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aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 1 systempower ? typical application 2.7v to 5.5v input supply enable buck enable ldo in_buck v out_buc k lx 4.7f c2 r1 r2 4.7f aat2500m 2.2h out_ldo fb_buck v out(ldo) c1 2.2f pgnd agnd in_ldo en_buck en_ldo 1f general description the aat2500m is a high efficiency 400ma step- down converter and 300ma low dropout (ldo) lin- ear regulator for applications where power efficien- cy and solution size are critical. the typical input power source can be a single-cell lithium-ion/poly- mer battery or a 5v or 3.3v power bus. the step-down converter is capable of delivering up to 400ma output current, uses a typical switching frequency of 1.8mhz to greatly reduce the size of external components, offers high speed turn-on and maintains a low 25a no load quiescent current. the ldo is capable of delivering up to 300ma out- put current. the aat2500m is available in the pb-free, space- saving 12-pin tsopjw package and is rated over the -40c to +85c operating temperature range. features ?v in range: 2.7v to 5.5v ? output current: step-down converter: 400ma ldo: 300ma ? low quiescent current 130a combined for both step-down converter plus ldo ? 90% efficient step-down converter (at 100ma) ? integrated power switches ? 100% duty cycle ? 1.8mhz switching frequency ? current limit protection ? automatic soft-start ? over temperature protection ? tsopjw-12 package ? -40c to +85c temperature range applications ? cellular phones ? digital cameras ? handheld instruments ? micro hard disc drives ? microprocessor / dsp core / io power ? optical storage devices ? pdas and handheld computers ? portable media players
aat2500m 400ma step-down converter and 300ma ldo 2 2500m.2007.06.1.0 pin descriptions pin configuration tsopjw-12 (top view) 1 2 3 4 5 6 12 11 10 9 8 7 lx pgnd en_buck en_ldo fb_buck out_ldo in_buck agnd agnd agnd agnd in_ldo pin # symbol function 1 lx step-down converter switching node. 2 pgnd power ground for step-down converter. 3 en_buck enable pin for step-down converter. 4 en_ldo enable pin for ldo. 5 fb_buck feedback input pin for step-down converter. regulated at 0.6v for adjustable version. 6 out_ldo ldo power output. 7 in_ldo input supply voltage for ldo. 8, 9, 10, 11 agnd analog signal ground. 12 in_buck input supply voltage for step-down converter.
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 3 absolute maximum ratings 1 thermal information symbol description value units ja thermal resistance 2 110 c/w p d maximum power dissipation 909 mw symbol description value units v p input voltage -0.3 to 6.0 v agnd, pgnd ground pins -0.3 to +0.3 v v en , v fb enable and feedback pins v in + 0.3 v i out maximum dc output current (continuous) 1000 ma t j operating temperature range -40 to 150 c t s storage temperature range -65 to 150 c t lead maximum soldering temperature (at leads, 10 sec) 300 c 1. stresses above those listed in absolute maximum ratings may cause permanent damage to the device. functional operation at c ondi- tions 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.
aat2500m 400ma step-down converter and 300ma ldo 4 2500m.2007.06.1.0 electrical characteristics 1 v in_buck = v in_ldo = 5.0v. t a = -40c to +85c unless noted otherwise. typical values are at t a = +25c. symbol description conditions min typ max units power supply v inbuck , input voltage 2.7 5.5 v v inldo v uvlo under-voltage lockout v in rising 2.7 v v in falling 2.35 v i q quiescent current v en = v in , no load 130 a i shdn shutdown current v en = gnd 1.0 a step-down converter v fb feedback voltage tolerance no load, t a = 25c 0.591 0.609 v `i out = 0 to 400ma; v in = 2.7 to 5.5v -3 +3 % i lxleak lx reverse leakage current v in = 5.5v, v lx = 0 to v in , v en = gnd -1.0 1.0 a i fb feedback leakage v fb = 1.0 v 0.2 a i lim p-channel current limit 1.2 a r ds(on)h high side switch on resistance 0.4 ? r ds(on)l low side switch on resistance 0.25 ? ? v out /v out load regulation i load = 0 to 400ma 0.25 % ? v out /v out line regulation v in = 2.7v to 5.5v 0.3 % f osc oscillator frequency 1.8 mhz t s start-up time from enable to output regulation 120 s ldo (v out = 3.3v) v out output voltage tolerance no load, 25c 3.24 3.30 3.36 v v out output voltage range i out = 0 to 300ma -3 3 % v in input voltage v out + 5.5 v v do 2 i out output current 300 ma i lim current limit 1a v do dropout voltage 3 i out = 300ma 160 240 mv ? v out /v out load regulation i load = 0 to 300ma 1.2 % ? v out /v out line regulation v in = 3.7v to 5.5v 0.6 % t s start-up time from enable to output regulation 100 s logic signals v en(l) enable threshold low 0.6 v v en(h) enable threshold high 1.5 v i en(h) enable current consumption -1.0 1.0 a t sd over-temperature shutdown 150 c threshold t hys over-temperature shutdown 15 c hysteresis 1. specification over the -40c to +85c operating temperature ranges is assured by design, characterization and correlation wi th sta- tistical process controls. 2. to calculate the minimum ldo input voltage, use the following equation: v in(min) = v out(max) + v do(max) . 3. v do is defined as v in - v out when v out is 98% of nominal.
