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general description the aat2556 is a member of analogictech's total power management ic? (tpmic?) product fam- ily. it is a fully integrated 500ma battery charger plus a 250ma step-down converter. the input volt- age range is 4v to 6.5v for the battery charger and 2.7v to 5.5v for the step-down converter, making it ideal for single-cell lithium-ion/polymer battery- powered applications. the battery charger is a complete constant current/ constant voltage linear charger. it offers an inte- grated pass device, reverse blocking protection, high current accuracy and voltage regulation, charge status, and charge termination. the charg- ing current is programmable via external resistor from 15ma to 500ma. in addition to standard fea- tures, the device offers over-voltage, current limit, and thermal protection. the step-down converter is a highly integrated converter operating at 1.5mhz of switching fre- quency, minimizing the size of external compo- nents while keeping switching losses low. it has independent input and enable pins. the output voltage ranges from 0.6v to the input voltage. the feedback and control deliver excellent load regula- tion and transient response with a small output inductor and capacitor. the aat2556 is available in a pb-free, thermally- enhanced tdfn33-12 package and is rated over the -40c to +85c temperature range. features ? battery charger: ? input voltage range: 4v to 6.5v ? programmable charging current up to 500ma ? highly integrated battery charger ? charging device ? reverse blocking diode ? step-down converter: ? input voltage range: 2.7v to 5.5v ? output voltage range: 0.6v to v in ? 250ma output current ? up to 96% efficiency ? 30a quiescent current ? 1.5mhz switching frequency ? 100s start-up time ? short-circuit, over-temperature, and current limit protection ? tdfn33-12 package ? -40c to +85c temperature range applications ? bluetooth? headsets ? cellular phones ? handheld instruments ? mp3 and portable music players ? pdas and handheld computers ? portable media players aat2556 battery charger and step-down converter for portable applications typical application 2556.2006.05.1.0 1 systempower ? c batt - adp iset gnd bat batt + adapter / usb input stat en_bat enable r set vin en_buck l= 3.3h fb lx r fb2 r fb1 c out v out system battery pack
pin descriptions pin configuration tdfn33-12 (top view) pin # symbol function 1 fb feedback input. this pin must be connected directly to an external resistor divider. nominal voltage is 0.6v. 2, 8, 10 gnd ground. 3 en_buck enable pin for the step-down converter. when connected to logic low, the step-down converter is disabled and it consumes less than 1a of current. when connected to logic high, it resumes normal operation. 4 en_bat enable pin for the battery charger. when internally pulled down, the battery charger is disabled and it consumes less than 1a of current. when connected to logic high, it resumes normal operation. 5 iset charge current set point. connect a resistor from this pin to ground. refer to typical curves for resistor selection. 6 bat battery charging and sensing. 7 stat charge status input. open drain status input. 9 adp input for usb/adapter charger. 11 lx output of the step-down converter. connect the inductor to this pin. internally, it is connected to the drain of both high- and low-side mosfets. 12 vin input voltage for the step-down converter. ep exposed paddle (bottom): connect to ground directly beneath the package. aat2556 battery charger and step-down converter for portable applications 2 2556.2006.05.1.0 fb gnd en_buck 1 en_bat iset bat vin lx gnd adp gnd sta t 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 in input voltage to gnd 6.0 v v adp adapter voltage to gnd -0.3 to 7.5 v v lx lx to gnd -0.3 to v in + 0.3 v v fb fb to gnd -0.3 to v in + 0.3 v v en en_bat and en_buck to gnd -0.3 to 6.0 v v x bat, iset and stat to gnd -0.3 to v adp + 0.3 v t j operating junction temperature range -40 to 150 c t lead maximum soldering temperature (at leads, 10 sec) 300 c aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 3 1. stresses above those listed in absolute maximum ratings may cause permanent damage to the device. functional operation at co nditions other than the operating conditions specified is not implied. only one absolute maximum rating should be applied at any one tim e. 2. mounted on an fr4 board.
