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general description the aat2505 is a member of analogictech's total power management ic? (tpmic?) product fam- ily. it is a low dropout (ldo) linear regulator and a step-down converter with an input voltage range of 2.7v to 5.5v, making it ideal for applications with single cell lithium-ion / polymer batteries. the ldo has an independent input pin and is capa- ble of delivering up to 300ma of current. the linear regulator has been designed for high-speed turn-on and turn-off performance, fast transient response, and good power supply rejection ratio (psrr). other features include low quiescent current, low dropout voltage, and a power-ok (pok) open drain output signaling when v out is in regulation. the 600ma step-down converter is designed to operate with 1.4mhz of switching frequency, mini- mizing external component size and cost while maintaining a low 27a no load quiescent current. peak current mode control with internal compen- sation provides a stable converter with a low equiv- alent series resistance (esr) ceramic output capacitor for extremely low output ripple. for maximum battery life with high voltage outputs, the step-down converter duty cycle increases to 100%. the output voltage is either fixed or adjustable with an integrated p- and n-channel mosfet power stage and 1.4mhz switching frequency. the aat2505 is available in a pb-free, 12-pin tdfn33 package and is rated over a temperature range of -40c to +85c. features ?v in range: 2.7v to 5.5v ? 300ma ldo ? 400mv dropout voltage at 300ma ? high accuracy: 1.5% ? fast line / load transient response ? power ok output ? 600ma step-down converter ? up to 98% efficiency ? 27a no load quiescent current ? shutdown current <1a ? low r ds(on) integrated power switches ? fast turn-on time (150s typical) ? low dropout 100% duty cycle ? 1.4mhz switching frequency ? internal soft start ? over-temperature and current limit protection ? tdfn33-12 package ? -40c to +85c temperature range applications ? cellular phones ? digital cameras ? handheld instruments ? microprocessor/dsp core/io power ? pdas and handheld computers ? portable media players aat2505 dual channel, step-down converter/linear regulator typical application 2505.2006.06.1.1 1 systempower ? efficiency (v out = 2.5v; l = 6.8h) output current (ma) efficiency (%) 60 70 80 90 100 0.1 1 10 100 1000 v in = 3.3v 6.8h l1 c1 4.7f 2.2f c4 10f c3 r3 100k pgnd 1 lx 2 vp 3 vcc 4 enldo 9 en 10 fb 11 sgnd 12 vldo 5 out 6 gnd 8 pok 7 aat2505 u1 l1 sumida cdrh3d16-4r7 c1 murata grm219r61a475ke19 c3 murata grm21br60j106ke19 v in = 2.7v to 5.5v 3.3v at 300ma 2.5v at 600ma
pin descriptions pin configuration tdfn33-12 (top view) pin # symbol function 1 pgnd step-down converter power ground return pin. connect to the output and input capacitor return. see section on pcb layout guidelines and evaluation board layout diagram. 2 lx power switching node. output switching node that connects to the output inductor. 3 vp step-down converter power stage supply voltage. must be closely decoupled to pgnd. 4 vcc step-down converter bias supply. connect to vp. 5 vldo ldo input voltage; should be decoupled with 1f or greater capacitor. 6 out 300ma ldo output pin. a 2.2f or greater output low-esr ceramic capacitor is required for stability. 7 pok power-ok output for the ldo. this open drain output is low when the out is out of regu- lation. connect a pull-up resistor from pok to out or vldo. when ldo is in shutdown (enldo = 0v), pok is pulled low. 8 gnd ldo ground connection pin. 9 enldo enable pin for ldo. when connected low, ldo is disabled and consumes less than 1a of current. 10 en step-down converter enable. when connected low, the step-down converter is disabled and consumes less than 1a. 11 fb step-down converter feedback input pin. for fixed output voltage versions, this pin is con- nected to the converter output, forcing the converter to regulate to the specific voltage. for adjustable output versions, an external resistive divider ties to this point and programs the output voltage to the desired value. 12 sgnd step-down converter signal ground. for external feedback, return the feedback resistive divider to this ground. for internal fixed version, tie to the point of load return. see sec- tion on pcb layout guidelines and evaluation board layout diagram. ep exposed paddle (bottom). use properly sized vias for thermal coupling to the ground plane. see section on pcb layout guidelines. aat2505 dual channel, step-down converter/linear regulator 2 2505.2006.06.1.1 pgnd lx vp 1 vcc vldo out sgnd fb en enldo gnd pok 2 3 4 5 6 12 11 10 9 8 7
absolute maximum ratings 1 thermal information symbol description value units p d maximum power dissipation 2.0 w ja thermal resistance 2 50 c/w symbol description value units v p , v ldo input voltages to gnd 6.0 v v lx lx to gnd -0.3 to v p + 0.3 v v fb fb to gnd -0.3 to v p + 0.3 v v en en to gnd -0.3 to 6.0 v t j operating junction temperature range -40 to 150 c t lead maximum soldering temperature (at leads, 10 sec) 300 c aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 3 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 with exposed paddle connected to ground plane.