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 5 typical characteristics no load quiescent current vs. input voltage (en_buck = en_ldo = v in ) input voltage (v) input current (a) 30 50 70 90 110 130 150 2.5 3 3.5 4 4.5 5 5.5 6 25c 85c -40c ldo turn-on time from enable (v in = 5v; v out = 3.3v; i out = 300ma) time (40s/div) enable voltage (top) (v) output voltage (bottom)(v) 0 2 4 6 -1 0 1 2 3 ldo turn-off response time (v in = 5v; v out = 3.3v; i out = 300ma) time (50ns/div) enable voltage (top) (v) output voltage (bottom)(v) 0 2 4 6 -1.0 0.0 1.0 2.0 3.0 ldo dropout voltage vs. output current output current (ma) dropout voltage (mv) 0 50 100 150 200 250 0 50 100 150 200 250 300 35 0 85c 25c -40c ldo dropout characteristics (v out = 3.3v) input voltage (v) output voltage (v) 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 i out = 100ma i out = 200ma i out = 300ma i out = 0.1ma i out = 10ma i out = 50ma ldo dropout voltage vs. temperature temperature (c) dropout voltage (mv) 0 30 60 90 120 150 180 210 -40 -20 0 20 40 60 80 100 i l = 300ma i l = 200ma i l = 100ma i l = 50ma
aat2500m 400ma step-down converter and 300ma ldo 6 2500m.2007.06.1.0 typical characteristics step-down converter switching frequency vs. input voltage (i out = 400ma) input voltage (v) frequency variation (%) -3 -2 -1 0 1 2 3 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5. 5 v out = 1.2v v out = 1.8v step-down converter switching frequency vs. temperature (v in = 5v; v out = 1.8v) temperature (c) switching frequency (mhz) 1.5 1.6 1.7 1.8 1.9 -40 -20 0 20 40 60 80 10 0 ldo v ih and v il vs. input voltage input voltage (v) v ih and v il (v) 0.6 0.7 0.8 0.9 1.0 1.1 1.2 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 v ih v il ldo load transient response (1ma to 300ma; v in = 5v; v out = 3.3v; c out = 4.7f) time (100s/div) output voltage (top) (v) output current (bottom) (a) 2.9 3.1 3.3 3.5 3.7 -0.2 0.0 0.2 0.4 300ma 1ma ldo line transient response (v in = 4v to 5v; v out = 3.3v; i out = 300ma; c out = 4.7f) time (40s/div) input voltage (top) (v) output voltage (bottom) (v) 4 5 3.1 3.3 3.5
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 7 typical characteristics step-down converter output ripple (v out = 1.8v; v in = 5v; i out = 1ma) time (10s/div) output voltage (top) (v) inductor current (bottom) (a) 1.79 1.80 1.81 0.0 0.1 0.2 step-down converter output ripple (v out = 1.8v; v in = 5v; i out = 400ma) time (200ns/div) output voltage (top) (v) inductor current (bottom) (a) 1.79 1.80 1.81 1.82 0.0 0.2 0.4 0.6 step-down converter efficiency vs. load (v out = 1.2v; l = 2.2h) output current (ma) efficiency (%) 20 30 40 50 60 70 80 90 100 0.1 1 10 100 1000 v in = 2.7v v in = 3.3v v in = 4.2v v in = 5v step-down converter dc regulation (v out = 1.2v; l = 2.2h) output current (ma) output error (%) -1.0 -0.5 0.0 0.5 1.0 0.1 1 10 100 1000 v in = 3.6v to 5.5v v in = 2.7v step-down converter efficiency vs. load (v out = 1.8v; l = 2.2h) output current (ma) efficiency (%) 20 30 40 50 60 70 80 90 100 0.1 1 10 100 1000 v in = 2.7v v in = 3.3v v in = 5.5v v in = 4.2v step-down converter dc regulation (v out = 1.8v; l = 2.2h) output current (ma) output error (%) -1.0 -0.5 0.0 0.5 1.0 0.1 1 10 100 1000 v in = 3.3v, 4.2v, 5.5v v in = 2.7v
aat2500m 400ma step-down converter and 300ma ldo 8 2500m.2007.06.1.0 typical characteristics step-down converter soft start (v in = 5v; v out = 1.8v; i out = 400ma; c ff = open) time (50s/div) inductor current (bottom) (a) enable voltage (top) (v) output voltage (middle) (v) -2 0 2 4 6 -0.2 0.0 0.2 0.4 step-down converter p-channel r ds(on)h vs. input voltage input voltage (v) r ds(on)h (m ? ? ) 300 400 500 600 700 2.5 3 3.5 4 4.5 5 5.5 6 120c 85c 100c 25c step-down converter n-channel r ds(on)l vs. input voltage input voltage (v) r ds(on)l (m ? ? ) 100 200 300 400 500 2.5 3 3.5 4 4.5 5 5.5 6 120c 85c 100c 25c step-down converter output voltage error vs. temperature (v in = 5v; v out = 1.8v; i out = 400ma) temperature (c) output voltage error (%) -1.0 -0.5 0.0 0.5 1.0 -50 -25 0 25 50 75 100 step-down converter output voltage error vs. temperature (v in = 5v; v out = 1.2v; i out = 400ma) temperature (c) output voltage error (%) -1.0 -0.5 0.0 0.5 1.0 -50 -25 0 25 50 75 100
