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RT8256 1 ds8256-01 march 2011 www.richtek.com ordering information pin configurations (top view) sop-8 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. 2a, 22v, 400khz step-down converter applications z distributive power systems z battery charger z dsl modems z pre-regulator for linear regulators general description the RT8256 is an as high voltage buck converter that can support the input voltage range from 4.75v to 22v and the output current can be up to 2a. current mode operation provides fast transient response and eases loop stabilization. the chip provides protection functions such as cycle-by- cycle current limiting and thermal shutdown protection. in shutdown mode, the regulator draws 25ua of supply current. the RT8256 is available in a sop-8 surface mount package. features z z z z z wide operating input range : 4.75v to 22v z z z z z adjustable output voltage range : 1.22v to 16v z z z z z output current up to 2a z z z z z 25ua low shutdown current z z z z z power mosfet : 0.18 z z z z z high efficiency up to 95% z z z z z 400khz fixed switching frequency z z z z z stable with low esr output ceramic capacitors z z z z z thermal shutdown protection z z z z z cycle-by-cycle over current protection z z z z z rohs compliant and halogen free typical application circuit en nc boot sw vin gnd comp fb 2 3 4 5 8 7 6 package type s : sop-8 RT8256 lead plating system g : green (halogen free and pb free) vin en gnd boot fb sw 8 6 3 4 2 l1 15h c b 10nf c out 22f r1 17k r2 10k v out 3.3v/2a c in 10f chip enable v in 4.75v to 22v RT8256 d1 b230 comp c c 1.5nf r c 10k c p nc 7 5
RT8256 2 ds8256-01 march 2011 www.richtek.com functional pin description function block diagram pin no. pin name pin function 1 nc no internal connection. 2 boot high side gate drive boost input. boot supplies the drive for the high side n-mosfet switch. connect a 10nf or greater capacitor from sw to boot to power the high side switch. 3 vin power input. v in supplies the power to the ic, as well as the step-down converter switches. bypass vin to gnd with a suitable large capacitor to eliminate noise on the input to the ic. 4 sw power switching output. sw is the switching node that supplies power to the output. connect the output lc filter from sw to the output load. note that a capacitor is required from sw to boot to power the high side switch. 5 gnd ground. 6 fb feedback input. fb senses the output voltage to regulate said voltage. the feedback reference voltage is 1.222v typically. 7 comp compensation node. comp is used to compensate the regulation control loop. connect a series rc network from comp to gnd to compensate the regulation control loop. in some cases, an additional capacitor from comp to gnd is required. 8 en enable input. en is a digital input that turns the regulator on or off. drive en higher than 1.4v to turn on the regulator, lower than 0.4v to turn it off. if the en pin is open, it will be pulled to high by internal circuit. table 1. recommended component selection v out (v) r1 (k ) r2 (k ) r c (k ) c c (nf) l1 ( h) c out ( f) 12 88.7 10 62 0.82 33 22 5 30 10 20 2.2 22 22 3.3 17 10 10 1.5 15 22 2.5 10.45 10 7.5 1.5 10 22 1.8 4.75 10 6 1.5 10 22 1.222 0 10 6 3.9 6.8 22 r q s q va + - + - + - + - ea uv comparator oscillator 400khz/120khz foldback control 0.6v internal regulator + - 1v 1a shutdown comparator current sense amplifier boot vin gnd sw fb en comp 1.222v 3v 10k va slope comp current comparator gm = 780a/v
RT8256 3 ds8256-01 march 2011 www.richtek.com electrical characteristics parameter symbol test conditions min typ max unit feedback reference voltage v fb 4.75v v in 22v 1.184 1.222 1.258 v high side switch-on resistance r ds(on)1 -- 0.18 -- low side switch-on resistance r ds(on)2 -- 10 -- switch leakage v en = 0v, v sw = 0v -- -- 10 a current limit i lim duty = 90%; v boot ? sw = 4.8v -- 2.7 -- a current sense transconductance g cs output current to v comp -- 2.5 -- a/v error amplifier tansconductance g m i c = 10 a -- 780 -- a/v oscillator frequency f sw 340 400 460 khz short circuit oscillation frequency v fb = 0v -- 120 -- khz maximum duty cycle d max v fb = 0.8v -- 90 -- % minimum on-time t on -- 100 -- ns under voltage lockout threshold rising 4 4.2 4.5 v under voltage lockout threshold hysteresis -- 300 -- mv en input low voltage -- -- 0.4 v en input high voltage 1.4 -- -- v enable pull up current -- 1 -- a (v in = 12v, t a = 25 c unless otherwise specified) absolute maximum ratings (note 1) z supply voltage, v in ----------------------------------------------------------------------------------------- 23v z switching voltage, sw ------------------------------------------------------------------------------------- ? 0.3v to (v in + 0.3v) z boot v oltage ------------------------------------------------------------------------------------------------ (v sw ? 0.3v) to (v sw + 6v) z the other pins ----------------------------------------------------------------------------------------------- ? 