electrical characteristics 1 v in = v ldo = v out(nom) + 1v for v out options greater than 1.5v. v in = v ldo = 2.5v for v out 1.5v. i out = 1ma, c out = 2.2f, c in = 1f, t a = -40c to +85c, unless otherwise noted. typical values are t a = 25c. symbol description conditions min typ max units ldo v out output voltage tolerance i out = 1ma t a = 25c -1.5 1.5 % to 300ma t a = -40c to +85c -2.5 2.5 v in input voltage v out + 5.5 v v do 2 v do dropout voltage 3, 4 i out = 300ma 400 600 mv v out / line regulation v in = v out + 1v to 5v 0.09 %/v v out * v in v out(line) dynamic line regulation i out = 300ma, v in = v out + 1v 2.5 mv to v out + 2v, t r /t f = 2s v out(load) dynamic load regulation i out = 1ma to 300ma, t r <5s 60 mv i out output current v out > 1.3v 300 ma i sc short-circuit current v out < 0.4v 1 a i qldo ldo quiescent current v in = 5v, no load, enldo = v in 70 125 a v pok pok trip threshold v out rising, t a = 25c 90 94 98 % of v out v pokhys pok hysteresis 1.0 % of v out v pok(ol) pok output voltage low i sink = 1ma 0.4 v i pok pok output leakage current v pok < 5.5v, v out in regulation 1.0 a i shdn shutdown current v in = 5v; enldo = gnd, 1.0 a en = sgnd = pgnd i out = 10ma 1khz 65 psrr power supply rejection ratio 10khz 45 db 1mhz 42 t sd over-temperature shutdown 145 c threshold t hys over-temperature shutdown 15 c hysteresis e n output noise 250 v rms t c output voltage temperature 22 ppm/c coefficient aat2505 dual channel, step-down converter/linear regulator 4 2505.2006.06.1.1 1. the aat2505 is guaranteed to meet performance specifications over the -40c to +85c operating temperature range and is assu red by design, characterization, and correlation with statistical process controls. 2. to calculate the minimum ldo input voltage, use the following equation: v in(min) = v out(max) + v do(max) , as long as v in 2.5v. 3. for v out < 2.1v, v do = 2.5 - v out . 4. v do is defined as v in - v out when v out is 98% of nominal.
