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aat1138 2a step-down converter with adjustable input current limit data sheet 1 skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 general description the aat1138 is a 2a synchronous step-down converter with an integrated current-limiting load switch designed for precise input current control applications. by guard- ing against excessive input current, the aat1138 enables the system designer to maximize the output current from the step-down converter while protecting the input supply. it is designed for protection of 5v usb ports and 3.3v supplies from heavy load transient conditions com- monly seen with high data rate modem applications. the aat1138 integrates a programmable current limited p-channel mosfet load switch to protect the input sup- ply against large currents which may cause the supply to fall out of regulation. current limit threshold is pro- grammed by an external resistor from set to ground. it can be adopted to control loads of the input supply that requires up to 1.4a. the aat1138?s internal step-down converter is a 2a, 1.2mhz constant frequency current mode pwm step- down converter. the step-down converter can run at 100% duty cycle for low dropout operation. the output voltage can be regulated as low as 0.6v. the aat1138 is available in a pb-free, low profile, 16-pin 3mm x 4mm tdfn package. the product is rated over the -40c to 85c temperature range. features ? ? v in range: 2.5v to 5.5 v ? ? v out range: 0.6v to v pvin ? ? system current limited load switch ? programmable over-current threshold ? < 1 s fast transient response to short current ? 145m typical r ds(on) ? ? step-down converter ? up to 2a output current ? 95% peak efficiency ? 1.2mhz switching frequency ? 135m low r ds(on) internal switches ? 100% duty cycle low dropout operation ? soft start ? ? under-voltage lockout ? ? independent enable pins ? ? <2 a shutdown current ? over-temperature and current limit protection ? ? low profile 16-pin 3mm x 4mm tdfn package ? ? -40c to 85c temperature range applications ? ? cellular phones ? ? mp3 players ? ? pdas and handheld computers ? ? portable media players ? ? usb devices typical application tdfn34-16 l 2.2h sw fb gnd sysin pgnd ensys c in 1f enbuck r2 267k r1 59.0k v in : 2.5v - 5.5v v out : 3.3v/ up to 2a load pulse sysout pvin svin c sysout ceramic cap 22f + large tantalum cap c outb 22f r set 11.3k set aat1138
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 2 pin descriptions pin # symbol function 1, 6, 15 gnd analog ground pin. 2, 3 sw switching node pin. connect the output inductor to this pin. 4, 5 pgnd power ground pin for the step-down converter. 7 sysin system input power. p-channel mosfet source. connect a 1 f capacitor from sysin to gnd. 8 ensys enable input for system power load switch. active low. 9 set input current limit set input. a resistor from set to ground sets the current limit for the input load switch. 10, 11 sysout system output power. p-channel mosfet drain. 12 enbuck enable input for step-down converter. active high. connect to pvin when enabling the step- down converter. do not leave enbuck oating. 13 pvin power supply input pin for step-down converter. must be closely decoupled to pgnd with a 22 f or greater ceramic capacitor. 14 svin analog supply input pin. provides bias for internal circuitry. connect to pvin. 16 fb feedback pin for step-down converter. connect fb to the center point of the external resistor divider. the feedback threshold voltage is 0.6v. ep exposed pad. must be connected to bare copper ground plane. pin configuration tdfn34-16 (top view) sw pgnd pgnd gnd sw 3 gnd sysin ensys svin pvin enbuck fb ep gnd sysout sysout set 4 5 1 2 6 7 8 14 13 12 16 15 11 10 9