0.3v to 6v z power dissipation, p d @ t a = 25 c sop-8 ---------------------------------------------------------------------------------------------------------- 0.833w z package thermal resistance (note 2) sop-8, ja ---------------------------------------------------------------------------------------------------- 120 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 (machine mode) ---------------------------------------------------------------------------------------- 200v recommended operating conditions (note 4) z supply voltage, v in ----------------------------------------------------------------------------------------- 22v z enable voltage, v en ----------------------------------------------------------------------------------------- 0v to 5.5v z junction temperature range ------------------------------------------------------------------------------ ? 40 c to 125 c z ambient temperature range ------------------------------------------------------------------------------ ? 40 c to 85 c to be continued
RT8256 4 ds8256-01 march 2011 www.richtek.com 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 the natural convection at t a = 25 c on a high effective four layers 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 shutdown current i shdn v en = 0v -- 25 50 a quiescent current i q v en = 2v, v fb = 1.5v -- 0.7 1 ma thermal shutdown t sd -- 150 -- c
RT8256 5 ds8256-01 march 2011 www.richtek.com typical operating characteristics efficiency vs. output current 0 10 20 30 40 50 60 70 80 90 100 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 output current (a) efficiency (%) frequency vs. input voltage 350 360 370 380 390 400 410 420 430 440 450 4 6 8 10121416182022 input voltage (v) frequency (khz) 1 v out = 3.3v, v in = 4.75v to 22v, i out = 0.3a output voltage vs. output current 3.261 3.264 3.267 3.270 3.273 3.276 3.279 3.282 3.285 3.288 3.291 3.294 3.297 3.300 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 output current (a) output voltage (v) v in = 12v v in = 22v v out = 3.3v v in = 5v v in = 12v v in = 22v v out = 3.3v reference voltage vs. input voltage 1.220 1.221 1.222 1.223 1.224 1.225 1.226 4 6 8 10121416182022 input voltage (v) reference voltage (v) v in = 4.75v to 22v, v out = 3.3v output voltage vs. temperature 3.200 3.225 3.250 3.275 3.300 3.325 3.350 3.375 3.400 -50-250 255075100125 temperature (c) output voltage (v) v in = 12v, v out = 3.3v frequency vs. temperature 350 360 370 380 390 400 410 420 430 440 450 -50-25 0 255075100125 temperature (c) frequency (khz) 1 v in = 12v v out = 3.3v, i out = 0.3a v in = 22v
RT8256 6 ds8256-01 march 2011 www.richtek.com current limit vs. input voltage 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 4 6 8 10121416182022 input voltage (v) current limit (a) v in = 4.75v to 22v, v out = 3.3v quiescent current vs. temperature 500 525 550 575 600 625 650 675 700 725 750 775 800 -50 -25 0 25 50 75 100 125 temperature (c) quiescent current (a ) v in = 22v v out = 3.3v v in = 12v shutdown current vs. temperature 0 10 20 30 40 50 60 -50 -25 0 25 50 75 100 125 temperature (c) shutdown current (a) 1 v in = 12v current limit vs. temperature 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25 4.50 -50 -25 0 25 50 75 100 125 temperature (c) current limit (a) v in = 12v v in = 22v v out = 3.3v time (100 s/div) load transient response v out (200mv/div) i out (1a/div) v in = 12v, v out = 3.3v, i out = 1a to 2a load transient response v out (200mv/div) i out (1a/div) v in = 12v, v out = 3.3v, i out = 0.1a to 2a time (100 s/div)
RT8256 7 ds8256-01 march 2011 www.richtek.com power off from en v en (2v/div) v out (2v/div) i out (2a/div) time (25ms/div) v in = 12v, v out = 3.3v, i out = 2a v en (2v/div) v out (2v/div) i out (2a/div) time (1ms/div) v in = 12v, v out = 3.3v, i out = 2a power on from en switching v sw (10v/div) v out (10mv/div) i sw (1a/div) time (1 s/div) v in = 12v, v out = 3.3v, i out = 2a power on from v in v in = 12v, v out = 3.3v, i out = 2a time (5ms/div) v in (5v/div) v out (2v/div) i in (1a/div)
RT8256 8 ds8256-01 march 2011 www.richtek.com application information the RT8256 is an asynchronous high voltage buck converter that can support the input voltage range from 4.75v to 22v and the output current can be up to 2a. output voltage setting the resistive divider allows the fb pin to sense the output voltage as shown in figure 1. figure 1. output voltage setting the output voltage is set by an external resistive divider according to the following equation : ?? + ?? ?? out fb r1 v = v1 r2 where v fb is the feedback reference voltage (1.222v typ.). external bootstrap diode connect a 10nf low esr ceramic capacitor between the boot pin and sw pin. this capacitor provides the gate driver voltage for the high side mosfet. it is recommended to add an external bootstrap diode between an external 5v and the boot pin for efficiency improvement when input voltage is lower than 5.5v or duty ratio is higher than 65%. the bootstrap diode can be a low cost one such as 1n4148 or bat54. the external 5v can be a 5v fixed input from system or a 5v output of the RT8256. figure 2. external bootstrap diode soft-start the RT8256 contains an internal soft-start clamp that gradually raises the output voltage. the soft-start time is designed by the internal capacitor. the typical soft-start time is 2ms. 