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 9 typical characteristics step-down converter line transient response (v in = 4v to 5v; v out = 1.8v; i out = 400ma; c out = 4.7f) time (40s/div) input voltage (top) (v) output voltage (bottom) (v) 4 5 6 1.5 1.6 1.7 1.8 step-down converter line regulation (v out = 1.2v; l = 2.2h) input voltage (v) accuracy (%) -1.00 -0.50 0.00 0.50 1.00 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 i out = 0.1ma to 400ma step-down converter load transient response (1ma to 400ma; v in = 5v; v out = 1.2v; c out = 4.7f) time (100s/div) output voltage (top) (v) output current (middle) (a) inductor current (bottom) (a) 1.2 1.4 -0.2 0.0 0.2 0.4 400ma 1ma step-down converter load transient response (1ma to 400ma; v in = 5v; v out = 1.2v; c out = 4.7f; c ff = 100pf) time (100s/div) output voltage (top) (v) output current (middle) (a) inductor current (bottom) (a) 1.2 1.4 -0.2 0.0 0.2 0.4 400ma 1ma step-down converter load transient response (1ma to 400ma; v in = 5v; v out = 1.8v; c out = 4.7f) time (100s/div) output voltage (top) (v) output current (middle) (a) inductor current (bottom) (a) 1.8 2.0 -0.2 0.0 0.2 0.4 400ma 1ma step-down converter load transient response (1ma to 400ma; v in = 5v; v out = 1.8v; c out = 4.7f; c ff = 100pf) time (100s/div) output voltage (top) (v) output current (middle) (a) inductor current (bottom) (a) 1.8 2.0 -0.2 0.0 0.2 0.4 400ma 1ma
aat2500m 400ma step-down converter and 300ma ldo 10 2500m.2007.06.1.0 functional block diagram en_buck in_buck lx pgnd bias oscillator en_ldo agnd r ldofb1 v cc r ldofb2 in_ldo out_ldo fb_buck control circuit v cc functional description the aat2500m is a high performance power man- agement ic comprised of a buck converter and a lin- ear regulator. the buck converter is a high efficien- cy converter capable of delivering up to 400ma. operating at 1.8mhz, the converter requires only three external power components (c in , c out , and l x ) and is stable with a ceramic output capacitor. the linear regulator delivers 300ma and is also sta- ble with ceramic capacitors. linear regulator the advanced circuit design of the linear regulator has been specifically optimized for very fast start- up and shutdown timing. this proprietary ldo has also been tailored for superior transient response characteristics. these traits are particularly impor- tant for applications that require fast power supply timing. the high-speed turn-on capability is enabled through implementation of a fast-start control cir-
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 11 cuit, which accelerates the power-up behavior of fundamental control and feedback circuits within the ldo regulator. fast turn-off time response is achieved by an active output pull-down circuit, which is enabled when the ldo regulator is placed in shutdown mode. this active fast shut- down circuit has no adverse effect on normal device operation. the ldo regulator output has been specifically optimized to function with low- cost, low-esr ceramic capacitors; however, the design will allow for operation over a wide range of capacitor types. the regulator comes with complete short-circuit and thermal protection. the combination of these two internal protection circuits gives a comprehen- sive safety system to guard against extreme adverse operating conditions. the regulator features an enable/disable function. this pin (en_ldo) is active high and is compatible with cmos logic. to assure the ldo regulator will switch on, the en_ldo turn-on control level must be greater than 1.5v. the ldo regulator will go into the disable shutdown mode when the voltage on the en_ldo pin falls below 0.6v. if the enable function is not needed in a specific application, it may be tied to v in_ldo to keep the ldo regulator in a continuously on state. the in_ldo input powers the internal reference, oscillator, and bias control blocks. for this reason, the in_ldo input must be connected to the input power source to provide power to both the ldo and step-down converter functions. when the regulator is in shutdown mode, an internal 1.5k ? resistor is connected between out and gnd. this is intended to