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 3 absolute maximum ratings 1 t a = 25c unless otherwise noted. symbol description value units v in sysin, pvin, svin to gnd -0.3 to 6.0 v v gnd pgnd, gnd -0.3 to 6.0 v v sw , v fb sw, fb to gnd -0.3 to v in + 0.3 v v set , v out set, sysout to gnd -0.3 to v in + 0.3 v v en ensys, enbuck to gnd -0.3 to v in + 0.3 v i max,sys maximum continuous current for sysout load switch 2 a t j operating junction temperature range -40 to 150 ? c t lead maximum soldering temperature (at leads, 10 sec) 300 c thermal characteristics symbol description value units ja maximum thermal resistance 68.86 c/w p d maximum power dissipation 2, 3 1.45 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. only one absolute maximum rating should be applied at any one time. 2. mounted on an fr4 board. 3. derate 20mw/c above 25c.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 4 electrical characteristics 1 v pvin = v svin = 3.6v, t a = -40c to 85c unless otherwise noted. typical values are at t a = 25c. symbol description conditions min typ max units v sysin input voltage 2.5 5.5 v t sd over-temperature threshold v sysin = 5v, t j increasing 135 o c v sysin = 5v, t j decreasing 105 step-down converter v out output voltage range 0.6 v pvin v i q input dc supply current active mode: v fb = 0.5v 300 500 a shutdown mode: v en = 0v, v pvin = 5.5v 0.1 1 a i fb feedback input bias current v fb = 0.65v 30 na v fb_acc feedback voltage accuracy 2 i load = 10ma, t a = 25c 0.588 0.60 0.612 v no load, -40c t a 0.582 0.60 0.618 v fb_tol feedback voltage tolerance v pvin = 2.5v to 5.5v, 10ma to 2.0a load -3.0 +3.0 % ? v linereg / ? v in line regulation v pvin = 2.5v to 5.5v, 10ma load, t a = 25c 0.10 0.20 %/v ? v loadreg / ? i out load regulation i load = 0a to 2a 0.2 %/a i lim current limit 2.5 3.5 a r ds(on)h high side switch on resistance v pvin = 3.6v 135 200 m r ds(on)l low side switch on resistance v pvin = 3.6v 95 150 m f osc oscillator frequency v fb = 0.6v 0.96 1.2 1.44 mhz t s startup time enable to output regulation 1.3 ms v il enbuck threshold low 0.3 v v ih enbuck threshold high 1.5 v i leak enbuck leakage current v en = 5.5v -1.0 1.0 a load switch i q quiescent current ensys = gnd, no load 925 a i q(off) off supply current ensys = 5v 1 a i sd(off) off switch current ensys = 5v, v sysin = 5v, v sysout = 0v 0.01 1 a v uvlo under-voltage lockout rising edge, 1% hysteresis 1.8 2.4 v r ds(on) on resistance v sysin = 5.0v, t a = 25c 145 180 m v sysin = 4.5v, t a = 25c 150 v sysin = 3.0v, t a = 25c 190 230 t crds on resistance temperature coef cient 2800 ppm/c i lim current limit r set = 6.8k 0.75 1 1.25 a i lim(min) minimum current limit 130 ma v ensys\(l) ensys input low voltage v sysin = 2.7v to 5.5v 0.8 v v ensys\(h) ensys input high voltage v sysin = 2.7v to < 4.2v 2.0 v v sysin 4.2v to 5.0v 2.4 i ensys\(sink) ensys input leakage v ensys = 5.5v 0.01 1 a t resp current limit response time v sysin = 5v 0.4 s t off turn-off time v sysin = 5v, r l = 10 412 s t on turn-on time v sysin = 5v, r l = 10 24 200 s 1. the aat1138 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. the regulated feedback voltage is tested in an internal test mode that connects v fb to the output of the error amplifier.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 5 typical characteristics step-down converter efficiency (v out = 1.8v; l = 22h; c out = c sysout = 22f) output current (ma) efficiency (%) 0.1 1 10 100 1000 10000 0 10 20 30 40 50 60 70 80 90 100 v sysout = 2.5v v sysout = 3.0v v sysout = 3.6v v sysout = 4.2v v sysout = 5.0v v sysout = 5.5v step-down converter efficiency (v out = 3.3v; l = 22h; c out = c sysout = 22f) output current (ma) efficiency (%) 0.1 1 10 100 1000 10000 0 10 20 30 40 50 60 70 80 90 100 v sysout = 3.6v v sysout = 3.7v v sysout = 4.2v v sysout = 4.5v v sysout = 5.0v v sysout = 5.5v step-down converter load regulation (v out = 1.8v; l = 22h; c out = c sysout = 22f) output current (ma) output error (%) 0.1 400 800 1200 1600 2000 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 v sysout = 2.5v v sysout = 3.0v v sysout = 3.6v v sysout = 4.2v v sysout = 5.0v v sysout = 5.5v step-down converter load regulation (v out = 3.3v; l = 22h; c out = c sysout = 22f) output current (ma) output error (%) 0 400 800 1200 1600 2000 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 v sysout = 3.7v v sysout = 4.2v v sysout = 4.5v v sysout = 5.0v v sysout = 5.5v step-down converter line regulation vs. input voltage (v out = 1.8v) input voltage (v ) output error (% ) 2.5 3.1 3.7 4.3 4.9 5.5 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 i load = 1ma i load = 0.5a i load = 1a i load = 1.5a i load = 2a
