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aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 1 www.analogictech.com general description the aat2784 is a 3-channel 1.8mhz step-down con- verter for applications where power efficiency and solu- tion size are critical. the input voltage range is 2.7v to 5.5v and the outputs are adjustable from 0.6v to v in . channel 3 delivers up to 1.5a output current and chan- nels 1 and 2 deliver up to 300ma each. the aat2784 uses a high switching frequency to minimize the size of external components. the aat2784 requires a minimum of external components to realize a high efficiency triple- output buck converter minimizing solution cost and pcb footprint. each of the 3 regulators has an independent enable pin, adjustable output voltage and operates with low no load quiescent current, providing high efficiency over the entire load range. the aat2784 is available in a pb-free 16 pin tdfn34 package, and is rated over the -40c to +85c operating temperature range. features ? v in range: 2.7 to 5.5v ? output voltage range: 0.6v to v in ? output current: ? channel 3: 1.5a ? channel 1: 300ma ? channel 2: 300ma ? highly efficient step-down converters ? low r ds(on) integrated power switches ? 100% duty cycle ? 1.8 mhz switching frequency ? internal soft start ? fast 150 s turn-on time ? over-temperature protection ? current limit protection ? tdfn34-16 package ? -40c to 85c temperature range applications ? cellular and smart phones ? digital cameras ? handheld instruments ? mass storage systems ? microprocessor / dsp core / io power ? pdas and handheld computers ? portable media players ? usb devices ? wireless lan typical application l3 1.5 h c5 10 f aat2784 agnd vp3 en 3 pgnd fb3 lx3 c2 10 f vin : 2. 7 C 5 .5v vout3: 1.2v 1.5 a r6 59 . 0k r5 59 . 0k c4 4.7f fb2 lx 2 vout2: 3.3v 300 ma r4 59 .0k r3 267 k l1 4.7h l2 4.7h c3 4. 7 f fb1 lx1 r2 59 .0 k r1 267 k vp 1_ 2 en 1 c1 10 f en2 vout1: 3. 3v 300 ma pgnd in
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2 2784.2007.11.1.1 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2 2784.2007.11.1.1 www.analogictech.com pin descriptions pin # symbol function 1 pgnd2 power ground return pin 2. connect to the output and input capacitor return. 2 fb2 feedback input pin for channel 2. connect an external resistor divider to this pin to program the output volt- age to the desired value. 3 en1 enable pin for channel 1. active high. 4 en2 enable pin for channel 2. active high. 5 agnd signal ground. 6 in input supply pin for device. supplies bias for the internal circuitry. 7 en3 enable pin for channel 3. active high. 8 fb3 feedback input pin for channel 3. connect an external resistor divider to this pin to program the output volt- age to the desired value. 9 pgnd3 power ground return for channel 3. connect to the output and input capacitor return. 10 lx3 power switching node for channel 3. output switching node connects to the output inductor. 11 vp3 input power supply pin for channel 3. must be closely decoupled. 12 fb1 feedback input pin for channel 1. connect an external resistor divider to this pin to program the output volt- age to the desired value. 13 pgnd1 power ground return for channel 1. connect to the output and input capacitor return. 14 lx1 power switching node for channel 1. output switching node connects to the output inductor. 15 vp1_2 input power supply pin for channels 1 and 2. must be closely decoupled. 16 lx2 power switching node for channel 2. output switching node connects to the output inductor. ep ep exposed pad. connect to ground directly under the device. use properly sized vias for thermal coupling to the ground plane. see section on pcb layout guidelines. pin configuration tdfn34-16 (top view) en1 en2 agnd pgnd2 fb2 3 in en3 fb3 lx1 pgnd1 fb1 lx2 vp1_2 vp3 lx3 pgnd3 4 5 1 2 6 7 8 14 13 12 16 15 11 10 9
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 3 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 3 www.analogictech.com absolute maximum ratings 1 symbol description value units v in , v p input voltages to agnd/pgnd 6.0 v v lx lx1, lx2, lx3 to agnd/pgnd -0.3 to v in + 0.3 v v fb fb1, fb2, fb3 to agnd/pgnd -0.3 to v in + 0.3 v v en en1, en2, en3 to agnd/pgnd -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 thermal information symbol description value units p d maximum power dissipation 2 2.0 w ja thermal resistance 3 50 c/w 1. stresses above those listed in absolute maximum ratings may cause permanent damage to the device. functional operation at c onditions 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 ambient temperature.