electrical characteristics 1 i out = 600ma; typical values are t a = 25c, v in = v cc = v p = 3.6v. symbol description conditions min typ max units buck converter v in input voltage 2.7 5.5 v v in rising 2.7 v v uvlo uvlo threshold hysteresis 100 mv v in falling 1.8 v v out output voltage tolerance i out = 0 to 600ma, -3.0 3.0 % v in = 2.7v to 5.5v v out output voltage range 0.6 v in v i qbuck step-down converter enldo = gnd, no load, 27 70 a quiescent current 0.6v adjustable version i shdn shutdown current en = sgnd = pgnd, 1.0 a enldo = gnd i lim p-channel current limit 800 ma r ds(on)h high side switch on resistance 0.45 r ds(on)l low side switch on resistance 0.40 i lxlk lx leakage current v in = 5.5v, v lx = 0 to v in , 1.0 a en = sgnd = pgnd v linereg line regulation v in = 2.7v to 5.5v 0.1 %/v v fb fb threshold voltage accuracy 0.6v output, no load, 591 600 609 mv t a = 25c i fb fb leakage current 0.6v output 0.2 a r fb fb impedance > 0.6v output 250 k t s start-up time from enable to output 150 s regulation f osc oscillator frequency t a = 25c 1.0 1.4 2.0 mhz t sd over-temperature shutdown 140 c threshold t hys over-temperature shutdown 15 c hysteresis logic signals v en(l) enable threshold low 0.6 v v en(h) enable threshold high 1.4 v i en(h) leakage current -1.0 1.0 a aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 5 1. the aat2505 is guaranteed to meet performance specifications over the -40c to +85c operating temperature range and is assu red by design, characterization, and correlation with statistical process controls.
typical characteristics unless otherwise noted, v in = 5v, t a = 25c. aat2505 dual channel, step-down converter/linear regulator 6 2505.2006.06.1.1 ldo initial power-up response time (en = gnd; enldo = v in ) time (400s/div) v enldo (5v/div) v out (1v/div) ldo output voltage vs. temperature (en = gnd; enldo = v in ) temperature ( c) output voltage variation (%) -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 ldo ground current vs. input voltage (en = gnd; enldo = v in ) 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 2 2.5 3 3.5 4.5 45 input voltage (v) ground current ( i out =0ma i out =10ma i out =50ma i out =150ma i out =300ma ldo dropout voltage vs. output current (en = gnd; enldo = v in ) 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 output current (ma) dropout voltage (mv) 85 c 25 c -40 c ldo dropout characteristics (en = gnd; enldo = v in ) 2.00 2.20 2.40 2.60 2.80 3.00 3.20 2.70 2.80 2.90 3.00 3.10 3.20 3.30 input voltage (v) output voltage (v) i out = 300ma i out = 150ma i out = 100ma i out = 50ma i out = 10ma i out = 0ma ldo dropout voltage vs. temperature (en = gnd; enldo = v in ) 0 60 120 180 240 300 360 420 480 540 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 temperature ( i l = 300ma i l = 150ma i l = 100ma i l = 50ma
typical characteristics unless otherwise noted, v in = 5v, t a = 25c. aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 7 ldo load transient response 300ma (en = gnd; enldo = v in ) 2.10 2.20 2.30 2.40 2.50 2.60 2.70 2.80 2.90 3.00 time (10s/div) output voltage (v) -100 0 100 200 300 400 500 600 700 800 output current (ma) v out i out ldo load transient response (en = gnd; enldo = v in ) 2.60 2.65 2.70 2.75 2.80 2.85 2.90 time (100s/div) output voltage (v) -100 0 100 200 300 400 500 output current (ma) v out i out ldo line transient response (en = gnd; enldo = v in ) 2.98 2.99 3.00 3.01 3.02 3.03 3.04 time (100s/div) input voltage (v) 0 1 2 3 4 5 6 output voltage (v) v in v out ldo turn-on time from enable (v in present) (en = gnd; enldo = v in ) time (5s/div) v in = 4v v out = 1v/div v enldo = 5v/div ldo turn-off response time (en = gnd; enldo = v in ) time (50s/div) v enldo (5v/div) v out (1v/div)