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 6 typical characteristics step-down converter feedback voltage vs. temperature (v sysout = 3.6v) temperature (c) v fb (v) -40 -15 10 35 60 85 0.591 0.594 0.597 0.600 0.603 0.606 0.609 step-down converter feedback voltage vs. input voltage input voltage (v ) v fb (v ) 2.5 3.1 3.7 4.3 4.9 5.5 2.8 3.4 4.0 4.6 5.2 0.594 0.596 0.598 0.600 0.602 0.604 0.606 step-down converter v enh and v enl vs. temperature (v sysout = 3.6v) temperature (c) v enh and v enl (v) -40 -15 10 35 60 85 0.32 0.36 0.40 0.44 0.48 0.52 v enh v enl step-down converter v enh and v enl vs. input voltage input voltage (v ) v enh and v enl (v ) 2.5 3.1 3.7 4.3 4.9 5.5 2.8 3.4 4.0 4.6 5.2 0.35 0.38 0.41 0.44 0.47 0.50 v enh v enl frequency vs. temperature temperature (c) frequency (mhz) -40 -20 0 20 40 60 80 1.14 1.16 1.18 1.20 1.22 1.24 1.26 step-down converter en leakage vs. temperature temperature (c) i enbuck (a) -40 -15 10 35 60 85 0.0 0.2 0.4 0.6 0.8 1.0
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 7 typical characteristics step-down converter p-channel r ds(on) vs. input voltage input voltage (v ) r ds(on)h (m ) 2.5 3.1 3.7 4.3 4.9 5.5 2.8 3.4 4.0 4.6 5.2 80 100 120 140 160 180 200 t = -40c t = 25c t = 85c step-down converter n-channel r ds(on) vs. input voltage input voltage (v ) r ds(on)l (m ) 2.5 3.1 3.7 4.3 4.9 5.5 2.8 3.4 4.0 4.6 5.2 50 70 90 110 130 150 t = -40c t = 25c t = 85c step-down converter output ripple (v sysout = 5.0v, v out = 1.8v, i load = 2a; c sysout = c out = 22f, l = 2.2h) time (800ns/div) v out (20mv/div) i inductor (1a/div) v sw (2v/div) 0 0 0 step-down converter output ripple (v sysout = 3.6v, v out = 1.8v, i load = 10ma; c sysout = c out = 22f, l = 2.2h) time (2s/div) v out (10mv/div) i inductor (100ma/div) v sw (2v/div) 0 0 0 step-down converter output ripple (v sysout = 5.0v, v out = 3.3v, i load = 2a; c sysout = c out = 22f, l = 2.2h) time (800ns/div) v out (20mv/div) i inductor (1a/div) v sw (2v/div) 0 0 0 step-down converter output ripple (v sysout = 5.0v, v out = 3.3v, i load = 10ma; c sysout = c out = 22f, l = 2.2h) time (2s/div) v out (10mv/div) i inductor (100ma/div) v sw (2v/div) 0 0 0
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 8 typical characteristics step-down converter output ripple (v sysout = 5.0v, v out = 3.8v, i load = 2a; c sysout = c out = 22f, l = 2.2h) time (800ns/div) v out (20mv/div) i inductor (1a/div) v sw (2v/div) 0 0 0 step-down converter output ripple (v sysout = 5.0v, v out = 3.8v, i load = 10ma; c sysout = c out = 22f, l = 2.2h) time (2s/div) v out (10mv/div) i inductor (100ma/div) v sw (2v/div) 0 0 0 step-down converter load transient (10ma to 2a; v sysout = 3.6v, v out = 1.8v; c out = 22f, c ff = 100pf, l = 2.2h) time (40s/div) v out (200mv/div) i load (1a/div) 10ma 0 step-down converter load transient (10ma to 2a; v sysout = 5.0v, v out = 3.3v; c out = 22f, c ff = 100pf, l = 2.2h) time (40s/div) v out (200mv/div) i load (1a/div) 10ma 0 step-down converter line transient (v sysout = 3.6v to 4.2v; v out = 1.8v; i load = 2a; c out = 22f, c ff = 100pf, l = 2.2h) time (100s/div) v sysout (2v/div) v out (50mv/div) 0 0 step-down converter line transient (v sysout = 4.5v to 5.0v; v out = 3.3v; i load = 2a; c out = 22f, c ff = 100pf, l = 2.2h) time (100s/div) v sysout (2v/div) v out (50mv/div) 0 0
