View RT8272_4731561.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

RT8272 1 ds8272-02 march 2011 www.richtek.com ordering information pin configurations (top view) 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. 3a, 24v, 1.2mhz step-down converter applications z distributive power systems z battery charger z dsl modems z pre-regulator for linear regulators general description the RT8272 is a high voltage buck converter that can support the input voltage range from 4.75v to 24v and the output current can be up to 3a. 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 25 a of supply current. the RT8272 is available in a sop-8 (exposed pad) surface mount package. features z z z z z wide operating input range : 4.75v to 24v z z z z z adjustable output voltage range : 0.92v to 15v z z z z z output current up to 3a z z z z z 25 a low shutdown current z z z z z power mosfet : 0.1 z z z z z high efficiency up to 95% z z z z z 1.2mhz 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 sop-8 (exposed pad) ss boot vin gnd sw fb en comp 2 3 4 5 8 7 6 gnd 9 package type sp : sop-8 (exposed pad-option 1) RT8272 lead plating system g : green (halogen free and pb free) vin en gnd boot fb sw 7 5 2 3 1 l 4.7h 10nf 47f r1 25.8k r2 10k v out 3.3v/3a 10f chip enable v in 4.75v to 24v RT8272 d1 b330 ss 8 c ss 0.1f comp c c 2.2nf r c 39k c p nc 6 4, 9 (exposed pad) c boot c out c in
RT8272 2 ds8272-02 march 2011 www.richtek.com functional pin description pin no. pin name pin function 1 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. 2 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. 3 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. 4, 9 (exposed pad) gnd ground. the exposed pad must be soldered to a large pcb and connected to gnd for maximum power dissipation. 5 fb feedback input. fb senses the output voltage to regulate said voltage. the feedback reference voltage is 0.92v typically. 6 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. 7 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. 8 ss soft-start control input. ss controls the soft start period. connect a capacitor from ss to gnd to set the soft-start period. a 0.1 f capacitor sets the soft-start period to 10ms. v out (v) r1 (k ) r2 (k ) r c (k ) c c (nf ) c out ( f) l ( h) 15 153 10 82 0.82 47 10 10 100 10 62 1.2 47 10 8 77 10 62 1.5 47 6.8 5 43 10 41 2.2 47 6.8 3.3 25.8 10 39 2.2 47 4.7 2.5 17 10 21 2.2 47 4.7 1.8 9.1 10 15 2.2 47 2.2 1.2 3 10 15 2.2 47 2.2 table 1. recommended component selection
RT8272 3 ds8272-02 march 2011 www.richtek.com function block diagram absolute maximum ratings (note 1) z supply voltage, v in ----------------------------------------------------------------------------------------- ? 0.3v to 26v z switching voltage, sw ------------------------------------------------------------------------------------- ? 0.3v to (v in + 0.3v) z boot v oltage ------------------------------------------------------------------------------------------------ (v sw ? 0.3v) to (v sw + 6v) z all other pins voltage -------------------------------------------------------------------------------------- ? 0.3v to 6v z power dissipation, p d @ t a = 25 c sop-8 (exposed pad) -------------------------------------------------------------------------------------- 1.333w z package thermal resistance (note 2) sop-8 (exposed pad), ja -------------------------------------------------------------------------------- 75 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 ----------------------------------------------------------------------------------------- 4.75v to 24v 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 va + - + - + - uv comparator oscillator 1.2mhz/440khz foldback control 0.5v internal regulator + - 1v 1a shutdown comparator current sense amplifier boot vin gnd sw fb en comp 3v 10k va v cc v cc slope comp current comparator ss 10a v cc + - ea 0.92v gm = 680a/v + logic