typical characteristics unless otherwise noted, v in = 5v, t a = 25c. aat2505 dual channel, step-down converter/linear regulator 8 2505.2006.06.1.1 step-down converter efficiency vs. load (v out = 3.3v; l = 10 h; enldo = gnd) output current (ma) efficiency (%) 60 70 80 90 100 0.1 1 10 100 1000 v in = 3.9v v in = 4.2v ldo enldo vs. v in 1.050 1.075 1.100 1.125 1.150 1.175 1.200 1.225 1.250 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) v ih v il ldo over-current protection (en = gnd; enldo = v in ) time (50ms/div) output current (ma) -200 0 200 400 600 800 1000 1200 step-down converter dc regulation (v out = 2.5v; l = 6.8h; enldo = gnd) output current (ma) output error (%) -1.0 -0.5 0.0 0.5 1.0 0.1 1 10 100 1000 v in = 4.2v v in = 3.6v v in = 3.0v v in = 5.0v step-down converter efficiency vs. load (v out = 2.5v; l = 10 h; enldo = gnd) output current (ma) efficiency (%) 60 70 80 90 100 0.1 1 10 100 1000 v in = 3.0v v in = 3.6v v in = 3.3v step-down converter dc regulation (v out = 3.3v; l = 6.8h; enldo = gnd) output current (ma) output error (%) -1.0 -0.5 0.0 0.5 1.0 0.1 1 10 100 1000 v in = 5.0v v in = 4.2v
aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 9 typical characteristics unless otherwise noted, v in = 5v, t a = 25c. step-down converter input current vs. input voltage (v o = 1.8v; en = v in ; enldo = gnd) input voltage (v) input current ( a) 85 c 25 c -40 c 15 20 25 30 35 2.5 3.0 3.5 4.0 4.5 5.0 5.5 step-down converter switching frequency vs. temperature (v in = 3.6v; v o = 1.5v; en = v in ; enldo = gnd) temperature ( c) frequency variation (%) -15.0 -12.0 -9.0 -6.0 -3.0 0.0 3.0 6.0 9.0 12.0 15.0 -40 -20 0 20 40 60 80 100 step-down converter output voltage error vs. temperature (v in = 3.6v; v o = 1.5v; en = v in ; enldo = gnd) temperature ( c) output error (%) -2.0 -1.0 0.0 1.0 2.0 -40 -20 0 20 40 60 80 100 step-down converter frequency vs. input voltage (v out = 1.8v; en = v in ; enldo = gnd) input voltage (v) frequency variation (%) -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 step-down converter dc regulation (v out = 1.8v; l = 4.7h; enldo = gnd) 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 v in = 4.2v v in = 2.7v step-down converter efficiency vs. load (v out = 1.5v; l = 4.7 h; enldo = gnd) output current (ma) efficiency (%) 50 60 70 80 90 100 0.1 1 10 100 1000 v in = 3.6v v in = 4.2v v in = 2.7v
aat2505 dual channel, step-down converter/linear regulator 10 2505.2006.06.1.1 typical characteristics unless otherwise noted, v in = 5v, t a = 25c. step-down converter line regulation (v out = 1.8v) input voltage (v) accuracy (%) -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 0.40 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 i out = 400ma i out = 1ma i out = 10ma step-down converter line transient (v out = 1.8v @ 400ma) output voltage (top) (v) input voltage (bottom) (v) time (25s/div) 1.74 1.76 1.78 1.80 1.82 1.84 2.6 3.6 4.6 5.6 6.6 7.6 step-down converter load transient response (1ma to 300ma; v in = 3.6v; v out = 2.5v; c1 = 4.7f; enldo = gnd) output voltage (top) (v) load and inductor current (200ma/div) (bottom) time (50s/div) 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 step-down converter load transient response (1ma to 300ma; v in = 3.6v; v out = 1.8v; c1 = 4.7f; enldo = gnd) output voltage (top) (v) load and inductor current (200ma/div) (bottom) time (50s/div) 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 step-down converter n-channel r ds(on) vs. input voltage (en = v in ; enldo = gnd) input voltage (v) r ds(on) (m ) 300 350 400 450 500 550 600 650 700 750 2.5 3 .0 3.5 4 .0 4.5 5 .0 5.5 25 c 120 c 100 c 85 c step-down converter p-channel r ds(on) vs. input voltage (en = v in ; enldo = gnd) input voltage (v) r ds(on) (m ) 300 350 400 450 500 550 600 650 700 750 2.5 3 .0 3.5 4 .0 4.5 5 .0 5.5 25 c 120 c 100 c 85 c
typical characteristics unless otherwise noted, v in = 5v, t a = 25c. aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 11 step-down converter output ripple (v in = 3.6v; v out = 1.8v; 400ma; en = v in ; enldo = gnd) output voltage (ac coupled) (top) (mv) inductor current (bottom) (a) time (250ns/div) -120 -100 -80 -60 -40 -20 0 20 40 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 step-down converter soft start (v in = 3.6v; v out = 1.8v; 400ma; en = v in ; enldo = gnd) time (100 s/div) enable and output voltage (top) (v) inductor current (bottom) (a) -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 v en i l v o