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 9 typical characteristics step-down converter start-up sequence (v sysout = 3.6v; v out = 1.8v; i load = 2a, c sysout = c out = 22f, l = 2.2h) time (400s/div) en (2v/div) i sysout (500ma/div) v out (2v/div) 0 0 0 step-down converter start-up sequence (v sysout = 5.0v; v out = 3.3v; i load = 2a, c sysout = c out = 22f, l = 2.2h) time (400s/div) en (2v/div) i sysout (500ma/div) v out (2v/div) 0 0 0 quiescent current vs. input voltage input voltage (v ) i q (a ) 2.5 3.1 3.7 4.3 4.9 5.5 2.8 3.4 4.0 4.6 5.2 0 800 1600 2400 3200 4000 v out = 1.8v v out = 3.3v v out = 3.8v quiescent current vs. temperature (v in = 5.0v) temperature (c) i q (a) -40 -15 10 35 60 85 320 350 380 410 440 470 500 v out = 1.8v v out = 3.3v v out = 3.8v shutdown current vs. temperature temperature (c) i sd (a) -40 -15 10 35 60 85 0.00 0.02 0.04 0.06 0.08 0.10
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 10 typical characteristics switch turn on (v in = 5v; r l = 10; c sysout = 22f) time (4s/div) en (2v/div) v out (2v/div) 0 0 switch turn off (v in = 5v; r l = 10; c sysout = 22f) time (4s/div) en (2v/div) v out (2v/div) 0 0 switch short circuit response (v in = 5v; r load = 0.3) time (1s/div) i out (2a/div) v out (5v/div) v in (2v/div) 0 0 0 switch r set vs. i lim (v in = 5.0v; v in - v out = 0.5v) i lim (ma) r set (k) 100 1000 10000 1 10 100 switch current limit vs. temperature (v in = 5.0v; r set = 11.3k) temperature (c) i lim (ma) -40 -15 10 35 60 85 650 660 670 680 690 700 switch r ds(on) vs. input voltage input voltage (v ) r ds(on) ( ) 2.5 3.1 3.7 4.3 4.9 5.5 2.8 3.4 4.0 4.6 5.2 0.14 0.16 0.18 0.20 0.22
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 11 typical characteristics switch v en(h) and v en(l) vs. input voltage input voltage (v ) v en(h) and v en(l) ( ) 2.5 3.1 3.7 4.3 4.9 5.5 2.8 3.4 4.0 4.6 5.2 0.9 1.2 1.5 1.8 2.1 2.4 v en(h) v en(l) switch v enh and v enl vs. temperature (v in = 5.0v; r set = 11.3k) temperature (c) v enh and v enl (v) -40 -15 10 35 60 85 1.5 1.7 1.9 2.1 2.3 2.5 v enh v enl switch en leakage vs. temperature temperature (c) i sysen (a) -40 -20 0 20 40 60 80 0.00 0.04 0.08 0.12 0.16 0.20
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 12 functional block diagram step-down converter 600mv reference fb sw pgnd enbuck pvin svin gnd en vp load switch control sysout en vp ensys 1.2v ref. current limit set current limit functional description the aa t1138 is a high performance 2a 1.2mhz synchro- nous step-down converter with a programmable current limited p-channel load switch. it is designed with the goal of high performance with precise input current con- trol under up to 2a load pulse on the step-down con- verter output. the p-channel load switch is adopted to limit the system input current. the current limit value is programmed by external resistor between set and gnd. its fast tran- sient response time make it protect the system input power ideally. the 2a step-down converter employs internal error amplifier and compensation. it provides excellent tran- sient response, load and line regulation. its output volt- age is programmed by an external resistor divider from 0.6v to converter input voltage. soft start eliminates any output voltage overshoot when the enable or input volt- age 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 aat1138's input voltage range is 2.5v to 5.5v. two independent enable pins control the load switch and step-down converter separately. the converter efficiency has been optimized for all load conditions, ranging from no load to 2a. step-down converter control loop the internal dc-dc converter of the aat1138 is a peak current mode synchronous step-down converter. the current through the p-channel mosfet (high side) is sensed for current loop control, as well as short circuit and overload protection. a slope compensation signal is added to the sensed current to maintain stability for duty cycles greater than 50%. the peak current mode loop