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 4 2784.2007.11.1.1 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 4 2784.2007.11.1.1 www.analogictech.com electrical characteristics 1 v in = v p = 3.6v; t a = -40c to 85c, unless noted otherwise. typical values are at t a = 25c. symbol description conditions min typ max units v in input voltage 2.7 5.5 v v out output voltage tolerance i out1 = 0 to 1.5a; i out2,3 = 0 to 300ma; v in = 2.7 to 5.5v -3.0 3.0 % v out output voltage range 0.6 v in v i q1,2 quiescent current channels 1, 2 per channel, no load 50 100 a i q3 quiescent current channel 3 no load 45 90 a i shdn shutdown current v en1 = v en2 = v en3 = gnd 1.0 a i lx _ leak lx reverse leakage current v in open, v lx = 5.5v; v en = 0v 1.0 a i lx _ leak lx leakage current v in = 5.5v, v lx = 0 to v in 1.0 a i fb feedback leakage v fb = 1.0v 0.2 a i lim1,2 p-channel current limit 1.8 a i lim3 p-channel current limit 3.81 a r ds(on)h1,2 high side switch on-resistance 480 m r ds(on)l1,2 low side switch on-resistance 400 m r ds(on)h3 high side switch on-resistance 150 m r ds(on)l3 low side switch on-resistance 120 m v loadreg load regulation i load1,2 = 0 to 300 ma; i load3 = 0 to 1.5a 0.8 % v linereg line regulation v in = 2.7 to 5.5v 0.5 % f osc1,2 oscillator frequency channels 1, 2 1.8 mhz f osc3 oscillator frequency channel 3 1.8 mhz t s start-up time from enable to output regulation 150 s t sd over-temperature shutdown threshold 140 c t hys over-temperature shutdown hysteresis 15 c v il enable threshold low 0.6 v v ih enable threshold high 1.4 v i en enable input current v in = v en = 5.5v -1.0 1.0 a 1. the aat2784 is guaranteed to meet performance specifications over the ?40 c to +85 c operating temperature range, and is assured by design, characterization and correla- tion with statistical process controls.
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 5 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 5 www.analogictech.com typical characteristics efficiency vs. output current (channels 1 and 2; v out = 3.3v) output current (ma) efficiency (%) 20 30 40 50 60 70 80 90 100 0.1 1 10 100 1000 v in = 3.6v v in = 4.2v v in = 5.0v load regulation (channels 1 and 2; v out = 3.3v) output current (ma) output error (%) -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 0.1 1 10 100 1000 v in = 3.6v v in = 4.2v v in = 5.0v efficiency vs. output current (channel 3; v out = 1.2v) output current (ma) efficiency (%) 0 10 20 30 40 50 60 70 80 90 100 0.1 1 10 100 1000 10000 v in = 4.2v v in = 3.6v v in = 2.7v load regulation (channel 3; v out = 1.2v) output current (ma) output error (%) -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 0.1 1 10 100 1000 10000 v in = 2.7v v in = 3.6v v in = 4.2v switching frequency vs. input voltage input voltage (v) switching frequency (%) -10 -8 -6 -4 -2 0 2 4 6 8 10 2.3 2.8 3.3 3.8 4.3 4.8 5.3 5. 8 channel 3 channels 1 and 2 output error vs. temperature temperature (c) output error (%) -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 -40 -15 10 35 60 85 channels 1 and 2 channel 3
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 6 2784.2007.11.1.1 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 6 2784.2007.11.1.1 www.analogictech.com typical characteristics quiescent current vs. input voltage (channels 1 and 2; v out = 3.3v; no load; open loop) input voltage (v) supply current (a) 0 10 20 30 40 50 60 70 80 90 100 3.2 3.5 3.8 4.1 4.4 4.7 5 5.3 5.6 85c 25c -40c quiescent current vs. input voltage (channel 3; v out = 1.2v; no load; open loop) input voltage (v) supply current (a) 0 10 20 30 40 50 60 70 80 90 100 2.6 2.9 3.2 3.5 3.8 4.1 4.4 4.7 5 5.3 5. 6 85c 25c -40c p-channel on-resistance vs. input voltage (channels 1 and 2; v out = 3.3v) input voltage (v) on-resistance (m ) 300 400 500 600 700 800 900 1000 3.2 3.6 4 4.4 4.8 5.2 5.6 25c 85c 100c p-channel on-resistance vs. input voltage (channel 3; v out = 1.2v) input voltage (v) switch on-resistance (m ) 0 50 100 150 200 250 300 2.6 3.1 3.6 4.1 4.6 5.1 5. 6 . 25c 85c 100c v ih vs. input voltage input voltage (v) v ih (v) 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 2.6 3.1 3.6 4.1 4.6 5.1 5. 6 85c 25c -40c v il vs. input voltage input voltage (v) v il (v) 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 2.6 3.1 3.6 4.1 4.6 5.1 5.6 25c -40c 85c