discharge c out when the ldo regulator is disabled. the internal 1.5k ? resistor has no adverse impact on device turn-on time. step-down converter the aat2500m buck is a constant frequency peak current mode pwm converter with internal com- pensation. it is designed to operate with an input voltage range of 2.7v to 5.5v. the output voltage ranges from 0.6v to the input voltage. the 0.6v fixed model shown in figure 1 is also the adjustable version and is externally programmable with a resistive divider, as shown in figure 2. the converter mosfet power stage is sized for 400ma load capability with up to 92% efficiency. light load efficiency is close to 80% at a 500a load. figure 1: aat2500m fixed output. figure 2: aat2500m with adjustable step-down output and enhanced transient response. l1 4. 7h c1 4.7f c4 4. 7f c1 10 f agnd agnd agnd agnd pgnd fb_buck lx vp _buck in_ldo out_ldo en_ ldo en_ buck aat2500m 1 5 11 10 9 8 2 6 4 3 7 12 vout _buck vout_ldo vin l1 4. 7uh c1 4. 7f r1 c4 4. 7f c1 10 f r2 59 k c8 100 pf agnd agnd agnd agnd pgnd fb_buck lx vp _buck in_ldo out_ldo en_ ldo en_ buck aat2500m 1 5 11 10 9 8 2 6 4 3 7 12 vout_ buc k vout_ldo vin
aat2500m 400ma step-down converter and 300ma ldo 12 2500m.2007.06.1.0 soft start the aat2500m soft-start control prevents output voltage overshoot and limits inrush current when either the input power or the enable input is applied. when pulled low, the enable input forces the converter into a low-power, non-switching state with a bias current of less than 1a. low dropout operation for conditions where the input voltage drops to the output voltage level, the converter duty cycle increases to 100%. as 100% duty cycle is approached, the minimum off-time initially forces the high side on-time to exceed the 1.8mhz clock cycle and reduce the effective switching frequency. once the input drops below the level where the out- put can be regulated, the high side p-channel mosfet is turned on continuously for 100% duty cycle. at 100% duty cycle, the output voltage tracks the input voltage minus the ir drop of the high side p-channel mosfet r ds(on) . low supply the under-voltage lockout (uvlo) guarantees suf- ficient v in bias and proper operation of all internal circuitry prior to activation. fault protection for overload conditions, the peak inductor current is limited. thermal protection disables switching when the internal dissipation or ambient temperature becomes excessive. the junction over-temperature threshold is 150c with 15c of hysteresis. applications information ldo regulator input and output capacitors: an input capacitor is not required for basic operation of the linear reg- ulator. however, if the aat2500m is physically located at a reasonable distance from an input power source, an input capacitor (c3) will be need- ed for stable operation. typically, a 1f or larger capacitor is recommended for c3 in most applica- tions. c3 should be located as closely to the input voltage (in_ldo) pin as practically possible. an input capacitor greater than 1f will offer supe- rior input line transient response and maximize power supply ripple rejection. ceramic, tantalum, or aluminum electrolytic capacitors may be select- ed for c3. there is no specific capacitor esr requirement for c3. however, for 300ma ldo reg- ulator output operation, ceramic capacitors are rec- ommended for c3 due to their inherent capability over tantalum capacitors to withstand input current surges from low impedance sources such as bat- teries in portable devices. for proper load voltage regulation and operational stability, a capacitor is required between the out_ldo and agnd pins. the output capacitor (c4) connection to the ldo regulator ground pin should be made as directly as practically possible for maximum device performance. since the regu- lator has been designed to function with very low esr capacitors, ceramic capacitors in the 1.0f to 10f range are recommended for best perform- ance. applications utilizing the exceptionally low output noise and optimum power supply ripple rejection should use 2.2f or greater for c4. in low output current applications, where output load is less than 10ma, the minimum value for c4 can be as low as 0.47f.