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 13 appears as a voltage-programmed current source in par- allel with the output capacitor. the output of the voltage error amplifier programs the current mode loop for the necessary peak switch current to force a constant output voltage for all load and line conditions. internal loop compensation terminates the transconductance voltage error amplifier output. for fixed voltage versions, the error amplifier reference voltage is internally set to pro- gram the converter output voltage. for the adjustable output, the error amplifier reference is fixed at 0.6v. current limit the aat1138 includes two kinds of current limit. one is input current limit by the load switch; the other is induc- tor current limit by the high-side mosfet current sense loop of the step-down converter. for overload conditions, the input current is limited by r set and the peak inductor current is limited to 3.5a. to minimize power dissipation and stresses under current limit and short-circuit condi- tions, step-down converter switching is terminated after entering current limit for a series of pulses. the termina- tion lasts for seven consecutive clock cycles after a cur- rent limit has been sensed during a series of four con- secutive clock cycles. over-temperature protection thermal protection completely disables load switch and step-down converter switching when internal dissipation becomes excessive. the junction over-temperature threshold is 125c with 10c of hysteresis. once an over-temperature or over-current fault condition is removed, the output voltage automatically recovers. enable/soft start aat1138 has two independent enable pins: ensys and enbuck. when ensys is pulled high, the current limit load switch is turned off and sysout drops to zero. when enbuck is pulled low, the step-down converter is forced into the low-power, no-switching state. soft start of the step-down converter limits the current surge seen at the input and eliminates output voltage overshoot. dropout operation when the step-down converter input voltage v pvin is close to the value of the output voltage, the main switch is allowed to remain on for more than one switching cycle and increases the duty cycle until it reaches 100%. the duty cycle d of a step-down converter is defined as: v out v in d = t on f osc 100% = 100% where t on is the main switch on time and f osc is the oscil- lator frequency 1.2mhz. the output voltage then is the input voltage minus the voltage drop across the main switch and the inductor. at low input supply voltage, the r ds(on) of the p-channel mosfet increases, and the effi- ciency of the converter decreases. caution must be exer- cised to ensure the heat dissipated does not exceed the maximum junction temperature of the ic.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 14 application information setting the load switch current limit the aat1138's load switch current limit can be pro- grammed by an external resistor r set from set 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 sysout, plus the operating temperature. together, these three factors add up to a 25% tolerance (see load switch i lim specification in electrical characteristics section). figure 1 illustrates a cold device with a statistically higher current limit and a hot device with a statistically lower current limit, both with r set equal to 10.3k . a 10.3k r set resistor sets the typical current limit to 0.665a. this figure shows that the actual current limit will be at least 0.5a and no greater than 0.83a. v sysout (v v ) i sysout (a) 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.83a 0.5a at 85c at -40c figure 1: load switch current limit (r set = 10.3k ). though the relationship between typical i lim and r set is not linear throughout the current limit setting range, there is constant coefficient between them within a small enough current limit range for the system designer to select a suitable r set value. table 1 shows the current limit range using a standard 1% metal film resistor. to determine r set , start with the maximum allowable input current from sysin as minimum i lim and multiply by 1.33 to derive the typical i lim value. next, refer to table 1 to find the constant coefficient i lim range which includes the calculated