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1 AAT2138 high current step-down converter with hi g h-precision pro g rammable current limited load switch data shee t skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 general description the AAT2138 is a high current synchronous step-down converter with an integrated current-limited load switch designed for high precision current control applications. by guarding against excessive input current, AAT2138 enables the system designer to maximize the output cur- rent from the step-down converter while protecting the input supply. it is designed for protection of usb ports from heavy load transient conditions commonly seen with high data rate modem applications. AAT2138 integrates a high precision programmable cur- rent limited p-channel mosfet load switch to protect the input supply against large currents which may cause the supply to fall out of regulation. the current limit thresh- old is programmed up to 3a by an external resistor between rset and ground. the AAT2138 is a 2.8mhz current mode step-down con- verter. it utilizes a tiny 1 h inductor and internal com- pensation to enable it to have an extremely small foot print. the regulator supports 100% duty cycle operation for dropout conditions. an external clock in the range of 1mhz to 3mhz can be fed into mode as a synchronous signal for the step-down regulator. this feature offers the flexibility of fine tuning the switching frequency and optimizing performance in the application. AAT2138 is available in a pb-free, low profile, 14-pin 3mm x 3mm tdfn package. the product is rated over the -40c to 85c temperature range. features ? ? input voltage range: 2.7v to 5.5 v ? output voltage range: down to 3.0v ? current-limited load switch - 10% high precision - programmable current limit range: 500ma to 3a ? high-current step-down converter - efficiency up to 95% - 85m high-side; 50m low-side fets - 2.8mhz switching frequency - 100% low dropout operation - no external compensation required - pfm/pwm or forced pwm modes - 1mhz to 3mhz external clock support ? soft start ? independent enable pins for switch and converter ? over-temperature and over load protection ? -40c to +85c temperature range ? pb-free, low-profile, 3mmx3mm 14-pin tdfn package applications ? wireless modem data cards ? portable hard drives ? portable memory card readers ? barcode scanners ? credit card readers ? general usb powered devices typical application mode enreg regin sysin sw fb pgnd agnd v in 2.7v-5.5v AAT2138 1f 22f c in c out 274k 59k r2 r1 rset ensys on off pwm pfm/pwm 3.4v/3 a 1 h off on c in 10f bulk capacitor r set v out 0.1 f c in
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 2 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t pin descriptions pin # symbol function 1, 2 pgnd power ground. pgnd is internally connected to the source of the low-side n-channel mosfet. 3, 4 regin input power supply for the step-down regulator. connect regin to the input power source. bypass regin to pgnd with a 4.7 f or greater ceramic capacitor. regin internally connects to the source of the load switch output and the step-down regulator?s high-side p-channel mosfet (see the functional block diagram). 5 enreg step-down regulator enable input. a logic high enables the AAT2138 switching regulator. a logic low forces the AAT2138 into shutdown mode, placing the output into a high-impedance state and reducing the quiescent current to less than 1 a. do not leave enreg oating. 6 rset load switch current-limit adjustment. connect a resistor between rset and ground to set the input current limit threshold. 7 agnd analog ground. agnd is internally connected to the analog ground of the control circuitry. 8 ensys load switch enable. active low input. a logic low enables the AAT2138 load switch. a logic high disables the AAT2138 load switch. do not leave ensys oating. 9fb feedback input / output voltage sense. fb senses the output voltage for regulation control. for adjustable output versions, connect a resistive divider network from the output to fb to gnd to set the output voltage accordingly. the fb regulation threshold is 0.6v. 10 mode regulator operating mode. pull mode high to force the regulator into forced pwm operation. pull mode low to allow automatic pulse-skipping under light-load operation. an external clock in the range of 1mhz to 3mhz can be connected into mode as a synchronous clock signal for the step-down converter to bypass the internal oscillator. 