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 13 equivalent series resistance: esr is a very important characteristic to consider when selecting a capacitor. esr is the internal series resistance asso- ciated with a capacitor that includes lead resistance, internal connections, size and area, material compo- sition, and ambient temperature. typically, capacitor esr is measured in milliohms for ceramic capacitors and can range to more than several ohms for tanta- lum or aluminum electrolytic capacitors. step-down converter inductor selection: the step-down converter uses peak current mode control with slope com- pensation 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 require- ments. the internal slope compensation for the adjustable and low-voltage fixed versions of the aat2500m is 0.24a/sec. this equates to a slope compensation that is 35% of the inductor current down slope for a 1.5v output and 2.2h inductor. this is the internal slope compensation for the adjustable (v o = 0.6v) version or low output volt- age fixed versions. when externally programming the 0.6v version to 2.5v, the calculated inductance is 3.75h. in this case, a standard 4.7h value is selected. for high output voltage fixed versions (2.5v and above), m = 0.48a/sec. table 1 displays inductor values for the aat2500m fixed and adjustable options. 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 saturation 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 loss- es 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. the 2.2h cdrh3d16 series inductor selected from sumida has a 59m ? dcr and a 1.3a dc cur- rent rating. at full load, the inductor dc loss is 9.4mw which gives a 1.5% loss in efficiency for a 400ma, 1.5v output. 0.35 ? v o l = = 1.5 ? v o = 1.5 ? 2.5v = 3.75h m 0.35 ? v o 0.24a sec a sec a a sec 0.35 ? v o m = = = 0.24 l 0.35 ? 1.5v 2.2h a sec table 1: inductor values. configuration output voltage inductor slope compensation 0.6v adjustable with 0.6v to 2.0v 2.2h 0.24a/sec external resistive divider 2.5v 4.7h 0.24a/sec fixed output 0.6v to 2.0v 2.2h 0.24a/sec 2.5v to 3.3v 2.2h 0.48a/sec
aat2500m 400ma step-down converter and 300ma ldo 14 2500m.2007.06.1.0 input capacitor select a 4.7f to 10f x7r or x5r ceramic capac- itor for the input. to estimate the required input capacitor size, determine the acceptable input rip- ple level (v pp ) and solve for c2. the calculated value varies with input voltage and is a maximum when v in_buck is double the output voltage (v o ). always examine the ceramic capacitor dc voltage coefficient characteristics when selecting the prop- er value. for example, the capacitance of a 10f, 6.3v, x5r ceramic capacitor with 5.0v dc applied is actually about 6f. the maximum input capacitor rms current is: the input capacitor rms ripple current varies with the input and output voltage and will always be less than or equal to half of the total dc load (output) current. for v in = 2 v o the term appears in both the input volt- age ripple and input capacitor rms current equa- tions and is a maximum when v in_buck is twice v out_buck . this is why the input voltage ripple and the input capacitor rms current ripple are a maxi- mum at 50% duty cycle. the input capacitor provides a low impedance loop for the edges of pulsed current drawn by the aat2500m. low esr/esl x7r and x5r ceramic capacitors are ideal for this function. to minimize stray inductance, the capacitor should be placed as closely 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 (c2) can be seen in the evaluation board layout in figure 3. a laboratory test set-up typically consists of two long wires running from the bench power supply to the evaluation board input voltage pins. the induc- tance of these wires, along with the low-esr ceramic input capacitor, can create a high q net- work that may affect converter performance. this problem often becomes apparent in the form of excessive ringing in the output voltage during load transients. errors in the loop phase and gain meas- urements can also result. since the inductance of a short pcb trace feeding the input voltage is significantly lower than the power leads from the bench power supply, most applications do not exhibit this problem. ?? 1 - ?? v o v in v o v in i o rms(max) i 2 = ?? 1 - = d (1 - d) = 0.5 2 = ?? v o v in v o v in 1 2 ?? i rms = i o 1 - ?? v o v in v o v in c in(min) = 1 ?? - esr 4 f osc ?? v pp i o ?? 1 - = for v in = 2 v o ?? v o v in v o v in 1 4 ?? 1 - ?? v o v in c in = v o v in ?? - esr f osc ?? v pp i o