i lim value and get the constant coefficient value c. then calculate the r set by the following formula: r set = r set_max - (i lim - i lim_range_low ) c r set_max is the maximum standard r set resistance at the certain constant coefficient current limit range. i lim is the calculated typical current limit. i lim_range_low is the low terminal of the current limit range. c is the coefficient of the current limit range. example: select r set for 500ma current limit. the typical current limit is i lim = 500 1.33 = 665ma. the constant coefficient current range is 600ma to 700ma and c = 25000. therefore: r set = 13k - (665ma - 600ma) 25000 = 11.375k select a standard 11.3k resistor for r set . considering the 25% tolerance, the current limit will be greater than 500ma but less than 831ma. i lim typ. (ma) constant coef cient c of r set / i lim 1% standard r set (k ) 200 186000 40.2 250 120000 30.9 300 56000 24.9 350 50000 22.1 400 36000 19.6 450 32000 17.8 500 30000 16.2 550 34000 14.7 600 25000 13.0 700 16300 10.5 800 13700 8.87 900 6900 7.50 1000 7700 6.81 1100 5500 6.04 1200 5000 5.49 1300 3500 4.99 1400 4.64 table 1: current limit r set values. dropout voltage dropout voltage is determined by r ds(on) and the current passing through it. aat1138 load switch maximum r ds(on) is 180m for usb applicationa and the step-down converter high side r ds(on) is maximum 200m . so for a 500ma load switch current limit setting (the load switch always limits below 831ma as described above), the load switch maximum dropout voltage can be calculated by: v dropout_switch = 831ma 180m = 0.15v
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 15 to 2a step-down converter, the maximum dropout volt- age is: v dropout_buck = 2a 200m = 0.4v operation in heavy load pulse when a heavy load pulse is applied to the output of aat1138 as typical application shows, the input current is limited to the value of i lim determined by r set . at this time, c sysout has the important role of providing enough current and voltage to the step-down converter input to bring it into regulation. the duty cycle of the heavy load pulse should not exceed the maximum value which allows sufficient time to charge c sysout from v in and balance the capacitor charging and discharging to make the operation normally. figure 2 shows the operation waveform at 5v v in and 3.8v v out with a 2a load pulse applied when r set = 11.5k , c in = 1 f, c sysout = 22 f ceramic capacitor + 4x330 f tantalum capacitor, and c outb = 22 f. time (1ms/div) v out (200mv/div) i out (2a/div) v sysout (ac coupled) (1v/div) i in (500ma/div) v in (100mv/div) 5v 0a 0a 3.8v figure 2: aat1138 operation waveform when 2a 217hz 12.5% load pulse is applied. capacitor selection c sysout selection c sysout 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 (v pvin and v svin ) is lower than the v out plus the dropout voltage, the aat1138 enters dropout mode. c sysout minimum value can be calculated by the following steps: first, calculate the allowed maximum delta voltage on c sysout to keep v out in regulation: ?v sysout = 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 = v out i out (v in - v dropout_switch ) next, calculate the maximum current c sysout should pro- vide: i csysout = i buckin - i lim finally, derive the c sysout at certain load on period t on . c sysout_min = i csysout t on v sysout example: a 2a 217hz 12.5% load pulse is applied on 3.8v v out in 5v v in and 500ma load switch current limit. under the condition, v dropout_switch is 0.15v. v dropout_buck is 0.4v. therefore: ?v sysout = 5 - 3.8 - 0.15 - 0.4 = 0.65v