aat2505 dual channel, step-down converter/linear regulator 12 2505.2006.06.1.1 functional block diagram note: internal resistor divider included for 1.2v versions. for low voltage versions, the feedback pin is tied directly to the error amplifier input. functional description the aat2505 is a high performance power man- agement ic comprised of a buck converter and a linear regulator. the high efficiency buck convert- er is capable of delivering up to 600ma. designed to operate at 1.4mhz, the converter requires only three external components (c in , c out , and l x ) and is stable with a ceramic output capacitor. the lin- ear regulator delivers 300ma and also is stable with a ceramic output capacitor. linear regulator the advanced circuit design of the linear regulator has been specifically optimized for very fast start-up and shutdown timing. this proprietary cmos ldo has also been tailored for superior transient response characteristics. these traits are particularly important for applications that require fast power supply timing. the high-speed turn-on capability is enabled through implementation of a fast-start control cir- 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 shutdown cir- cuit has no adverse effect on normal device opera- tion. the ldo regulator output has been specifi- cally optimized to function with low-cost, low-esr ceramic capacitors; however, the design will allow for operation over a wide range of capacitor types. other features include an integrated power-ok comparator which indicates when the output is out of regulation. the pok open-drain output is low when out is 6% below its nominal regulation voltage. the open-drain signal is held low when the linear regula- tor is in shutdown mode. the regulator comes with complete short-circuit and thermal protection. the combination of these two internal protection circuits gives a comprehensive safety system to guard against extreme adverse operating conditions. en lx error amp. logic dh dl pgnd vp fb gnd voltage reference voltage reference out over-current protection control logic vldo fast start control enldo sgnd vcc see note error amp. - + 94% pok
the regulator features an enable/disable function. this pin (enldo) is active high and is compatible with cmos logic. to assure the ldo regulator will switch on, the enldo 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 pin falls below 0.6v. if the enable function is not needed in a specific application, it may be tied to v in to keep the ldo regulator in a continuously on state. when the regulator is in shutdown mode, an inter- nal 20k resistor is connected between out and gnd. this is intended to discharge c out when the ldo regulator is disabled. the internal 20k resis- tor has no adverse impact on device turn-on time. step-down converter the aat2505 buck is a constant frequency peak current mode pwm converter with internal compen- sation. 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 for the internally fixed version (see figure 1) , and up to 3.3v for the exter- nally adjustable version (see figure 2). the 0.6v fixed model is also the adjustable version and is externally programmable with a resistive divider. the converter mosfet power stage is sized for 600ma load capability with up to 96% efficiency. light load efficiency exceeds 80% at a 500a load. soft start the aat2505 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. a startup time of 150s is achieved across the operating range. 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 1.4mhz 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 140c with 15c of hysteresis. aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 13 figure 1: aat2505 fixed output. figure 2: aat2505 with adjustable step-down output and enhanced transient response. l1 4.7f c1 v in r1 59k r2 4.7f 4.7h c4 10f c3 v outldo v outbuck 100pf c8 pgnd 1 lx 2 vp 3 vcc 4 enldo 9 en 10 fb 11 sgnd 12 vldo 5 out 6 gnd 8 pok 7 aat2505 u1 r3 100k l1 4.7h 4.7f c1 v outbuck v in 4.7f c4 10f c3 v outldo pgnd 1 lx 2 vp 3 vcc 4 enldo 9 en 10 fb 11 sgnd 12 vldo 5 out 6 gnd 8 pok 7 aat2505 u1 r3 100k