11, 12 sysin system load switch input. sysin connects to the source of the p-channel load switch that lim- its the system input supply current. bias input supply for the system and step-down regulator. sysin supplies power to the ana- log and logic control circuitry of the AAT2138. 13, 14 sw inductor switching node. sw is internally connected to the source of the high-side p-channel mosfet and the drain of the low-side n-channel mosfet. externally connected to the power inductor as shown in the typical application circuit. ep gnd (ep) exposed pad. connect directly to the ground plane to reduce the thermal impedance. pin configuration tdfn33-14 (top view) 1 2 3 4 8 7 6 5 14 13 9 10 11 12 regin enreg agnd sw fb sysin rset regin sysin pgnd sw pgnd mode ensys
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 3 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t absolute maximum ratings 1 symbol description value units v in sysin, regin to pgnd -0.3 to +6 v v sw sw to pgnd -0.3 to (regin + 0.3) v fb fb to agnd -0.3 to (sysin + 0.3) v rset rset to agnd -0.3 to (v in + 0.3) v en ensys, enreg to agnd -0.3 to (v in + 0.3) i sysin load switch maximum rms current capability 4.0 a v gnd pgnd to agnd -0.3 to +0.3 v t j operating junction temperature range -40 to 150 c t a operating ambient temperature range -40 to 85 t lead maximum soldering temperature (at leads, 10 sec) 300 thermal characteristics symbol description value units ja maximum thermal resistance 60 c/w p d maximum power dissipation 2, 3 1.67 w 1. stresses above those listed in absolute maximum ratings may cause permanent damage to the device. functional operation at co nditions other than the operating conditions specified is not implied. 2. mounted on an fr4 board. 3. derate 70mw/c above 25c.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 4 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t electrical characteristics 1 v sysin = 5.0v, enreg = sysin, ensys = agnd = pgnd, t a = -40c to 85c unless otherwise noted. typical values are at t a = 25c. symbol description conditions min typ max units v sysin system input voltage range sysin, regin 2.7 5.5 v v uvlo(sysin) system input under-voltage lockout sysin rising 2.7 hysteresis 0.2 v uvlo(regin) regulator input under-voltage lockout regin rising 2 2.7 hysteresis 0.2 v dis(sysin) regulator input discharge threshold sysin rising 3.15 hysteresis 0.15 r dischrg discharge mosfet on-resistance v sysin = v regin = 3.6v 8 t shdn over-temperature shutdown threshold hysteresis = 15c 150 c i q no load supply current no load current; not switching 90 a i shdn shutdown current enreg = sw = gnd, 1 system input load switch r ds(on) load switch on-resistance v sysin = 5.0v, t a = 25c 85 m isysilim current limit accuracy r set = 6.4k , v regin < v uvlo(regin), t a = 20 c to 85 c -10 5 10 % short-circuit current-limit threshold r set = 6.4k , v regin < v uvlo(regin), t a = 20 c to 85 c 400 ma adjustable current-limit range t a = 25 c 0.5 3 a t ilim current-limit response time 5 s t ss(sys) load switch soft-start period 1.5 ms v ih_ensys ensys input logic threshold high 1.5 v v il_ensys ensys input logic threshold low 0.4 i ensys ensys input current v en = 0v or < v in + 0.3v -1.0 1.0 a step-down regulator v out output voltage range 3.0 v in v i fb fb leakage current v fb = 0.65v 50 na v fb fb regulation threshold 3 10ma load, t a = +25 c 588 600 612 mv no load, t a = -40 c to +85 c 582 600 618 vout /i out load regulation i out = 0 to 2.5a, t a = 25c 0.20 %/a vout /v in line regulation v in = 3.6v to 5.5v, v out = 3.3v, t a = 25c, i out = 10ma 0.10 0.30 %/v i limpk high-side p-channel mosfet current limit 3.0 4.0 a r ds(on)hi high-side p-channel mosfet on- resistance v regin = 3.6v 85 m r ds(on)lo low-side n-channel mosfet on- resistance v regin = 3.6v 50 m f osc internal oscillator frequency 2.4 2.8 mhz t ss(reg) step-down regulator soft-start period 4 c out = 100 f1ms t off minimum off-time 50 ns v ih_enreg enreg input logic threshold high 1.5 v v il_enreg enreg input logic threshold low 0.4 v i enreg enreg input current v en = 0v or 5.5v < v in + 0.3v -1.0 1.0 a 1. the AAT2138 is guaranteed to meet performance specifications over the -40c to +85c operating temperature range and is assu red by design, characterization, and correla- tion with statistical process controls. 