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 16 considering the step-down converter at 2a 3.8v v out is 90%. i buckin = = 1.74 a 3.8 2 (5 - 0.15) 90% i csysout = 1.74 - 0.5 = 1.24 a t on is 576 s for a 217hz 12.5% duty cycle load pulse. c sysout_min = = 1099f 1.24a 576s 0.65v considering 20% capacitance tolerance, the minimum capacitance should be 1319 f. so select 4x330 f tanta- lum capacitor as c sysout , as well as an additional 22 f ceramic capacitor to closely filter the input voltage v sysout of the step-down converter on the pcb board. c outb selection the value of output capacitance is generally selected to limit output voltage ripple to the level required by the specification. since the ripple current in the output inductor is usually determined by l, v out and v in , the series impedance of the capacitor primarily determines the output voltage ripple. the three elements of the capacitor that contribute to its impedance (and output voltage ripple) are equivalent series resistance (esr), equivalent series inductance (esl), and capacitance (c). the formula below gives the general output voltage rip- ple calculation: ?v out esr + v out (v in - v out ) v in f osc l 1 8 f osc c out the output voltage droop due to a load transient is dominated by the capacitance of the output capacitor. during a step increase in load current, the output capac- itor alone supplies the load current until the loop responds. within three switching cycles, the loop responds and the inductor current increases to match the load current demand. the relationship of the output voltage droop during the three switching cycles to the output capacitance can be estimated by: c out = 3 i load v droop f osc in many practical designs, to get the required esr, a capacitor with much more capacitance than is needed must be selected. for both continuous and discontinuous inductor current mode operation, the esr of the c out needed to limit the ripple to v o , v peak-to-peak is: esr v o i l esl can be a problem by causing ringing in the low megahertz region but can be controlled by choosing low esl capacitors, limiting lead length (pcb and capacitor), and replacing one large device with several smaller ones connected in parallel. in conclusion, in order to meet the requirement of low output voltage ripple and regulation loop stability, ceram- ic capacitors with x5r or x7r dielectrics are recommend- ed due to their low esr and high ripple current ratings. a 22 f ceramic capacitor can satisfy most applications. inductor selection for most designs, the aat1138 operates with inductor values of 2.0 h to 6.8 h. inductors with low inductance values are physically smaller but generate higher induc- tor current ripple leading to higher output voltage ripple. refer to the ?capacitor selection? section of this datasheet for the output ripple calculation. the inductor ripple cur- rent can be derived from the following equation: ?i l v out (v in - v out ) v in f osc l large value inductors lower ripple current and small value inductors result in high ripple currents. choose inductor ripple current approximately 30% of the maxi- mum load current 2a, or ? i l = 600ma 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. the dc current rating of the inductor should be at least equal to the maximum load current plus half the inductor rip- ple current to prevent core saturation (2a + 300ma).
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 17 adjustable output resistor selection for applications requiring an adjustable output voltage, the 0.6v version can be externally programmed. resistors r1 and r2 of figure 3 program the output to regulate at a voltage higher than 0.6v. 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 and the r1+r2 should be less than 1.5m . the external resistor sets the output voltage according to the following equation: v out = 0.6 1 + r1 r2 r1 = -1 r2 v out 0.6 table 4 summarizes the standard 1% metal film resistor values for various output voltages with r2 set to 59k . v out (v) r2 = 59k r1 (k ) 1.0 39.2 1.2 59.0 1.5 88.7 1.8 118 2.0 137 2.5 187 3.3 267 3.6 294 3.8 316 4.2 357 table 4: resistor selections for different output voltage settings. manufacturer part number value voltage tolerance temp. co. esr (m ) case murata grm21br60j226me39 22 f 6.3 20% x5r 26 0805 avx tajd337m006r 330 f 6.3 20% x5r 400 7343 tpsd337m006r0150 330 f 6.3 20% x5r 150 7343 tajd477m006r 470 f 6.3 