aat2505 dual channel, step-down converter/linear regulator 14 2505.2006.06.1.1 applications information linear regulator input and output capacitors: an input capacitor is not required for basic operation of the linear reg- ulator. however, if the aat2505 is physically locat- ed more than three centimeters from an input power source, a c in capacitor will be needed for stable operation. typically, a 1f or larger capaci- tor is recommended for c in in most applications. c in should be located as closely to the device v in 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 c in . there is no specific capacitor esr requirement for c in . however, for 300ma ldo reg- ulator output operation, ceramic capacitors are rec- ommended for c in 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 out and gnd. the c out capacitor connection to the ldo regulator ground pin should be made as directly as practically possible for maximum device perform- ance. since the regulator has been designed to function with very low esr capacitors, ceramic capacitors in the 1.0f to 10f range are recom- mended for best performance. applications utilizing the exceptionally low output noise and optimum power supply ripple rejection should use 2.2f or greater for c out . in low output current applications, where output load is less than 10ma, the minimum value for c out can be as low as 0.47f. 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 capaci- tors and can range to more than several ohms for tantalum or aluminum electrolytic capacitors. ceramic capacitor materials: ceramic capacitors less than 0.1f are typically made from npo or c0g materials. npo and c0g materials generally have tight tolerance and are very stable over tem- perature. larger capacitor values are usually com- posed of x7r, x5r, z5u, or y5v dielectric materi- als. large ceramic capacitors (i.e., greater than 2.2f) are often available in low-cost y5v and z5u dielectrics. these two material types are not rec- ommended for use with the regulator, since the capacitor tolerance can vary more than 50% over the operating temperature range of the device. a 2.2f y5v capacitor could be reduced to 1f over temperature; this could cause problems for circuit operation. x7r and x5r dielectrics are much more desirable. the temperature tolerance of x7r dielec- tric is better than 15%. capacitor area is another contributor to esr. capacitors that are physically large in size will have a lower esr when compared to a smaller sized capacitor of an equivalent material and capaci- tance value. these larger devices can improve cir- cuit transient response when compared to an equal value capacitor in a smaller package size. consult capacitor vendor datasheets carefully when select- ing capacitors for ldo regulators. 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 aat2505 is 0.24a/sec. this equates to a slope compensation that is 75% of the inductor current down slope for a 1.5v output and 4.7h inductor. 0.75 ? v o m = = = 0.24 l 0.75 ? 1.5v 4.7 h a sec
this is the internal slope compensation for the adjustable (0.6v) version or low-voltage fixed ver- sions. when externally programming the 0.6v ver- sion to 2.5v, the calculated inductance is 7.5h. in this case, a standard 6.8h value is selected. for high-voltage fixed versions (2.5v and above), m = 0.48a/sec. table 1 displays inductor values for the aat2505 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 4.7h cdrh3d16 series inductor selected from sumida has a 105m dcr and a 900ma dc current rating. at full load, the inductor dc loss is 17mw which gives a 2.8% loss in efficiency for a 400ma, 1.5v output. 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 c. the calculated value varies with input voltage and is a maximum when v in is double the output voltage. 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: aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 15 table 1: inductor values. configuration output voltage inductor 0.6v adjustable with 1v, 1.2v 2.2h external feedback 1.5v, 1.8v 4.7h 2.5v, 3.3v 6.8h fixed output 0.6v to 3.3v 4.7h ?? 1 - = for v in = 2 v obuck ?? v obuck v in v obuck v in 1 4 ?? 1 - ?? v obuck v in c in = v obuck v in ?? - esr f s ?? v pp i obuck 0.75 ? v o l = = 3 ? v o = 3 ? 2.5v = 7.5 h m 0.75 ? v o 0.24a sec a sec a a sec ?? i rms = i obuck 1 - ?? v obuck v in v obuck v in c in(min) = 1 ?? - esr 4 f s ?? v pp i obuck
aat2505 dual channel, step-down converter/linear regulator 16 2505.2006.06.1.1 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 current. for v in = 2 x v obuck the term appears in both the input voltage ripple and input capacitor rms cur- rent equations and is a maximum when v obuck is twice v in . 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 aat2505. 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 measurements 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. 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. this dampens the high q network and stabilizes the system. output capacitor the output capacitor limits the output ripple and provides holdup during large load transitions. a 4.7f to 10f x5r or x7r ceramic capacitor pro- vides sufficient bulk capacitance to stabilize the output during large load transitions and has the esr and esl characteristics necessary for low output ripple. figure 3: aat2505 evaluation board top side. figure 4: aat2505 evaluation board bottom side. ?? 1 - ?? v obuck v in v obuck v in i obuck rms(max) i 2 = ?? 1 - = d (1 - d) = 0.5 2 = ?? v obuck v in v obuck v in 1 2
aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 17 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 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 resistor selection for applications requiring an adjustable output volt- age, the 0.6v version can be externally pro- grammed. resistors r1 and r2 of figure 5 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 suggested 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 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 aat2505, combined 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 3.3 267 1000 ?? ?? r1 = -1 r2 = - 1 59k = 88.5k v out v ref ?? ?? 1.5v 0.6v 1 23 v out (v in(max) - v out ) rms(max) i l f s v in(max) = c out = 3 i load v droop f s
figure 5: aat2505 evaluation board schematic. aat2505 dual channel, step-down converter/linear regulator 18 2505.2006.06.1.1 thermal calculations there are three types of losses associated with the aat2505 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. 1. for step-down converter, enhanced transient configuration c8 = 100pf and c1 = 10f. table 3 l1 4.7f 1 c1 10f c2 gnd v in1 1 2 3 buck enable lx1 gnd table 3 r1 59k r3 100k r2 pgnd 1 lx 2 vp 3 vcc 4 enldo 9 en 10 fb 11 sgnd 12 in 5 out 6 gnd 8 pok 7 aat2505 u1 4.7f c4 10f c3 1 2 3 ldo enable v outldo v outbuck 1 2 3 ldo input 0.01f c7 c8 1 n/a c9 1 pok p total = i obuck 2 r dson(h) + i oldo (v in - v oldo ) + (i qbuck + i qldo ) v in p total i obuck 2 (r dson(h) v obuck + r dson(l) [v in - v obuck ]) v in = + (t sw f s i obuck + i qbuck + i qldo ) v in + i oldo (v in - v oldo )
given the total losses, the maximum junction tem- perature can be derived from the ja for the tdfn33-12 package which is 50c/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 vp and pgnd, as shown in figure 4. 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 pin. this prevents noise from being coupled into the high impedance feed- back 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. 5. for good thermal coupling, pcb vias are required from the pad for the tdfn paddle to the ground plane. the via diameter should be 0.3mm to 0.33mm and positioned on a 1.2mm grid. aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 19 t j(max) = p total ja + t amb
aat2505 dual channel, step-down converter/linear regulator 20 2505.2006.06.1.1 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 s = 1.4mhz t amb = 85c 1.8v buck output inductor (use 4.7h; see table 1) for sumida inductor cdrh3d16, 4.7h, dcr = 105m . 1.8v buck output capacitor v droop = 0.1v 1 23 1 1.8v (4.2v - 1.8v) 4.7 h 1.4mhz 4.2v 23 rms i l1 f s v in(max) = 3 i load v droop f s 3 0.3a 0.1v 1.4mhz c out = = = 6.4 f; use 10 f = 45marms (v o ) (v in(max) - v o ) = p esr = esr i rms 2 = 5m (45ma) 2 = 10 w v o v o 1.8 v 1.8v i l1 = ? 1 - = ? 1 - = 156m a l1 ? f s v in 4.7 h ? 1.4mhz 4.2v i pkl1 = i o + i l1 = 0.4a + 0.068a = 0.468a 2 p l1 = i o 2 ? dcr = 0.4a 2 ? 105m = 17mw ? ? ? ? ? ? ? ? l1 = 3 ? v o2 = 3 ? 1.8v = 5.4h sec a sec a