RT8272 4 ds8272-02 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 51-7 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. electrical characteristics parameter symbol test conditions min typ max unit feedback reference voltage v fb 4.75v v in 24v 0.902 0.92 0.938 v high side switch-on resistance r ds(on)1 -- 0.1 0.16 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 = 80%; v boot ? sw = 4.8v -- 4.1 -- a current sense transconductance g cs output current to v comp -- 4 -- a/v error amplifier tansconductance g m i c = 10 a 500 680 900 a/v oscillator frequency f sw -- 1.2 -- mhz short circuit oscillation frequency v fb = 0v -- 440 -- khz maximum duty cycle d max v fb = 0.8v -- 80 -- % minimum on-time t on -- 90 -- ns under voltage lockout threshold rising 3.8 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 0.15 1 2.65 a shutdown current i shdn v en = 0v -- 25 -- a quiescent current i q v en = 2v, v fb = 1v -- 0.8 1 ma soft-start current i ss -- 10 -- a soft-start period c ss = 0.1 f -- 10 -- ms thermal shutdown t sd -- 150 -- c (v in = 12v, t a = 25 c unless otherwise specified)
RT8272 5 ds8272-02 march 2011 www.richtek.com efficiency vs. output current 0 10 20 30 40 50 60 70 80 90 100 00.511.5 22.53 output current (a) efficiency (%) frequency vs. temperature 1.00 1.05 1.10 1.15 1.20 1.25 1.30 -50-25 0 25 50 75100125 temperature (c) frequency (mhz) 1 frequency vs. input voltage 1.17 1.18 1.19 1.20 1.21 1.22 1.23 4 6 8 1012141618202224 input voltage (v) frequency (mhz) 1 typical operating characteristics v in = 24v v in = 12v v out = 3.3v reference voltage vs. input voltage 0.915 0.916 0.917 0.918 0.919 0.920 0.921 0.922 0.923 0.924 0.925 4 6 8 1012141618202224 input voltage (v) reference voltage (v) reference voltage vs. temperature 0.9 0.905 0.91 0.915 0.92 0.925 0.93 0.935 0.94 -50-250 255075100125 temperature (c) reference voltage (v) output voltage vs. output current 3.300 3.305 3.310 3.315 3.320 3.325 3.330 3.335 3.340 3.345 3.350 00.511.5 22.53 output current (a) output voltage (v) v in = 12v v in = 12v v out = 3.3v, i out = 0.3a v in = 12v, v out = 3.3v, i out = 0.3a
RT8272 6 ds8272-02 march 2011 www.richtek.com quiescent current vs. temperature 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 -50 -25 0 25 50 75 100 125 temperature (c) quiescent current (ma ) v in = 12v, v out = 3.3v current limit vs. duty cycle 3.0 3.4 3.8 4.2 4.6 5.0 5.4 5.8 6.2 10 20 30 40 50 60 70 80 duty cycle (%) current limit (a) uvlo vs. temperature 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 -50-250 255075100125 temperature ( c) uvlo (v) rising falling load transient response time (100 s/div) i out (2a/div) v out (50mv/div) v in = 12v, v out = 3.3v i out = 1.5a to 3a load transient response time (100 s/div) i out (2a/div) v out (100mv/div) v in = 12v, v out = 3.3v i out = 0a to 3a enable threshold vs. temperature 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 -50-250 255075100125 temperature ( c) enable threshold (v ) rising falling
RT8272 7 ds8272-02 march 2011 www.richtek.com output ripple time (500ns/div) i l (2a/div) v out (5mv/div) v in = 12v, v out = 3.3v i out = 3a v lx (10v/div) power on from en pin time (2.5ms/div) i in (500ma/div) v in = 12v, v out = 3.3v i out = 3a v out (1v/div) v en (5v/div) power off from en pin time (100 s/div) i in (500ma/div) v in = 12v, v out = 3.3v i out = 3a v out (1v/div) v en (5v/div)
RT8272 8 ds8272-02 march 2011 www.richtek.com application information the RT8272 is an asynchronous high voltage buck converter that can support the input voltage range from 4.75v to 24v and the output current can be up to 3a. 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 (0.92v typ.). figure 2. external bootstrap diode soft-start the RT8272 contains an external soft-start clamp that gradually raises the output voltage. the soft-start timming can be programed by the external capacitor between ss pin and gnd. the chip provides a 10 a charge current for the external capacitor. if 0.1 f capacitor is used to set the soft-start and it ? s period will be 10ms(typ.). 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. RT8272 gnd fb r1 r2 v out sw boot 5v RT8272 10nf 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 RT8272.