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 7 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 7 www.analogictech.com typical characteristics load transient (channels 1 and 2; v in = 3.6v; i out = 100 to 300ma; v out = 3.3v) time (50s/div) output voltage (top) (v) output current (middle) (ma) inductor current (bottom) (ma) -0.5 0 0.5 100ma 300ma 100ma 300ma load transient (channels 1 and 2; v in = 5v; i out = 1 to 300ma; v out = 3.3v) time (50s/div) output voltage (top) (v) output current (middle) (ma) inductor current (bottom) (ma) -0.5 0 0.5 1ma 1ma 300ma 300ma load transient (channel 3; v in = 3.6v; i out = 0.5 to 1.5a; v out = 1.2v; no c ff ) time (50s/div) output voltage (top) (v) output current (middle) (a) inductor current (bottom) (a) -0.5 0 0.5 0.5a 0.5a 1.5a 1.5a load transient (channel 3; v in = 5v; i out = 0.5 to 1.5a; v out = 1.2v; no c ff ) time (50s/div) output voltage (top) (v) output current (middle) (a) inductor current (bottom) (a) -0.5 0 0.5 0.5a 0.5a 1.5a 1.5a soft start (channels 1 and 2; v in = 5v; v out = 3.3v; i out = 50ma) time (50s/div) enable voltage (top) (v) output voltage (middle) (v) inductor current (bottom) (a) 0 1 2 3 4 0 0.5 1 soft start (channel 3; v in = 5v; v out = 1.2v; i out = 1ma) time (50s/div) enable voltage (top) (v) output voltage (middle) (v) inductor current (bottom) (a) 0 1 2 3 4 0 0.5 1
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 8 2784.2007.11.1.1 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 8 2784.2007.11.1.1 www.analogictech.com typical characteristics soft start (channel 3; v in = 5v; v out = 1.2v; i out = 1.5a) time (50s/div) enable voltage (top) (v) output voltage (middle) (v) inductor current (bottom) (a) 0 1 2 3 4 0 0.5 1 1.5 line transient (channels 1 and 2; v out = 3.3v; v in = 3.6 to 4.2v; i out = 300ma) time (100ms/div) input voltage (top) (v) output voltage (bottom) (v) 3.6 4.2 4.8 -0.04 -0.02 0 0.02 0.04 line transient (channel 3; v out = 1.2v; v in = 3.6 to 4.2v; i out = 1.5a) time (50s/div) input voltage (top) (v) output voltage (bottom) (v) 3.5 4 4.5 5 -0.04 -0.02 0 0.02 0.04 line regulation (channels 1 and 2; v out = 3.3v) input voltage (v) accuracy (%) -2 -1.5 -1 -0.5 0 0.5 1 3.2 3.7 4.2 4.7 5.2 5.7 i out = 10ma i out = 300ma i out = 100ma line regulation (channel 3; v out = 1.2v) input voltage (v) accuracy (%) -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 2.6 3.1 3.6 4.1 4.6 5.1 5.6 i out = 10ma i out = 1000ma i out = 1500ma i out = 100ma
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 9 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 9 www.analogictech.com typical characteristics output ripple (channels 1 and 2; v out = 3.3v; v in = 4.6v; i out = 1ma) time (400ns/div) output voltage (top) (v) inductor current (bottom) (a) -0.01 0 0.01 0 0.1 0.2 output ripple (channels 1 and 2; v out = 3.3v; v in = 4.6v; i out = 300ma) time (400ns/div) output voltage (top) (v) inductor current (bottom) (a) -0.01 0 0.01 0 0.1 0.2 0.3 0.4 output ripple (channel 3; v out = 1.2v; v in = 4.6v; i out = 1.5a) time (400ns/div) output voltage (top) (v) inductor current (bottom) (a) -0.01 0 0.01 0 0.5 1 1.5 2 output ripple (channels 1 and 2; v out = 3.3v; v in = 3.6v; i out = 300ma) time (400ns/div) output voltage (top) (v) inductor current (bottom) (a) -0.01 0 0.01 0 0.1 0.2 0.3 0.4 output ripple (channel 3; v out = 1.2v; v in = 3.6v; i out = 1.5a) time (400ns/div) output voltage (top) (v) inductor current (bottom) (a) -0.01 0 0.01 0 0.5 1 1.5 2 output ripple (channels 1 and 2; v out = 3.3v; v in = 5v; i out = 300ma) time (400ns/div) output voltage (top) (v) inductor current (bottom) (a) -0.01 0 0.01 0 0.1 0.2 0.3 0.4
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 10 2784.2007.11.1.1 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 10 2784.2007.11.1.1 www.analogictech.com typical characteristics output ripple (channel 3; v out = 1.2v; v in = 5v; i out = 1.5a) time (400ns/div) output voltage (top) (v) inductor current (bottom) (a) -0.01 0 0.01 0 0.5 1 1.5 2 output ripple (channel 3; v out = 1.2v; v in = 4.2v; i out = 1ma) time (400ns/div) output voltage (top) (v) inductor current (bottom) (a) -0.02 0 0.02 0.04 0 0.2 0.4
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 11 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 11 www.analogictech.com functional description the aat2784 is a high performance power management ic comprised of 3 buck converters. each channel has an independent enable pin. operating at a switching fre- quency of 1.8mhz, the converter requires a minimum of small external components, reducing the solution cost and pcb footprint. all converters operate with an input voltage range of 2.7v to 5.5v. the output voltage range is 0.6v to v in and is adjustable with an external resistor divider. channel 3 power devices are sized for 1.5a output cur- rent. channels 1 and 2 power devices are sized for 300ma output current while maintaining over 85% effi- ciency at full load. peak efficiency is above 95%. light load efficiency is maintained at greater than 80% down to 85% of full load current. all channels have excellent transient response, load and line regulation. transient response time is typically less than 20 s. soft start limits the current surge seen at the input and eliminates output voltage overshoot. the enable inputs, when pulled low, force the respective converter into a low power non-switching state consuming less than 1 a of current. for overload conditions, the peak input current is limit- ed. also, thermal protection completely disables switch- ing if internal dissipation becomes excessive, thus pro- tecting the device from damage. the junction over- temperature threshold is 140 ? c with 15 ? c of hysteresis. under-voltage lockout (uvlo) guarantees sufficient v in bias and proper operation of all internal circuits prior to activation. control loop the aat2784 is a peak current mode step-down con- verter. the current through the p-channel mosfet (high side) is sensed for current loop control, as well as short- functional block diagram logic comp. error amp control logic logic error amp control logic osc ot osc comp. vp3 lx 3 pgnd3 vp1_2 lx2 pgnd2 lx1 pgnd1 fb2 en2 fb1 en1 fb3 en3 logic error amp control logic voltage ref comp. in agnd
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 12 2784.2007.11.1.1 www.analogictech.com circuit and overload protection. a fixed slope compensa- tion signal is added to the sensed current to maintain stability for duty cycles greater than 50%. the peak cur- rent mode loop appears as a voltage-programmed cur- rent source in parallel with the output capacitor. the output of the voltage error amplifier programs the cur- rent mode loop for the necessary peak switch current to force a constant output voltage for all load and line con- ditions. internal loop compensation terminates the transconductance voltage error amplifier output. the reference voltage is internally set to program the con- verter output voltage greater than or equal to 0.6v. soft start/enable soft start limits the current surge seen at the input and eliminates output voltage overshoot. when pulled low, the enable input forces the aat2784 into a low-power, non-switching state. the total input current during shut- down is less than 1 a. low dropout operation for conditions where the input voltage drops to the out- put voltage level, the converter duty cycle increases to 100%. as the converter approaches the 100% duty cycle, the minimum off time initially forces the high side in time to exceed the 1.8mhz clock cycle and reduce the effective switching frequency. once the input drops below the level where the converter can regulate the output, the high side p-channel mosfet is enabled con- tinuously for 100% duty cycle. at 100% duty cycle the output voltage tracks the input voltage minus the i*r drop of the high side p-channel mosfet. current limit and over-temperature protection for overload conditions, the peak input current is limit- ed. to minimize power dissipation and stresses under current limit and short-circuit conditions, switching is terminated after entering current limit for a series of pulses. switching is terminated for seven consecutive clock cycles after a current limit has been sensed for a series of four consecutive clock cycles. thermal protec- tion completely disables switching when internal dissipa- tion becomes excessive. the junction over-temperature threshold is 140c with 15c of hysteresis. once an over-temperature or over-current fault condition is removed, the output voltage automatically recovers. under-voltage lockout internal bias of all circuits is controlled via the v in input. under-voltage lockout (uvlo) guarantees sufficient v in bias and proper operation of all internal circuitry prior to activation. component selection inductor selection: channels 1 and 2 the step-down converter uses peak current mode con- trol with slope compensation to maintain stability for duty cycles greater than 50%. the output inductor value must be selected so the inductor current down slope meets the internal slope compensation requirements. the internal slope compensation for the adjustable and low voltage fixed versions of channels 1 and 2 is 0.6a/ . this equates to a slope compensation that is 75% of the inductor current down slope for a 1.8v output and 2.2 h inductor. 0.75 ? v o m = = = 0.6 l 0.75 ? 1.8v 2.2h a s 0.75 ? v o l = = = 4.1h m 0.75 ? 3.3v 0.6 a s in this case a standard 4.7 h value is selected. table 1 displays the suggested inductor values for channels 1 and 2. the 4.7 h cdrh2d11 series inductor selected from sumida has a 170m dcr and a 0.88a dc current rating. at full load the inductor dc loss is 15mw which corresponds to a 1.5% loss in efficiency for a 300ma, 3.3v output. for 4.7 h glf2518t4r7m series tdk inductor has a 260m worst case dcr and a 475ma dc current rating. at full 300ma load, the inductor dc loss is 23mw which gives less than 7% loss in efficiency for a 300ma, 3.3v output. inductor selection: channel 3 the internal slope compensation for the adjustable and low voltage fixed versions of channel 3 is 0.75a/ s. this equates to a slope compensation that is 75% of the inductor current down slope for a 1.8v output and 1.8 h inductor. 0.75 ? v o m = = = 0.75 l 0.75 ? 1.8v 1.8h a s
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 13 www.analogictech.com 0.75 ? v o l = = = 1.2h m 0.75 ? 1.2v 0.75 a s the inductor should be set equal to the output voltage numeric value in micro henries ( h). this guarantees that there is sufficient internal slope compensation. manufacturer?s specifications list both the inductor dc current rating, which is a thermal limitation, and the peak current rating, which is determined by the satura- tion characteristics. the inductor should not show any appreciable saturation under normal load conditions. some inductors may meet the peak and average current ratings yet result in excessive losses due to a high dcr. always consider the losses associated with the dcr and its effect on the total converter efficiency when selecting an inductor. for channel 3, the 1.5 h lqh32pn1r5nn0l series murata inductor has a 68.4m worst case dcr and a 1.75a dc current rating. at full 1.5a load, the inductor dc loss is 154mw which gives less than 5% loss in efficiency for a 1.5a, 1.2v output. input capacitor select a 10 f to 22 f x7r or x5r ceramic capacitor for the vp1_2 and vp3 inputs. to estimate the required input capacitor size, determine the acceptable input ripple level (v pp ) and solve for c in . the calculated value varies with input voltage and is a maximum when v in is double the output voltage. con guration output voltage inductor slope compensation 0.6v adjustable with external resistive divider 0.6v- 2.0v 2.2 h 0.6a/ s 2.5v 3.3 h 3.3v 4.7 h table 1: aat2784 inductor values. ?? 1 - ?? v o v in c in = v o v in ?? - esr f s ?? v pp i o ?? 1 - = for v in = 2 v o ?? v o v in v o v in 1 4 c in(min) = 1 ?? - esr 4 f s ?? v pp i o always examine the ceramic capacitor dc voltage coef- ficient characteristics when selecting the proper value. for example, the capacitance of a 10 f, 6.3v, x5r ceramic capacitor with 5.0v dc applied is actually about 6 f. the maximum input capacitor rms current is: ?? i rms = i o 1 - ?? v o v in v o v in 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. ?? 1 - = d (1 - d) = 0.5 2 = ?? v o v in v o v in 1 2 for v in = 2 v o i o rms(max) i 2 = the term appears in both the input voltage ripple and input capacitor rms current equations and is a maxi- mum when v o is twice v in . this is why the input voltage ripple and the input capacitor rms current ripple are a maximum at 50% duty cycle. the input capacitor pro- vides a low impedance loop for the edges of pulsed cur- rent drawn by the aat2784. low esr/esl x7r and x5r ceramic capacitors are ideal for this function. to mini- mize stray inductance, the capacitor should be placed as closely as possible to the ic. this keeps the high fre- quency content of the input current localized, minimizing emi and input voltage ripple. the proper placement of the input capacitor (c1) can be seen in the evaluation board layout in the layout section of this datasheet (see figure 2). 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 inductance of these wires, along with the low-esr ceramic input capacitor, can create a high q network that may affect converter performance. this problem often becomes apparent in the form of excessive ringing in the output voltage during load transients. errors in the loop phase and gain measurements can also result. since the induc- tance 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 prob- lem. in applications where the input power source lead inductance cannot be reduced to a level that does not
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 14 2784.2007.11.1.1 www.analogictech.com 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 damp- ens the high q network and stabilizes the system. output capacitor: channels 1 and 2 the output capacitor limits the output ripple and pro- vides holdup during large load transitions. a 4.7 f to 10 f x5r or x7r ceramic capacitor typically provides sufficient bulk capacitance to stabilize the output during large load transitions and has the esr and esl charac- teristics necessary for low output ripple. the output volt- age droop due to a load transient is dominated by the capacitance of the ceramic output capacitor. during a step increase in load current, the ceramic output capac- itor 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 voltage droop during the three switching cycles to the output capacitance can be estimated by: c out = 3 i load v droop f s once the average inductor current increases to the dc load level, the output voltage recovers. the above equa- tion establishes a limit on the minimum value for the output capacitor with respect to load transients. the internal voltage loop compensation also limits the mini- mum output capacitor value to 4.7 f. this is due to its effect on the loop crossover frequency (bandwidth), phase margin, and gain margin. increased output capac- itance will reduce the crossover frequency with greater phase margin. output capacitor: channel 3 the output capacitor limits the output ripple and pro- vides holdup during large load transitions. a 10 f to 22 f x5r or x7r ceramic capacitor typically provides sufficient bulk capacitance to stabilize the output during large load transitions and has the esr and esl charac- teristics necessary for low output ripple. adjustable output resistor selection the output voltage on the aat2784 is programmed with external resistors r1 and r2. to limit the bias current required for the external feedback resistor string while maintaining good noise immunity, the minimum sug- gested value for r2 is 59k . although a larger value will further reduce quiescent current, it will also increase the impedance of the feedback node, making it more sensi- tive to external noise and interference. table 2 summa- rizes 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. ?? r1 = - 1 r2 = - 1 59k = 267 k ?? v out v in ?? ?? 3.3v 0.6v v out (v) r2 = 59k r1 (k ) r2 = 221k 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 3.0 237 887 3.3 267 1000 table 2: aat2784 resistor values for various output voltages. thermal calculations there are three types of losses associated with the aat2784 step-down converter: switching losses, con- duction 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 con- duction mode (ccm), a simplified form of the losses is given by: p total i o 2 (r ds(on)h v o + r ds(on)l [v in - v o ]) v in = + (t sw f s i o + i q ) v in i q is the step-down converter quiescent current. the term t sw is used to estimate the full load step-down con-
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 15 www.analogictech.com verter switching losses. for the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to: p total = i o 2 r dson(h) + i q v in since r ds(on) , quiescent current, and switching losses all vary with input voltage, the total losses should be inves- tigated over the complete input voltage range. given the total losses, the maximum junction temperature can be derived from the ja for the tdfn34-16 package, which is 50c/w. t j(max) = p total ja + t amb layout the suggested pcb layout for the aat2784 is shown in figures 2 and 3. the following guidelines should be used to help ensure a proper layout. 1. the power input capacitors (c5 and c8) should be connected as closely as possible to vp1_2, vp3 and pgnd1,2,3 as shown in figure 2. due to the pin placement of vp1_2 and vp3 for all converters, proper decoupling is not possible with just one input capacitor. 2. c1 and r7 are optional low pass filter components for the in supply pin for the device if additional noise decupling is required in a noisy system 3. c2 and l1, c6 and l2, c10 and l3 should be con- nected as closely as possible. the connection of l1, 2, 3 to the lx1, 2, 3 pin should be as short as possible. 4. the feedback trace or fb pin should be separate from any power trace and connect as closely as pos- sible to the load point. sensing along a high-current load trace will degrade dc load regulation. 5. the resistance of the trace from the load returns to pgnd1, 2 and 3 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 signal ground and the power ground. 6. connect unused signal pins to ground to avoid unwanted noise coupling. 7. for good thermal coupling, pcb vias are required from the pad for the tdfn paddle to the bottom ground plane. the via diameter should be 0.3mm to 0.33mm and positioned on a 1.2mm grid. evaluation board schematic 1 1 1 v out2 v out1 v out3 1 3 2 en1 1 3 2 en2 1 3 2 en3 c5 10f c4 10f c1 10f c8 10f c10 10f c2 4.7f c6 4.7f 4.7h l1 4.7h l2 1.5h l3 r6 59k r5 59k r3 133k r4 29.4k r1 133k r2 29.4k 1 lx2 1 lx3 1 lx1 r7 0 1 pgnd 1 pgnd 1 v in pgnd2 1 fb2 2 en1 3 en2 4 gnd 5 vin 6 en3 7 fb3 8 pgnd3 9 lx3 10 vp3 11 fb1 12 pgnd1 13 lx1 14 vp1_2 15 lx2 16 u1 aat2784 c3 100pf c7 100pf c9 56pf figure 1: aat2784 evaluation board schematic.
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 16 2784.2007.11.1.1 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 16 2784.2007.11.1.1 www.analogictech.com evaluation board layout figure 2: aat2784 evaluation board figure 3: aat2784 evaluation board component side layout solder side layout component part number manufacturer description u1 aat2784 aati 3-channel step-down dc/dc converter l1, l2 cdrx2d11 sumida 4.7 h 0.88a 170m (3.2x3.2x1.2)mm shielded l3 lqh32pn1r5nn0l murata 1.5 h series murata inductor has a 68.4m worst case dcr and a 1.75a dc c1, c4 generic 10 f (optional) c2, c6 gmr219r61a475ke19 murata 4.7 f 10v 0805 c5, c8, c10 gmr21br60j106ke19 murata 10 f 6.3v 0805 c9 generic 56pf 6.3v 0402 r1, r3 generic 133k 0402 r2, r4 generic 29.4k 0402 r5, r6 generic 59k 0402 r7 generic 0 table 3: aat2784 evaluation board bill of materials.
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 17 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 17 www.analogictech.com design example specifications v o3 1.2v @ 1.5a (adjustable using 0.6v version), pulsed load i load = 1.5a v o1 3.3v @ 300ma (adjustable using 0.6v version), pulsed load i load = 300ma v o2 3.3v @ 300ma (adjustable using 0.6v version), pulsed load i load = 300ma v in 2.7v to 4.2v (3.6v nominal) f s 1.8 mhz t amb 85c channel 3 output inductor 0.75 ? v o l = = = 1.2h m 0.75 ? 1.2v 0.75 a s ; use 1.5 h. (see table 4). select murata lqh32pn1r5nn0l 1.5 h 1.75a dc current rating dcr = 68m . v o3 v o3 1.5 v 1.5v i 3 = 1 - = ? 1 - = 357m a l ? f v in 1.5h ? 1.8mhz 4.2v i pk3 = 1.5a + 0.36a = 1.86a p l3 = i o3 2 ? dcr = 1.5a 2 ? 68m = 153mw ? ? ? ? ? ? ? ? channels 1 and 2 output inductors 0.75 ? v o l1 = l2 = = = 4.1h m 0.75 ? 3.3v 0.6 a s ; use 4.7 h. (see table 4) select sumida cdrh2d11 4.7 h 0.88a dc current rating dcr = 170m . v o1 v o1 3.3 v 3.3v i 1 = i 2 = 1 - = ? 1 - = 84m a l ? f v in 4.7h ? 1.8mhz 4.2v i pk1 = i pk2 = 0.3a + 0.084a = 0.384a p l1 = p l2 = i o1 2 ? dcr = 0.3 2 ? 170m = 15.3mw ? ? ? ? ? ? ? ?
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 18 2784.2007.11.1.1 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 18 2784.2007.11.1.1 www.analogictech.com channel 3 output capacitor 1 23 1 1.2v (4.2v - 1.2v) 1.5h 1.8mhz 4.2v 23 rms(max) i l f s v in(max) = 3 i load1 v droop f s 3 1.5a 0.2v 1.8mhz c out3 = = = 12.5f; use 22f = 92m a v out (v in(max) - v out ) = p esr = esr i rms 2 = 5m 92ma 2 = 0.042mw channels 1 and 2 output capacitors 1 23 1 3.3v (4.2v - 3.3v) 4.7h 1.8mhz 4.2v 23 rms(max) i l f s v in(max) = 3 i load1 v droop f s 3 0.3a 0.2v 1.8mhz c out1 = c out2 = = = 2.5f; use 4.7f = 24m a v out1 (v in(max) - v out1 ) = p esr = esr i rms 2 = 5m 24ma 2 = 3w channel 3 input capacitor input ripple v pp = 33mv c in3 = = = 9.3f; use 10f 1 ?? - esr 4 f s ?? v pp i o3 1 ?? - 5m 4 1.8mhz ?? 33mv 1.5a i o rms(max) i p esr = esr i rms 2 = 5m (0.75a) 2 = 3mw 2 = = 0.75a channels 1 and 2 input capacitors input ripple v pp = 15mv c in1 = c in2 = = = 6.9f; use 10f 1 ?? - esr 4 f s ?? v pp i o1 + i o2 1 ?? - 5m 4 1.8mhz ?? 15mv 0.6a i o rms(max) i p esr = esr i rms 2 = 5m (0.3a) 2 = 0.45mw 2 = = 0.3a
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 19 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 19 www.analogictech.com aat2784 losses total loss can be estimated by calculating the dropout (v in = v o ) losses where the power mosfets r ds (on) will be at the maximum value. all values assume an 85oc ambient temperature and a 120oc junction temperature with the tdfn 50c/w package. p loss = i o3 2 r ds(on)h1 +2 (i o1 2 r ds(on)h2,3 ) = 1.5a2 120m +2 (0.3a2 400m ) = 0.342w t j(max) = t amb + ja *p loss = 85oc + 50c*0.324w = 101c. manufacturer part number inductance ( h) max dc current (a) dcr ( ) size (mm) lxwxh type sumida cdrh2d11 1.5 1.48 0.068 3.2x3.2x1.2 shielded sumida cdrh2d11 2.2 1.27 0.098 3.2x3.2x1.2 shielded sumida cdrh2d11 3.3 1.02 0.123 3.2x3.2x1.2 shielded sumida cdrh2d11 4.7 0.88 0.170 3.2x3.2x1.2 shielded taiyo yuden cbc2518t 1.0 1.2 0.08 2.5x1.8x1.8 wire wound chip taiyo yuden cbc2518t 2.2 1.1 0.13 2.5x1.8x1.8 wire wound chip taiyo yuden cbc2518t 4.7 0.92 0.2 2.5x1.8x1.8 wire wound chip taiyo yuden cbc2016t 2.2 0.83 0.2 2.0x1.6x1.6 wire wound chip table 3: typical surface mount inductors.
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 20 2784.2007.11.1.1 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 20 2784.2007.11.1.1 www.analogictech.com ordering information package voltage marking 1 part number (tape and reel) 2 channel 1 channel 2 channel 3 tdfn34-16 0.6 0.6 0.6 zcxyy AAT2784IRN-AAA-T1 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. legend voltage code adjustable (0.6v) a 1. xyy = assembly and date code. 2. sample stock is generally held on all part numbers listed in bold.
aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 21 www.analogictech.com aat2784 3-channel step-down dc/dc converter systempower tm product datasheet 2784.2007.11.1.1 21 www.analogictech.com advanced analogic technologies, inc. 3230 scott boulevard, santa clara, ca 95054 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 speci cations or to discontinue any product or service without notice. except as provided in analogictech?s terms and conditions of sale, analogictech assumes no liability whatsoever, and analogictech disclaims any express or implied warranty re lating to the sale and/or use of analogictech products including liability or warranties relating to tness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right . in order to minimize risks associated with the customer?s applications, adequate 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 analogictech deems necessary to support this warranty. speci c testing of all parameters of each device is not necessarily performed. analogictech and the analogictech logo are trademarks of advanced analogic technologies incorporated. all other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders. package information tdfn34-16 3.000 all dimensions in millimeters. 1. 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 connection.

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