1.8v buck input capacitor input ripple v pp = 25mv aat2505 total losses aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 21 t j(max) = t amb + ja p loss = 85 c + (50 c/w) 395mw = 105 c p total + (t sw f s i obuck + i qbuck + i qldo ) v in + (v in - v oldo ) i oldo i obuck 2 (r dson(h) v obuck + r dson(l) [v in - v obuck ] ) v in = = + (5ns 1.4mhz 0.4a + 50a +125a) 4.2v + (4.2v - 3.3v) 0.3a = 395mw 0.4 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 = = 0.2arms c in = = = 4.75 f 1 ?? - esr 4 f s ?? v pp i obuck 1 ?? - 5m 4 1.4mhz ?? 25mv 0.4a
aat2505 dual channel, step-down converter/linear regulator 22 2505.2006.06.1.1 table 3: evaluation board component values. table 4: typical surface mount inductors. inductance max dc dcr size (mm) manufacturer part number (h) current (a) ( ) lxwxh type sumida cdrh3d16-4r7 4.7 0.90 0.11 4.0x4.0x1.8 shielded sumida cdrh3d16/hp100 10 0.84 0.23 4.0x4.0x1.8 shielded murata lqh32cn4r7m23 4.7 0.45 0.20 2.5x3.2x2.0 non-shielded murata lqh32cn4r7m33 4.7 0.65 0.15 2.5x3.2x2.0 non-shielded murata lqh32cn4r7m53 4.7 0.65 0.15 2.5x3.2x1.55 non-shielded coilcraft lpo6610-472 4.7 1.10 0.20 5.5x6.6x1.0 1mm coilcraft lpo3310-472 4.7 0.80 0.27 3.3x3.3x1.0 1mm coiltronics sdrc10-4r7 4.7 1.53 0.117 4.5x3.6x1.0 1mm shielded coiltronics sdr10-4r7 4.7 1.30 0.122 5.7x4.4x1.0 1mm shielded coiltronics sd3118-4r7 4.7 0.98 0.122 3.1x3.1x1.85 shielded coiltronics sd18-4r7 4.7 1.77 0.082 5.2x5.2x1.8 shielded v out (v) r1 (k ) r1 (k ) l1 (h) adjustable version r2 = 59k r2 = 221k 1 1 (0.6v device) 0.8 19.6 75.0 4.7 0.9 29.4 113 4.7 1.0 39.2 150 4.7 1.1 49.9 187 4.7 1.2 59.0 221 4.7 1.3 68.1 261 4.7 1.4 78.7 301 4.7 1.5 88.7 332 4.7 1.8 118 442 4.7 1.85 124 464 4.7 2.0 137 523 4.7 or 6.8 2.5 187 715 10 3.3 267 1000 10 v out (v) r1 (k ) l1 (h) fixed version r2 not used 0.6-3.3v 0 4.7 1. for reduced quiescent current r2 = 221k .
aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 23 table 5: surface mount capacitors. manufacturer part number value voltage temp. co. case murata grm219r61a475ke19 4.7f 10v x5r 0805 murata grm21br60j106ke19 10f 6.3v x5r 0805
aat2505 dual channel, step-down converter/linear regulator 24 2505.2006.06.1.1 ordering information voltage package buck converter ldo marking 1 part number (tape and reel) 2 tdfn33-12 adj. 2.8v poxyy aat2505iwp-aq-t1 tdfn33-12 adj. 2.6v ppxyy AAT2505IWP-AO-T1 1. xyy = assembly and date code. 2. sample stock is generally held on part numbers listed in bold . 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 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.
tdfn33-12 all dimensions in millimeters. aat2505 dual channel, step-down converter/linear regulator 2505.2006.06.1.1 25 top view bottom view detail "b" detail "a" side view 3.00 1 1 2.40 1 1 1 1 1 +
aat2505 dual channel, step-down converter/linear regulator 26 2505.2006.06.1.1 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 specifi cations or to discontinue any product or service without notice. customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information b eing relied on is current and complete. all products are sold sub- ject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. analogictech warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with anal ogictech?s standard warranty. testing and other quality con- trol techniques are utilized to the extent analogictech deems necessary to support this warranty. specific testing of all param eters of each device is not necessarily performed. analogictech and the analogictech logo are trademarks of advanced analogic technologies incorporated. all other brand and produ ct names appearing in this document are regis- tered trademarks or trademarks of their respective holders.

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