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. out out l in vv i = 1 fl v ??? ? ?? ??? ? ??? ? having a lower ripple current reduces not only the esr losses in the output capacitors but also the output voltage ripple. high frequency with small ripple current can achieve highest efficiency operation. however, it requires a large inductor to achieve this goal. for the ripple current selection, the value of i l = 0.4(i max ) will be a reasonable starting point. the large st ripple current occurs at the highest v in . to guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation : out out l(max) in(max) vv l = 1 fi v ??? ? ? ??? ? ??? ? inductor core selection the inductor type must be selected once the value for l is known. generally speaking, high efficiency converters can not afford the core loss found in low cost powdered iron cores. so, the more expensive ferrite or mollypermalloy cores will be a better choice. the selected inductance rather than the core size for a fixed inductor value is the key for actual core loss. as the inductance increases, core losses decrease. unfortunately, increase of the inductance requires more turns of wire and therefore the copper losses will increase. ferrite designs are preferred at high switching frequency due to the characteristics of very low core losses. so, design goals can focus on the reduction of copper loss and the saturation prevention. RT8256 gnd fb r1 r2 v out sw boot 5v RT8256 10nf
RT8256 9 ds8256-01 march 2011 www.richtek.com ferrite core material saturates ? hard ? , which means that inductance collapses abruptly when the peak design current is exceeded. the previous situation results in an abrupt increase in inductor ripple current and consequent output voltage ripple. do not allow the core to saturate! different core materials and shapes will change the size/ current and price/current relationship of an inductor. toroid or shielded pot cores in ferrite or permalloy materials are small and do not radiate energy. however, they are usually more expensive than the similar powdered iron inductors. the rule for inductor choice mainly depends on the price vs. size requirement and any radiated field/ emi requirements. 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 a minimum voltage drop and recovery times. schottky diode is recommended and it should be able to handle those current. the reverse voltage rating of the diode should be greater than the maximum input voltage, and current rating should be greater than the maximum load current. for more detail, please refer to table 4. c in and c out selection the input capacitance, c in, is needed to filter the trapezoidal current at the source of the high side mosfet. to prevent large ripple current, a low esr input capacitor sized for the maximum rms current should be used. the rms current is given by : this formula has a maximum at v in = 2v out , where i rms = i out /2. this simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. choose a capacitor rated at a higher temperature than required. several capacitors may also be paralleled to meet size or height requirements in the design. for the input capacitor, a 10 f low esr ceramic capacitor is recommended. for the recommended capacitor, please refer to table 3 for more detail. the selection of c out is determined by the required esr to minimize 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 as described in a later section. the output ripple, v out , is determined by : the output ripple will be 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 available in surface mount packages. special polymer capacitors offer very low esr value. however, it provides lower capacitance density than other types. although tantalum capacitors have the highest capacitance density, it is important to only use types that pass the surge test for use in switching power supplies. aluminum electrolytic capacitors have significantly higher esr. however, it can be used in cost-sensitive applications for ripple current rating and long term reliability considerations. ceramic capacitors have excellent low esr characteristics but can have a high voltage coefficient and audible piezoelectric effects. the high q of ceramic capacitors with trace inductance can also lead to significant ringing. higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. their high ripple current, high voltage rating and low esr make them ideal for switching regulator applications. however, care must be taken when these capacitors are used at input and output. when a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, v in . at best, this ringing can couple to the output and be mistaken as loop instability. at worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at v in large enough to damage the part. out in rms out(max) in out v v i = i 1 vv ? out l out 1 viesr 8fc ?? ?? + ?? ??