20% x5r 400 7343 tpsd477m006r0150 470 f 6.3 20% x5r 150 7343 tajd687m006r 680 f 6.3 20% x5r 500 7343 tpsd687m006r0100 680 f 6.3 20% x5r 100 7343 kemet t491d337m006at 330 f 6.3 20% x5r 400 7343 t495d337m006ate100 330 f 6.3 20% x5r 100 7343 t491d477m006at 470 f 6.3 20% x5r 400 7343 t495d477m006ate150 470 f 6.3 20% x5r 150 7343 t491d687m006zt 680 f 6.3 20% x5r 500 7343 t495d687m006zte150 680 f 6.3 20% x5r 150 7343 table 2: surface mount capacitors. manufacturer part number inductance ( h) saturation current (a) dcr typ. (m ) size (mm) lxwxh type sumida cdrh5d16 2.2 3.0 28.7 5.8x5.8x1.8 shielded cdrh5d16 3.3 2.6 35.6 5.8x5.8x1.8 shielded cdrh8d28 4.7 3.4 19 8.3x8.3x3.0 shielded coiltronics sd53 2.0 3.3 23 5.2x5.2x3.0 shielded sd53 3.3 2.6 29 5.2x5.2x3.0 shielded sd53 4.7 2.1 39 5.2x5.2x3.0 shielded table 3: surface mount inductors.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 18 l1 2.2h r1 316k r2 59k c31 22f c1 1f c4 100pf vin sysout ensys enbuck c21 22f c25 330f c22 330f c23 330f c24 330f c32 330f vout sysout 10 set 9 ep gnd 1 sw 3 sw 2 pgnd 4 pgnd 5 gnd 6 sysin 7 ensys 8 sysout 11 pvin 13 enbuck 12 svin 14 fb 16 gnd 15 u1 aat1138 gnd r set 11.5k sw figure 3: aat1138 evaluation board schematic. layout guidance when laying out the pc board, the following layout guidelines should be followed to ensure proper operation of the aat1138: 1. the exposed pad (ep) must be reliably soldered to the gnd plane. a gnd pad below ep is strongly rec- ommended. 2. the power traces, including the gnd trace, the sw trace and the sysin, sysout trace should be kept short, direct and wide to allow large current flow. the l1 connection to the sw pins should be as short as possible. do not put any signal lines under the inductor. 3. the input capacitor (c1 and c21) should connect as closely as possible to sysin and sysout and gnd to get good power filtering. 4. keep the switching node, sw away from the sensi- tive fb node. 5. the feedback trace should be separate from any power trace and connect as closely as possible to the load point. sensing along a high-current load trace will degrade dc load regulation. external feedback resistors should be placed as closely as possible to the fb pin to minimize the length of the high imped- ance feedback trace. 6. the resistance of the trace from the load return to gnd should be kept to a minimum. this will help to minimize any error in dc regulation due to differ- ences in the potential of the internal signal ground and the power ground.
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 19 figure 4: aat1138 evaluation board figure 5: aat1138 evaluation board layout top layer. layout bottom layer. designation part number description manufacturer u1 aat1138irn-0.6-t1 2a buck with current limit skyworks c1 grm21br71e105k cap ceramic 1 f 0805 x7r 25v 10% murata c4 grm1885c1h101j cap ceramic 100pf 0603 cog 50v 5% c21, c31 grm21br60j226m cap ceramic 22 f 0805 x5r 6.3v 20% c22, c23, c24, c25, c32 tpsy337m006r0150 cap tan 330 f y case 6.3v 20% avx l1 cdrh5d16-2r2 power inductor 2.2 h 3.0a smd sumida r1 rc0603fr-07316kl res 316k 1/10w 1% 0603 smd yageo r2 rc0603fr-0759kl res 59k 1/10w 1% 0603 smd rset rc0603fr-0711k5l res 11.5k 1/10w 1% 0603 smd table 5: aat1138 evaluation board bill of materials (bom).
skyworks solutions, inc. ? phone [781] 376-3000 ? fax [781] 376-3100 ? sales@skyworksinc.com ? www.skyworksinc.com 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 20 ordering information output voltage package marking 1 part number (tape and reel) 2 adjustable 0.6v tdfn34-16 7bxyy aat1138irn-0.6-t1 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 tdfn34-16 3 3.000 + 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 201978b ? skyworks proprietary information ? products and product information are subject to change without notice. ? marc h 15, 2013 aat1138 2a step-down converter with adjustable input current limit data sheet 21 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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