2. also used for load switch current-limit fold-back threshold 3. the regulated feedback voltage is tested in an internal test mode that connects v fb to the output of the error amplifier. 4. this time period will keep the regulator start-up current around 300ma to 400ma.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 5 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t typical characteristics v regin = 3.8v v regin = 4.2v v regin = 4.5v v regin = 4.8v v regin = 5.0v v regin = 5.5v 0 10 20 30 40 50 60 70 80 90 100 1 10 100 1000 10000 step-down converter efficiency (v out = 3.4v) output current (ma) efficiency (%) 0 10 20 30 40 50 60 70 80 90 100 1 10 100 1000 10000 v in = 4.2v v in = 4.5v v in = 4.8v v in = 5.0v v in = 5.2v v in = 5.5v step-down converter efficiency (v out = 3.65v) output current (ma) efficiency (%) 0 10 20 30 40 50 60 70 80 90 100 1 10 100 1000 10000 v regin = 4.2v v regin = 4.5v v regin = 4.8v v regin = 5.0v v regin = 5.2v v regin = 5.5v step-down converter efficiency (v out = 3.8v) output current (ma) efficiency (%) -1.2 -0.8 -0.4 0 0.4 0.8 1.2 0 500 1000 1500 2000 2500 3000 v regin = 3.8v v regin = 4.2v v regin = 4.5v v regin = 4.8v v regin = 5.0v v regin = 5.5v step-down converter load regulation (v out = 3.4v) output current (ma) output error (%) -1.2 -0.8 -0.4 0 0.4 0.8 1.2 0 500 1000 1500 2000 2500 3000 v regin = 4.2v v regin = 4.5v v regin = 4.8v v regin = 5.0v v regin = 5.2v v regin = 5.5v step-down converter load regulation (v out = 3.8v) output current (ma) output error (%) -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 3.8 4 4.2 4.4 4.6 4.8 5 5.2 5.4 1ma 100ma 500ma 1000ma 1500ma 2000ma 2500ma step-down converter line regulation (v out = 3.4v) input voltage (v) output error (%)
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 6 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t typical characteristics -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 4.2 4.4 4.6 4.8 5 5.2 5.4 1ma 100ma 500ma 1000ma 1500ma 2000ma 2500ma step-down converter line regulation (v out = 3.8v) input voltage (v) output error (%) 2.6 2.65 2.7 2.75 2.8 2.85 2.9 -40 -15 10 35 60 85 frequency vs temperature temperature(c) frequency (mhz) 0.591 0.594 0.597 0.600 0.603 0.606 0.609 -40 -15 10 35 60 85 feedback voltage vs temperature temperature (oc) v fb (v) 0.591 0.594 0.597 0.600 0.603 0.606 0.609 3.1 3.4 3.7 4.0 4.3 4.6 4.9 5.2 5.5 feedback voltage vs input voltage input voltage (v) v fb (v) 0.6 0.7 0.8 0.9 1 1.1 1.2 3.8 4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 v enh v enl step-down converter enable threshold vs input voltage input voltage (v) v enreg (v) 0.6 0.7 0.8 0.9 1 1.1 1.2 -40 -15 10 35 60 85 v enh v enl step-down converter enable threshold vs temperature temperature (c) v enreg (v)
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 7 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t typical characteristics 40 50 60 70 80 90 100 110 120 3.1 3.5 3.9 4.3 4.7 5.1 5.5 85c 25c -40c no load quiescent current vs input voltage (open loop) input voltage (v) quiescent current (a) 50 60 70 80 90 100 110 120 -40 -15 10 35 60 85 no load quiescent current vs temperature (open loop, v sysin = 5v) temperature (c) quiescent current (a) 0 3 6 9 12 15 18 3.7 4 4.3 4.6 4.9 5.2 5.5 85c 25c -40c no load quiescent current vs input voltage (close loop, v out = 3.4v) input voltage (v) quiescent current (ma) 8 10 12 14 16 -40 -15 10 35 60 85 no load quiescent current vs temperature (close loop, v sysin = 5v, v out = 3.4v) temperature (c) quiescent current (ma) 40 58 76 94 112 130 3.1 3.5 3.9 4.3 4.7 5.1 5.5 85c 25c -40c step-down converter p-channel r ds(on) vs input voltage input voltage (v) r ds(on) (m) 20 30 40 50 60 70 80 3.1 3.5 3.9 4.3 4.7 5.1 5.5 85c 25c -40c step-down converter n-channel r ds(on) vs input voltage input voltage (v) r ds(on) (m)
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 8 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t typical characteristics v enregin 2v/div v out 2v/div i regin 1a/div 0v 0v 0a soft start (v sysin = v regin = 5v, c out = 22f, l=1h, v out = 3.4v, i out = 2.5a) time 1ms/div soft start (v sysin = v regin = 5v, c out = 22f,v out = 3.4v, l = 1h, i out = 0a) time 1ms/div v enregin 2v/div v out 2v/div i regin 50ma/div 0v 0a 0v output ripple (v sysin = v regin = 5v, pfm mode, l=1h, c out = 22f, c regin = 10f + 0.1f,v out = 3.4v,i out = 10ma) time 400ns/div v sw 2v/div v out(ac) 10mv/div 0a 0v 0v i in 200ma/div v out(ac) 10mv/div i in 200ma/div 0a 0v 0v v sw 2v/div output ripple (v sysin = v regin = 5v, pwm mode, l = 1h, c out = 22f, c regin = 10f + 0.1f, v out = 3.4v, i out = 10ma) time 400ns/div output ripple (v sysin = v regin = 5v, pwm mode, l = 1h, c out = 22f, c regin = 10f + 0.1f, v out = 3.4v, i out = 2a) time 400ns/div v sw 2v/div i in 1a/div v out(ac) 10mv/div 0v 0v 0a 100ma 1a step-down converter load transient (v sysin = v regin = 5v, v out = 3.8v , c cout = 22f, l = 1h, c ff = 22pf, c regin = 10f + 0.1f) time 100s/div i load 500ma/div v out(ac) 100mv/div 0v 0a
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 9 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t typical characteristics 100 2.5a ma step-down converter load transient (v sysin = v regin = 5v,v out = 3.8v, c cout = 22f, l = 1h, c ff = 22pf, c regin = 10f + 0.1f) time 100s/div v out(ac) 200mv/div i load 1a/div 0v 0a 100ma step-down converter load transient (v sysin = v regin = 5v, v out = 3.4v, c cout = 22f, l = 1h, c ff = 22pf, c regin = 10f + 0.1f) time 100s/div v out(ac) 100mv/div i load 500ma/div 0v 0a 1a step-down converter linetransient (i out = 100ma, v out = 3.8v, c out = 22f, l = 1h, c ff = 22pf, c regin = 10f + 0.1f) time 100s/div v regin 1v/div 0v 0v v out(ac) 200mv/div 4.2v 5v 0.6 0.7 0.8 0.9 1 1.1 1.2 3.1 3.5 3.9 4.3 4.7 5.1 5.5 v enh v enl load switch enable threshold vs input voltage input voltage (v) v ensys (v) 5v 4.2v step-down converter linetransient (i out = 100ma, v out = 3.4v, c out = 22f, l = 1h, c ff = 22pf, c regin = 10f + 0.1f) time 100s/div v regin 1v/div v out(ac) 200mv/div 0v 0v 2.5a 100ma step-down converter load transient (v sysin = v regin = 5v, v out = 3.4v, c cout = 22f, l = 1h, c ff = 22pf, c regin = 10f + 0.1f) time 100s/div v out(ac) 200mv/div i load 1a/div 0v 0a
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 10 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t typical characteristics 0.7 0.8 0.9 1 1.1 1.2 1.3 -40 -15 10 35 60 85 v enh v enl load switch enable threshold vs temperature temperature (c) v ensys (v) 40 60 80 100 120 140 3.1 3.4 3.7 4 4.3 4.6 4.9 5.2 5.5 85c 25c -40c load switch r ds(on) vs input voltage input voltage (v) r ds(on) (m) load switch turn on (v sysin = 5v, c sysin = 1f, c regin = 10f + 0.1f, rl = 100) (v out = 3.4v) time 400s/div v out 2v/div i in 500ma/div v ensys 2v/div v regin 2v/div 0a 0v 0v 0v regin discharge function (v sysin from 5v to 3v, c sysin = 1f, c regin = 10f + 0.1f) time 400s/div v sysin 2v/div v regin 2v/div 0v 0v load switch turn on (v sysin = 5v, c sysin = 1f, c regin = 10f + 0.1f + 680f, r l = 100) (v out = 3.4v) time 2ms/div v out 2v/div i in 500ma/div v ensys 2v/div v regin 5v/div 0a 0v 0v 0v load switch turn off (v sysin = 5v, c sysin = 1f, c regin = 10f + 0.1f, r l = 100) (v out = 3.4v) time 1ms/div v out 2v/div i in 50ma /div v ensys 2v/div v regin 2v/div 0a 0v 0v 0v
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 11 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t functional description the AAT2138 is a high performance, 2.8mhz, synchro- nous step-down converter with a high-precision, pro- grammable current limited p-channel load switch up to 3a. the p-channel load switch is adopted to limit the system input current. the current limit value is programmed by external resistor between rset and gnd. its fast tran- sient response time makes it ideal to protect the system from input power surges. the AAT2138 employs internal error amplifier and com- pensation. it provides excellent transient response, load and line regulation. its output voltage is programmed by an external resistor divider from 3.0v to converter input voltage. soft start eliminates any output voltage over- shoot when the enable or input voltage is applied. dropout mode makes the converter increase the switch duty cycle to 100% and the output voltage tracks the input voltage minus the r ds(on) drop of the p-channel high-side mosfet of the converter. the AAT2138's input voltage range is 2.7v to 5.5v. two independent enable pins control the load switch and step-down converter separately. the converter efficiency has been optimized for a 1 h inductor. control scheme the AAT2138 uses a peak current-mode step-down con- trol scheme. the converter senses the current through the high-side p-channel mosfet for current loop control as well as overload protection. a fixed slope compensa- tion signal is added to the sensed current to maintain stability for duty cycles greater than 50%. the input current of the step-down converter is limited by the load switch. the feedback amplifier compares the fb voltage against the 0.6v reference voltage. the error amplifier?s trans- conductance output is internally compensated, and pro- grams the current-mode loop for the necessary peak switch current to force a constant output voltage over all load and line conditions. enable/soft start AAT2138 has two independent enable pins: ensys and enreg. when ensys is pulled high, the current limit load switch is turned off and regin drops to zero. when enreg is pulled low, the step-down converter is forced into the low-power, no-switching, high impedance state. the total input current during shutdown is less than 1ua. when ensys is pulled low, the system will turn on the load switch in a soft start process. to avoid a big inrush current during the startup, the regin voltage will be charged from sysin with limited input current under 500ma. the startup time depends on the capacitance between regin and gnd. when enreg is pulled high, the step-down converter is functional block diagram step-down converter 600mv reference fb sw pgnd enreg regin en vp load switch control sysin en vp ensys 1.0v ref. current limit rset current limit mode mode agnd discharge control
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 12 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t active with soft start. when the regin voltage is above uvlo and reaches 95% of v sysin voltage, the reference voltage is charged from 0 to 0.6v slowly by an internal current source to avoid a large input inrush current. during the soft start, the output voltage increases with the reference voltage. the soft start time is about 1ms with a 100 f output capacitor. regin discharge the voltage on the capacitor between regin and gnd is discharged quickly when the power on sysin is removed quickly (unplugged adaptor). when ensys is pulled low and sysin is below 3.0v, AAT2138 automatically turns on the discharge mosfet to discharge the regin capacitor storage. discharge resistance is typical 8ohm and the specific discharge time depends on the capaci- tance between regin and gnd. input under-voltage lockout under-voltage lockout (uvlo) guarantees sufficient v regin bias and proper operation of all internal circuitry prior to activation. mode function when the mode pin is pulled high, the part runs in forced pwm mode operation using the internal oscillator. when the mode pin is pulled low, a light-load mode operation (pfm/pwm) is designed to reduce the domi- nant switching losses at low output voltage and light- load condition. the step-down converter can also be synchronized to an external clock signal fed into the mode pin. in this case, the internal oscillator is bypassed. the frequency of the external clock must be in the range from 1mhz to 3mhz. protection circuitry the AAT2138 includes protection for overload and over- temperature conditions. the overload protection turns off the high-side switch when the inductor current exceeds the current-limit threshold (3a minimum). the AAT2138 includes thermal protection that disables the regulator and the load switch when the die tempera- ture reaches 150c. the regin pin is maintained in a high-impendance state. it automatically restarts when the temperature drops by 15c or more. application information load switch current limit setting the AAT2138's load switch current limit can be pro- grammed by an external resistor r set from rset to gnd. in most applications, the variation in i lim must be taken into account when determining r set . the i lim variation is due to processing variations from part to part, as well as variations in the voltages at sysin and regin, plus the operating temperature. together, these three factors add up to a 5% tolerance (see load switch i lim specification in electrical characteristics section). table 1 gives 1% standard metal film resistor example values for pmos current limit programming. i lim (a) r set (k ) 0.4 13.3 0.482 10 0.51 9.53 0.553 8.88 0.613 8.06 0.798 6.34 0.908 5.6 1.01 5.1 1.078 4.75 1.245 4.02 1.538 3.16 2.07 2.2 2.48 1.74 2.64 1.6 2.9 1.4 3 1.3 table 1: examples of 1% standard resistor value of r set . 0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 r set (k) i limit (a) figure 1: load switch current limit vs r set .
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 13 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t dropout voltage dropout voltage is determined by r ds(on) and the current passing through it. AAT2138 load switch typical r ds(on) is 80m for usb application. so, for a 750ma load switch current limit setting, the load switch dropout voltage can be calculated by: v dropout_switch = 750ma 80m = 60mv step-down converter inductor selection the step-down converter uses peak current-mode con- trol with slope compensation to maintain stability for duty cycles greater than 50%. the output inductor value must be selected so the inductor current down slope meets the internal slope compensation requirements. the inductor should be set equal to half the output volt- age numeric value in h. this guarantees that there is sufficient internal slope compensation. manufacturer?s specifications list both the inductor dc current rating, which is a thermal limitation, and the peak current rating, which is determined by the satura- tion characteristics. the inductor should not show any appreciable saturation under normal load conditions. some inductors may meet the peak and average current ratings yet result in excessive losses due to a high dcr. always consider the losses associated with the dcr and its effect on the total converter efficiency when selecting an inductor. c regin selection c regin is not only the load switch output capacitor but also the step-down converter input capacitor. it is designed to provide the additional input current and maintain the sysout voltage for the step-down con- verter when load switch limits the input current from sysin. if the input voltage of the step-down converter is lower than the v out plus the dropout voltage, the AAT2138 enters dropout mode. c regin minimum value can be calculated by the following steps: first, calculate the allowed maximum delta voltage on c regin to keep v out in regulation: v regin = v in - v out - v dropout_switch - v dropout_buck second, calculate the required input current at sysout for the step-down converter: i buckin = i out v out v sysin - v dropout_switch next, calculate the maximum current c regin should pro- vide: i cregin = i buckin - i lim finally, derive the c sysout at certain load on period t on c regin_min = t on i cregin ?v regin example : a 2a, 217hz, 12.5% load pulse is applied on 3.8v v out in 5v v in and 750ma load switch current limit, under the condition, v dropout_switch is 0.06v, v dropout_buck is 0.17v. therefore, considering the step-down converter at 2a, 3.8v, v out is 90%. v regin = 5 - 3.8 - 0.06 - 0.17 = 0.970v i buckin = 2 3.8 0.9 5 - 0.06 = 1.71a i cregin = 1.71 - 0.75 = 0.96a t on is 576 s for a 217hz, 12.5% duty cycle load pulse. c regin_min = 576 0.96 0.970 = 570f 0 0 0 i out 1a/div i sysin 1a/div v regin 2v/div v out 500mv/div 800s/div figure 2: AAT2138 operation waveform 2a, 577 s load pulse (applied) v in = 5v, v out = 3.8v.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 14 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t considering 20% capacitance tolerance, the minimum capacitance should be 684 f. so select a 680 f tanta- lum capacitor as c regin , as well as an additional 10 f ceramic capacitor to closely filter the input voltage v regin of the step-down converter on the pcb board. input capacitor selection select a 1 f x7r or x5r ceramic capacitor for the sys- tem input. to estimate the required input capacitor size, determine the acceptable input ripple level and solve for c in . the calculated value varies with input voltage and is a maximum when v in is twice the output voltage. c in = v out v in 1 - v out v in f s v pp i out - esr for v in = 2 v out, = v out v in 1 - v out v in 1 4 and c in(min) = 1 4 f s v pp i out - esr the input capacitor provides a low impedance loop for the edges of pulsed current drawn by the AAT2138. low esr/esl x7r and x5r ceramic capacitors are ideal for this function due to their low esr and esl. to minimize stray parasitic inductance, place the capacitor as close as possible to the ic. this keeps the high frequency content of the input current localized, minimizing emi and input voltage ripple. the proper placement of the input capacitor (c1) can be seen in the evaluation board layout in the layout section of this datasheet (see figure 4). a laboratory test set-up typically consists of two long wires running from the bench power supply to the eval- uation board input voltage pins. the inductance of these wires, along with the low-esr ceramic input capacitor, can create a high q network that may affect converter performance. this problem often becomes apparent in the form of excessive ringing in the output voltage dur- ing load transients. errors in the loop phase and gain measurements can also result. feedback resistor selection the output voltage on the AAT2138 is adjustable with external resistors r1 and r2. to limit the bias current required for the external feedback resistor string while maintaining good noise immunity, the minimum sug- gested value for r2 is 59k . although a larger value will further reduce quiescent current, it will also increase the impedance of the feedback node, making it more sensi- tive to external noise and interference. the maximum value of r1 should be below 1mohm to keep reference voltage normal and avoid noise coupling. the external resistor r1, combined with an external capacitor up to 22pf feed-forward capacitor (c4 in figure 3), delivers enhanced transient response for extreme pulsed load applications and reduces ripple in light load conditions. the addition of the feed forward capacitor typically requires a larger output capacitor c31-c32 for stability. the external resistors set the out- put voltage according to the following equation: r1 = - 1 r2 = v out v ref - 1 r2 v out 0.6v table 2 shows the standard 1% metal film resistor examples for different step-down ouput voltages v out (v) r2 = 59k , r1 (k ) 3.3 267 3.4 274 3.6 294 3.8 316 table 2: resistor selections for different output voltage settings.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 15 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t printed circuit board layout recommendations for best performance of the AAT2138, the following guidelines should be followed when designing the pcb layout: 1. reliably solder the exposed pad (ep) to the gnd plane. a gnd pad below ep is strongly recommended. 2. keep the power traces, including the gnd trace, the sw trace and the sysin, regin short, direct and wide to allow large current flow. keep the l connection to the sw pins as short as possible. do not put any sig- nal lines under the inductor. 3. connect the input capacitors (c1 and c21) as close as possible between sysin and regin and gnd to get good power filtering. 4. connect the input capacitor c23 as close as possible between regin and agnd to get good power filtering and load regulation. 5. keep the switching node, sw away from the sensitive fb node. 6. separate the feedback trace from any power trace and connect as close as possible to the load point. sensing along a high-current load trace will degrade dc load regulation. place external feedback resistors as close as possible to the fb pin to minimize the length of the high impedance feedback trace. 7. minimize the resistance of the trace from the load return to gnd. this will help minimize any error in dc regulation due to differences in the potential of the internal signal ground and the power ground. 1uh l1 r1 274k 59k r2 22f c31 22pf c4 sysin regin j2 1f c1 10f c21 680f c22 vout sysin rset sw j3 sysin j1 sysin mode 10 sw 13 fb 9 sw 14 rset 6 pgnd 1 pgnd 2 agnd 7 ensys 8 sysin 12 enreg 5 sysin 11 regin 3 regin 4 u1 AAT2138 2.7v-5.5v option c32 gnd gnd gnd gnd gnd 0.1f c23 3.4v gnd figure 3. AAT2138 evaluation board schematic.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 16 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t (a) top layer (b) bottom layer figure 4. AAT2138 evaluation board layout manufacturer part number value ( f) voltage (v) temperature range case size murata grm188r71h104ka93d 0.1 50 x7r 0603 grm21br71e105ka99l 1 25 x7r 0805 grm21br61c106ke15l 10 16 x5r 0805 grm21br60j226me39 22 6.3 x5r 0805 grm0335c1h220gd01d 22pf 50 coj 0603 kemet t495d687m006zte150 680 6.3 x5r 7343 table 4: recommended capacitor selection. manufacturer part number inductance ( h) max dc current (a) dcr (m ) size (mm) lxwxh type samsung cig22h1r0mne 1 3.3 83 2x2.5x1.2 shielded murata lqh44pn1r0npo 1 2.95 30 4x4x1.65 shielded table 3: recommended inductor selection component part number description manufacturer u1 AAT2138iwo-0.6 high current step-down converter with adjustable current limit load switch skyworks c1 grm21br71e105ka99l cap ceramic 1uf 0805 x7r 25v 10% murata c21 grm21br61c106ke15l cap ceramic 10uf 0805 x5r 16v 10% c23 grm21br71e105ka99l cap ceramic 0.1uf 0603 x7r 50v 10% c31 grm21br60j226me39 cap ceramic 22uf 0805 x5r 6.3v 10% c4 grm0335c1h220gd01d cap ceramic 22pf 0603 c0j 50v 10% c22 t495d687m006zte150 cap 680uf 7343 x5r 6.3v 20% kemet r1 rc0603fr-07274kl res 274k 1/10w 1% 0603 smd yageo r2 rc0603fr-0759kl res 59k 1/10w 1% 0603 smd rset rc0603fr-076k34l res 6.34k 1/10w 1% 0603 smd l cig22h1r0mne power inductor 1uh 3.3a 83m smd samsung table 5: evaluation board bom list