RT8256 10 ds8256-01 march 2011 www.richtek.com checking transient response the regulator loop response can be checked by looking at the load transient response. switching regulators take several cycles to respond to a step in load current. when a load step occurs, v out immediately shifts by an amount equal to i load (esr) and also begins to charge or discharge c out generating a feedback error signal for the regulator to return v out to its steady-state value. during this recovery time, v out can be monitored for overshoot or ringing that would indicate a stability problem. thermal considerations the maximum power dissipation depends on the thermal resistance of ic package, pcb layout, the rate of surroundings airflow and temperature difference between junction to ambient. the maximum power dissipation can be calculated by following formula : p d(max) = ( t j(max) ? t a ) / ja where t j(max) is the maximum operation junction temperature, t a is the ambient temperature and the ja is the junction to ambient thermal resistance. for recommended operating conditions specification of RT8256, the maximum junction temperature is 125 c. the junction to ambient thermal resistance ja for sop-8 package is 120 c/w on the standard jedec 51-7 four- layers thermal test board. the maximum power dissipation at t a = 25 c can be calculated by following formula : p d(max) = (125 c ? 25 c) / (120 c/w) = 0.833w for sop-8 packages the maximum power dissipation depends on operating ambient temperature for fixed t j(max) and thermal resistance ja . for RT8256 packages, the figure 3 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. layout consideration follow the pcb layout guidelines for optimal performance of the RT8256. ` keep 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 (vin and gnd). ` lx node is with high frequency voltage swing and should be kept at small area. keep sensitive components away from the lx node to prevent stray capacitive noise pick- up. ` place the feedback components to the fb pin as close as possible. ` the gnd should be connected to a strong ground plane for heat sinking and noise protection. figure 3. derating curves for RT8256 packages 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0255075100125 ambient temperature (c) power dissipation (w) four layer pcb sop-8
RT8256 11 ds8256-01 march 2011 www.richtek.com figure 4. pcb layout guide table 3. suggested capacitors for c in and c out component supplier series dimensions (mm) tdk slf12555t 12.5x12.5x5.5 taiyo yuden nr8040 8x8x4 tdk slf12565t 12.5x12.5x6.5 table 2. suggested inductors for typical application circuit component supplier series v rrm (v) i out (a) package diodes b330a 30 3 sma panjit sk23 30 2 do-214aa table 4. suggested diode location component supplier part no. capacitance ( f) case size c in murata grm31cr61e106k 10 1206 c in tdk c3225x5r1e106k 10 1206 c in taiyo yuden tmk316bj106ml 10 1206 c out murata grm32er61e226m 22 1210 c out tdk c3225x5r0j226m 22 1210 c out taiyo yuden emk325bj226mm 22 1210 v in v out gnd c in c b 2 3 4 5 8 7 6 en nc boot sw vin gnd comp fb gnd c p c c r c sw d1 v out c out l1 r1 r2 input capacitor must be placed as close to the ic as possible. sw should be connected to inductor by wide and short trace. keep sensitive components away from this trace. the feedback and compensation components must be connected as close to the device as possible.
RT8256 12 ds8256-01 march 2011 www.richtek.com 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 re sponsibility for any said applications i s assumed by richtek. 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 a b j f h m c d i 8-lead sop 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 3.988 0.150 0.157 c 1.346 1.753 0.053 0.069 d 0.330 0.508 0.013 0.020 f 1.194 1.346 0.047 0.053 h 0.170 0.254 0.007 0.010 i 0.050 0.254 0.002 0.010 j 5.791 6.200 0.228 0.244 m 0.400 1.270 0.016 0.050

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