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 15 in applications where the input power source lead inductance cannot be reduced to a level that does not affect the converter performance, a high esr tantalum or aluminum electrolytic should be placed in parallel with the low esr, esl bypass ceramic capacitor. this dampens the high q network and stabilizes the system. output capacitor the step-down converter output capacitor limits the output ripple and provides holdup during large load transitions. a 4.7f to 10f x5r or x7r ceramic capacitor typically provides sufficient bulk capaci- tance to stabilize the output during large load tran- sitions and has the esr and esl characteristics necessary for low output ripple. the output voltage droop due to a load transient is dominated by the capacitance of the ceramic out- put capacitor. during a step increase in load cur- rent, the ceramic output capacitor alone supplies the load current until the loop responds. within two or three switching cycles, the loop responds and the inductor current increases to match the load current demand. the relationship of the output volt- age droop during the three switching cycles to the output capacitance can be estimated by: once the average inductor current increases to the dc load level, the output voltage recovers. the above equation establishes a limit on the minimum value for the output capacitor with respect to load transients. the internal voltage loop compensation also limits the minimum output capacitor value to 4.7f. this is due to its effect on the loop crossover frequency (bandwidth), phase margin, and gain margin. increased output capacitance will reduce the crossover frequency with greater phase margin. the maximum output capacitor rms ripple current is given by: dissipation due to the rms current in the ceramic output capacitor esr is typically minimal, resulting in less than a few degrees rise in hot-spot temperature. adjustable output voltage resistor selection for applications requiring an adjustable output volt- age (v o or v out ), the 0.6v version can be externally programmed. resistors r1 and r2 of figure 5 pro- gram the output to regulate at a voltage higher than 0.6v. to limit the bias current required for the exter- nal feedback resistor string while maintaining good noise immunity, the minimum suggested value for r2 1 23 v out (v in(max) - v out ) rms(max) i l f osc v in(max) = c out = 3 ? i load v droop f osc figure 3: aat2500m evaluation board top side. figure 4: aat2500m evaluation board bottom side.
aat2500m 400ma step-down converter and 300ma ldo 16 2500m.2007.06.1.0 is 59k ? . although a larger value will further reduce quiescent current, it will also increase the impedance of the feedback node, making it more sensitive to external noise and interference. table 2 summarizes the resistor values for various output voltages with r2 set to either 59k ? for good noise immunity or 221k ? for reduced no load input current. the adjustable version of the aat2500m, com- bined with an external feedforward capacitor (c8 in figures 2 and 5), delivers enhanced transient response for extreme pulsed load applications. the addition of the feedforward capacitor typically requires a larger output capacitor c1 for stability. table 2: adjustable resistor values for use with 0.6v step-down converter. r2 = 59k ? r2 = 221k ? v out (v) r1 (k ? ) r1 (k ? ) 0.8 19.6 75 0.9 29.4 113 1.0 39.2 150 1.1 49.9 187 1.2 59.0 221 1.3 68.1 261 1.4 78.7 301 1.5 88.7 332 1.8 118 442 1.85 124 464 2.0 137 523 2.5 187 715 ?? ?? r1 = -1 r2 = - 1 59k ? = 88.5k ? v out v ref ?? ?? 1.5v 0.6v figure 5: aat2500m evaluation board schematic. l1 4.7h c1 4.7f c2 10f v out buck v out ldo gnd v in1 1 2 3 buck enable lx1 gnd table 2 r1 59k r2 lx 1 pgnd 2 en_buck 3 en_ldo 4 agnd 9 agnd 10 agnd 11 in_buck 12 fb_buck 5 out_ldo 6 agnd 8 in_ldo 7 u1 aat2500m c4 4.7f c3 10f 1 2 3 ldo enable 1 2 3 ldo input c7 0.01f c8 n/a c9 n/a