RT8272 9 ds8272-02 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 3. 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 2 for more detail. the input capacitor has to connect another 10 f ceramic capacitor between the input and ground when the input voltage is lower than 6.5v. 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 ?? ?? + ?? ??
RT8272 10 ds8272-02 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) 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 for continuous operation, do not exceed the maximum operation junction temperature 125 c. 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 RT8272, where the maximum junction temperature is 125 c. the junction to ambient thermal resistance ja is layout dependent. for sop-8 (exposed pad) packages, the thermal resistance ja is 75 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) / (75 c/w) = 1.333w for sop-8 (exposed pad) packages the maximum power dissipation depends on operating ambient temperature for fixed t j(max) and thermal resistance ja . for RT8272 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 RT8272. ` 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 analog components away from the lx node to prevent stray capacitive noise pick- up. ` connect feedback network behind the output capacitors. keep the loop area small. place the feedback components near the RT8272. ` connect all analog grounds to a command node and then connect the command node to the power ground behind the output capacitors. ` an example of pcb layout guide is shown in figure 4 for reference. figure 3. derating curves for RT8272 packages 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 25 50 75 100 125 ambient temperature maximum power dissipation (w) four layer pcb ( c)
RT8272 11 ds8272-02 march 2011 www.richtek.com figure 4. pcb layout guide table 3. suggested capacitors for c in and c out component supplier series inductance ( m h) dcr (m w ) current rating (a) dimensions (mm) tdk rlf7030 4.7 31 3.5 7.3 x 6.8 x 3.2 taiyo yuden nr8040 4.7 18 4.7 8 x 8 x 4 goternd gssr2 4.7 18 5.7 10 x 10 x 3.8 table 2. suggested inductors for typical application circuit component supplier series v rrm (v) i out (a) package diodes b330a 30 3 do-214ac diodes b340 40 3 do-214ab panjit sk33 30 3 do-214ab panjit sk34 40 3 do-214ab table 4. suggested diode component supplier part no. capacitance ( m f) case size murata grm31cr61e106k 10 1206 tdk c3225x5r1e106k 10 1206 taiyo yuden tmk316bj106ml 10 1206 murata grm31cr60j476m 47 1206 tdk c3225x5r0j476m 47 1210 taiyo yuden emk325bj476mm 47 1210 v in v out gnd c in cb 2 3 4 5 8 7 6 gnd ss boot vin gnd sw fb en comp gnd c ss c p c c r c sw d1 v out c out l input capacitor must be placed as close to the ic as possible. the output capacitor must be placed near the RT8272. sw should be connected to inductor by wide and short trace. keep sensitive components away from this trace. the resistor divider must be connected as close to the device as possible. the parallel distance between comp and fb traces must be as short as possible.
RT8272 12 ds8272-02 march 2011 www.richtek.com information that is provided by richtek technology corporation is believed to be accurate and reliable. richtek reserves the right to make any change in circuit design, specification or other related things if necessary without notice at any time. no third party intellectual property infringement of the applications should be guaranteed by users when integrating richtek products into any application. no legal responsibility for any said applications is 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 y x exposed thermal pad (bottom of package) 8-lead sop (exposed pad) 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 4.000 0.150 0.157 c 1.346 1.753 0.053 0.069 d 0.330 0.510 0.013 0.020 f 1.194 1.346 0.047 0.053 h 0.170 0.254 0.007 0.010 i 0.000 0.152 0.000 0.006 j 5.791 6.200 0.228 0.244 m 0.406 1.270 0.016 0.050 x 2.000 2.300 0.079 0.091 option 1 y 2.000 2.300 0.079 0.091 x 2.100 2.500 0.083 0.098 option 2 y 3.000 3.500 0.118 0.138

▲Up To Search▲

Price & Availability of RT8272

	To Download RT8272 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .