pgood) pgood is an open-drain output and requires a pull-up resistor. pgood is actively held low in soft-start, standby, and shutdown. it is released when the output voltage rises above 90% of nominal regulation point. the pgood signal goes low if the output is turned off or vout under 85% of setting. under voltage protection (uvp) the output voltage can be continuously monitored for under voltage protection. both high side and low side gate drivers will be forced to low if the output is less than 50% of its set voltage threshold. the uvp will be ignored for at least 1.5ms (typ.) after start up or a rising edge on the en threshold. remove the uvp fault latch by reseting the en pin and v in to restart the controller. over voltage protection (ovp) the RT7271A is latched once ovp is triggered and can only be released by toggling en threshold or cycling v in . there is a 20 s delay built into the over voltage protection circuit to prevent false transition. over current protection (ocp) the RT7271A provides over current protection by detecting high side mosfet peak inductor current. if the sensed peak inductor current is over the current limit threshold (12a typ.), the ocp will be triggered. when ocp is tripped, the RT7271A will keep the over current threshold level until the over current condition is removed. voltage rating and current rating are the key parameters when selecting an input capacitor. generally, selecting an input capacitor with voltage rating 1.5 times greater than the maximum input voltage is a conservatively safe design. the input capacitor is used to supply the input rms current, which can be approximately calculated using the following equation :
RT7271A 12 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. thermal shutdown (otp) the device implements an internal thermal shutdown function when the junction temperature exceeds 160 c. the thermal shutdown forces the device to stop switching when the junction temperature exceeds the thermal shutdown threshold. once the die temperature decreases below the hysteresis of 10 c, the device reinstates the power up sequence. 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 RT7271A, 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 wl-csp-12b 1.65x1.95 (bsc), the thermal resistance, ja , is 53 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 : p d(max) = (125 c ? 25 c) / (53 c/w) = 1.88w for wl-csp-12b 1.65x1.95 (bsc) 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 2 allow the designer to see the effect of rising ambient temperature on the maximum power dissipation. figure 2. derating curve of maximum power dissipation 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 25 50 75 100 125 ambient temperature (c) maximum power dissipation (w) 1 four-layer pcb layout considerations layout is very important in high frequency switching converter design. the pcb can radiate excessive noise and contribute to converter instability with improper layout. certain points must be considered before starting a layout using the RT7271A. make the traces of the main current paths as short and wide as possible. put the input capacitor as close as possible to the device pins (v in and gnd). lx node encounters high frequency voltage swings so it should be kept in a small area. keep sensitive components away from the lx node to prevent stray capacitive noise pick-up. ensure all feedback network connections are short and direct. place the feedback network as close to the chip as possible. the gnd pin should be connected to a strong ground plane for heat sinking and noise protection. an example of pcb layout guide is shown in figure 3 for reference.
RT7271A 13 ds7271a-00 february 2013 www.richtek.com ? copyright 2013 richtek technology corporation. all rights reserved. is a registered trademark of ric htek technology corporation. figure 3. pcb layout guide c out r1 c in r en r pg c f r2 r c c c l gnd vout 3v3 vin gnd gnd vout vin c b input capacitors must be placed as close to the ic as passable. the voltage divider and compensation components must be placed as close to the ic as passable. lx should be connected to inductor by wide and short trace. keep sensitive components away form this trace. the output capacitor must be placed near the ic. fb en agnd pgood comp boot vin gnd gnd lx lx lx a3 a1 a2 b3 b1 b2 c3 c1 c2 d3 d1 d2 c p
RT7271A 14 ds7271a-00 february 2013 www.richtek.com richtek technology corporation 5f, no. 20, taiyuen 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 12b wl-csp 1.65x1.95 package (bsc) min. max. min. max. a 0.700 0.800 0.028 0.031 a1 0.200 0.260 0.008 0.010 b 0.290 0.350 0.011 0.014 d 1.900 2.000 0.075 0.079 d1 e 1.600 1.700 0.063 0.067 e1 e e1 0.500 0.020 1.200 0.047 0.700 0.028 symbol dimensions in millimeters dimensions in inches 1.500 0.059

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