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 17 thermal calculations there are three types of losses associated with the aat2500m step-down converter: switching losses, conduction losses, and quiescent current losses. conduction losses are associated with the r ds(on) characteristics of the power output switching devices. switching losses are dominated by the gate charge of the power output switching devices. at full load, assuming continuous conduction mode (ccm), a simplified form of the step-down convert- er and ldo losses is given by: i qbuck is the step-down converter quiescent cur- rent and i qldo is the ldo quiescent current. the term t sw is used to estimate the full load step-down converter switching losses. for the condition where the buck converter is in dropout at 100% duty cycle, the total device dissi- pation reduces to: since r ds(on) , quiescent current, and switching losses all vary with input voltage, the total losses should be investigated over the complete input voltage range. given the total losses, the maximum junction tem- perature can be derived from the ja for the tsopjw-12 package which is 110c/w. pcb layout the following guidelines should be used to ensure a proper layout. 1. the input capacitor c2 should connect as closely as possible to in_buck and pgnd, as shown in figure 5. 2. the output capacitor and inductor should be connected as closely as possible. the connec- tion of the inductor to the lx pin should also be as short as possible. 3. 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. if external feedback resistors are used, they should be placed as closely as pos- sible to the fb_buck pin. this prevents noise from being coupled into the high impedance feedback node. 4. 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 differences in the potential of the internal sig- nal ground and the power ground. t j(max) = p total ja + t amb p total = i obuck 2 r dson(hs) + i oldo (v in - v oldo ) + (i qbuck + i qldo ) v in p total i obuck 2 (r dson(hs) v obuck + r dson(ls) [v in - v obuck ]) v in = + (t sw f osc i obuck + i qbuck + i qldo ) v in + i oldo (v in - v oldo )
aat2500m 400ma step-down converter and 300ma ldo 18 2500m.2007.06.1.0 step-down converter design example specifications v obuck = 1.8v @ 400ma (adjustable using 0.6v version), pulsed load ? i load = 300ma v oldo = 3.3v @ 300ma v in = 2.7v to 4.2v (3.6v nominal) f osc = 1.8mhz t amb = 85c 1.8v buck output inductor (see table 1) for sumida inductor cdrh3d16, 2.2h, dcr = 59m ? . 1.8v output capacitor v droop = 0.2v 1 23 1 1.8v (4.2v - 1.8v) 2.2h 1.8mhz 4.2v 23 rms i l1 f osc v in(max) = 3 ? i load v droop f osc 3 0.3a 0.2v 1.8mhz c out = = = 2.5f = 75ma rm s (v obuck ) (v in(max) - v obuck ) = p esr = esr i rms 2 = 5m ? (75ma) 2 = 28.1w v obuck v obuck 1.8 v 1.8v ? i l1 = ? 1 - = ? 1 - = 260m a l1 ? f v in 2.2h ? 1.8mhz 4.2v i pkl1 = i obuck + ? i l1 = 0.4a + 0.130a = 0.53a 2 p l1 = i obuck 2 ? dcr = (0.4a) 2 ? 59m ? = 9.4mw ? ? ? ? ? ? ? ? l1 = 1.5 ? v obuck = 1.5 ? 1.8v = 2.7h sec a sec a
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 19 input capacitor input ripple v pp = 25mv aat2500m losses t j(max) = t amb + ja p loss = 85 c + (110 c/w) 399mw = 129 c p total + (t sw f osc i obuck + i qbuck + i qldo ) v in + (v in - v ldo ) i ldo i obuck 2 (r dson(hs) v obuck + r dson(ls) [v in - v obuck ] ) v in = = + (5ns 1.8mhz 0.4a + 50 a +125 a) 4.2v + (4.2v - 3.3v) 0.3a = 399mw (0.4a) 2 (0.725 ? 1.8v + 0.7 ? [4.2v - 1.8v]) 4.2v i obuck rms i p = esr i rms 2 = 5m ? (0.2a) 2 = 0.2mw 2 = = 0.2a rms c in = = = 2.42f 1 ?? - esr 4 f osc ?? v pp i obuck 1 ?? - 5m ? 4 1.8mhz ?? 25mv 0.4a