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 17 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t ordering information output voltage package marking 1 part number (tape and reel) adjustable 0.6v tdfn33-14 s6xyy AAT2138iwo-0.6-t1 2 skyworks green? products are compliant with all applicable legislation and are halogen-free. for additional information, refer to skyworks de?ition of green , document number sq04-0074. package information tdfn33-14 3 top view bottom view 3.000 0.050 index area 3.000 0.050 detail "a" 1.650 0.050 2.500 0.050 0.203 ref 0.750 0.050 0.000 + 0.100 - 0.000 detail "a" side view 0.425 0.050 0.400 bsc 0.180 0.050 pin 1 indicator (optional) all dimensions in millimeters. 1. xyy = assembly and date code. 2. sample stock is generally held on part numbers listed in bold . 3. the leadless package family, which includes qfn, tqfn, dfn, tdfn and stdfn, has exposed copper (unplated) at the end of the lead terminals due to the manufacturing process. a solder fillet at the exposed copper edge cannot be guaranteed and is not required to ensure a proper bottom solder c onnection.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 202006c ? skyworks proprietary information ? products and product information are subject to change without notice. ? ma rch 18, 2013 18 AAT2138 high current step-down converter with high-precision programmable current limited load switch data shee t copyright ? 2012, 2013 skyworks solutions, inc. all rights reserved. information in this document is provided in connection with skyworks solutions, inc. (?skyworks?) products or services. these m aterials, including the information contained herein, are provided by skyworks as a service to its customers and may be used for informational purposes only by the customer. skyworks assumes no responsibility fo r errors or omissions in these materials or the information contained herein. sky- works may change its documentation, products, services, speci cations or product descriptions at any time, without notice. skyworks makes no commitment to update the materials or informati on and shall have no responsibility whatsoever for con icts, incompatibilities, or other dif culties arising from any future changes. no license, whether express, implied, by estoppel or otherwise, is granted to any intellectual property rights by this document . skyworks assumes no liability for any materials, products or information provided here- under, including the sale, distribution, reproduction or use of skyworks products, information or materials, except as may be p rovided in skyworks terms and conditions of sale. the materials, products and information are provided ?as is? without warranty of any kind, whether express, implied, statutory, or otherwise, including fitness for a particular purpose or use, merchantability, performance, quality or non-infringement of any intellectual property right; all such warranti es are hereby expressly disclaimed. skyworks does not warrant the accuracy or completeness of the information, text, graphics or other items contained within these materials. sk yworks shall not be liable for any damages, in- cluding but not limited to any special, indirect, incidental, statutory, or consequential damages, including without limitation , lost revenues or lost profits that may result from the use of the materials or information, whether or not the recipient of materials has been advised of the possibility of such damage. skyworks products are not intended for use in medical, lifesaving or life-sustaining applications, or other equipment in which the failure of the skyworks products could lead to personal injury, death, physical or en- vironmental damage. skyworks customers using or selling skyworks products for use in such applications do so at their own risk and agree to fully indemnify skyworks for any damages resulting from such improper use or sale. customers are responsible for their products and applications using skyworks products, which may deviate from published speci cations as a result of design defects, errors, or operation of products outside of pub- lished parameters or design speci cations. customers should include design and operating safeguards to minimize these and other risks. skyworks assumes no liabi lity for applications assistance, customer product design, or damage to any equipment resulting from the use of skyworks products outside of stated published speci cations or parameters. skyworks, the skyworks symbol, and ?breakthrough simplicity? are trademarks or registered trademarks of skyworks solutions, inc ., in the united states and other countries. third-party brands and names are for identi cation purposes only, and are the property of their respective owners. additional information, including relevant terms and co nditions, posted at www.skyworksinc.com, are incorporated by reference.

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