aat2500m 400ma step-down converter and 300ma ldo 20 2500m.2007.06.1.0 table 3: evaluation board component values. table 4: typical surface mount inductors. table 5: surface mount capacitors. manufacturer part number value voltage temp. co. case murata grm21br61a475ka73l 4.7f 10v x5r 0805 murata grm18br60j475ke19d 4.7f 6.3v x5r 0603 murata grm21br60j106ke19 10f 6.3v x5r 0805 murata grm21br60j226me39 22f 6.3v x5r 0805 inductance max dc dcr size (mm) manufacturer part number (h) current (a) ( ? ) lxwxh type sumida cdrh3d16-4r7 4.7 0.90 0.11 3.8x3.8x1.8 shielded sumida cdrh3d161hp-2r2 2.2 1.30 0.059 4.0x4.0x1.8 shielded murata lqh32cn4r7m23 4.7 0.45 0.20 2.5x3.2x2.0 non-shielded murata lqh32cn2r2m23 2.2 0.60 0.13 2.5x3.2x2.0 non-shielded coilcraft lpo3310-222 2.2 1.10 0.15 3.3x3.3x1.0 non-shielded coilcraft lpo3310-472 4.7 0.80 0.27 3.3x3.3x1.0 non-shielded coiltronics sd3118-4r7 4.7 0.98 0.122 3.1x3.1x1.85 shielded v out (v) r1 (k ? ) r1 (k ? ) l1 (h) adjustable version r2 = 59k ? r2 = 221k ? 1 (0.6v device) 0.8 19.6 75.0 2.2 0.9 29.4 113 2.2 1.0 39.2 150 2.2 1.1 49.9 187 2.2 1.2 59.0 221 2.2 1.3 68.1 261 2.2 1.4 78.7 301 2.2 1.5 88.7 332 2.2 1.8 118 442 2.2 1.85 124 464 2.2 2.0 137 523 2.2 or 3.3 2.5 187 715 4.7 v out (v) r1 (k ? ) l1 (h) fixed version r2 not used 0.6-3.3v 0 2.2 1. for reduced quiescent current r2 = 221k ? .
aat2500m 400ma step-down converter and 300ma ldo 2500m.2007.06.1.0 21 ordering information all analogictech products are offered in pb-free packaging. the term pb-free means semiconductor products that are in compliance with current rohs standards, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. for more information, please visit our website at http://www.analogictech.com/pbfree. voltage package buck converter ldo marking 1 part number (tape and reel) 2 tsopjw-12 adj 0.6v 3.3v xlxyy AAT2500MITP-AW-T1 legend voltage code adjustable a (0.6v) 0.9 b 1.2 e 1.5 g 1.8 i 1.9 y 2.5 n 2.6 o 2.7 p 2.8 q 2.85 r 2.9 s 3.0 t 3.3 w 4.2 c 1. xyy = assembly and date code. 2. sample stock is generally held on part numbers listed in bold . 3. contact sales for availability.
aat2500m 400ma step-down converter and 300ma ldo 22 2500m.2007.06.1.0 advanced analogic technologies, inc. 830 e. arques avenue, sunnyvale, ca 94085 phone (408) 737- 4600 fax (408) 737- 4611 ? advanced analogic technologies, inc. analogictech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an analogictech pr oduct. no circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. analogictech reserves the right to make changes to their products or specifications or to discontinue any product or service with- out notice. except as provided in analogictechs terms and conditions of sale, analogictech assumes no liability whatsoever, an d analogictech disclaims any express or implied war- ranty relating to the sale and/or use of analogictech products including liability or warranties relating to fitness for a part icular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. in order to minimize risks associated with the customers applications, adequa te design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. testing and other quality control techniques are utilized to the extent an alogictech deems necessary to support this warranty. specific testing of all parameters of each device is not necessarily performed. analogictech and the analogictech logo are trad emarks of advanced analogic technologies incorporated. all other brand and product names appearing in this document are registered trademarks or trademarks of their respective holder s. package information tsopjw-12 all dimensions in millimeters. 0.20 + 0.10 - 0.05 0.055 0.045 0.45 0.15 7 nom 4 4 3.00 0.10 2.40 0.10 2.85 0.20 0.50 bsc 0.50 bsc 0.50 bsc 0.50 bsc 0.50 bsc 0.15 0.05 0.9625 0.0375 1.00 + 0.10 - 0.065 0.04 ref 0.010 2.75 0.25

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