electrical characteristics 1 v in = 3.6v; t a = -40c to +85c, unless otherwise noted. typical values are t a = 25c. symbol description conditions min typ max units step-down converter v in input voltage 2.7 5.5 v v in rising 2.7 v v uvlo uvlo threshold hysteresis 200 mv v in falling 1.8 v v out output voltage tolerance 2 i out = 0 to 250ma, -3.0 3.0 % v in = 2.7v to 5.5v v out output voltage range 0.6 v in v i q quiescent current no load 30 a i shdn shutdown current en = gnd 1.0 a i lim p-channel current limit 600 ma r ds(on)h high-side switch on resistance 0.59 r ds(on)l low-side switch on resistance 0.42 i lxleak lx leakage current v in = 5.5v, v lx = 0 to v in 1.0 a v linereg / v in line regulation v in = 2.7v to 5.5v 0.2 %/v v fb feedback threshold voltage accuracy v in = 3.6v 0.597 0.606 0.615 v i fb fb leakage current v out = 1.0v 0.2 a f osc oscillator frequency 1.5 mhz t s startup time from enable to output 100 s regulation t sd over-temperature shutdown threshold 140 c t hys over-temperature shutdown hysteresis 15 c v en(l) enable threshold low 0.6 v v en(h) enable threshold high 1.4 v i en input low current v in = v en = 5.5v -1.0 1.0 a aat2556 battery charger and step-down converter for portable applications 4 2556.2006.05.1.0 1. the aat2556 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. output voltage tolerance is independent of feedback resistor network accuracy.
electrical characteristics 1 v adp = 5v; t a = -40c to +85c, unless otherwise noted. typical values are t a = 25c. symbol description conditions min typ max units battery charger operation v adp adapter voltage range 4.0 6.5 v v uvlo under-voltage lockout (uvlo) rising edge 3 4 v uvlo hysteresis 150 mv i op operating current charge current = 200ma 0.5 1 ma i shutdown shutdown current v bat = 4.25v, en = gnd 0.3 1 a i leakage reverse leakage current from bat pin v bat = 4v, adp pin open 0.4 2 a voltage regulation v bat _ eoc end of charge accuracy 4.158 4.20 4.242 v v ch /v ch output charge voltage tolerance 0.5 % v min preconditioning voltage threshold 2.85 3.0 3.15 v v rch battery recharge voltage threshold measured from v bat _ eoc -0.1 v current regulation i ch charge current programmable range 15 500 ma i ch /i ch charge current regulation tolerance 10 % v set iset pin voltage 2 v k i _ a current set factor: i ch /i set 800 charging devices r ds(on) charging transistor on resistance v adp = 5.5v 0.9 1.1 logic control/protection v en(h) input high threshold 1.6 v v en(l) input low threshold 0.4 v v s tat output low voltage stat pin sinks 4ma 0.4 v i s tat stat pin current sink capability 8 ma v ovp over-voltage protection threshold 4.4 v i tk /i chg pre-charge current i ch = 100ma 10 % i term /i chg charge termination threshold current 10 % aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 5 1. the aat2556 output charge voltage is specified over the 0 to 70c ambient temperature range; operation over the -25c to +8 5c temperature range is guaranteed by design.
typical characteristics ? step-down converter aat2556 battery charger and step-down converter for portable applications 6 2556.2006.05.1.0 line regulation (v out = 1.8v) input voltage (v) accuracy (%) -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 i out = 250ma i out = 10ma i out = 0ma i out = 50ma i out = 150ma soft start (v in = 3.6v; v out = 1.8v; i out = 250ma; c ff = 100pf) enable and output voltage (top) (v) inductor current (bottom) (a) time (100s/div) -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0 -0.2 -0.4 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 v en v o i l dc load regulation (v out = 1.2v; l = 1.5h) 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 = 5.5v v in = 2.7v v in = 4.2v v in = 3.6v efficiency vs. load (v out = 1.2v; l = 1.5h) output current (ma) efficiency (%) 30 40 50 60 70 80 90 100 0.1 1 10 100 1000 v in = 3.6v v in = 2.7v v in = 5.5v v in = 4.2v v in = 5.0v dc load regulation (v out = 1.8v; l = 3.3h) 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 = 2.7v v in = 5.5v v in = 5.0v efficiency vs. load (v out = 1.8v; l = 3.3h) output current (ma) efficiency (%) 40 50 60 70 80 90 100 0.1 1 10 100 1000 v in = 3.6v v in = 2.7v v in = 4.2v v in = 5.0v v in = 5.5v
typical characteristics ? step-down converter aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 7 n-channel r ds(on) vs. input voltage input voltage (v) r ds(on)l (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 6.0 120 c 100 c 85 c 25 c p-channel r ds(on) vs. input voltage input voltage (v) r ds(on)h (m ) 300 400 500 600 700 800 900 1000 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 120 c 100 c 85 c 25 c no load quiescent current vs. input voltage input voltage (v) supply current (a) 10 15 20 25 30 35 40 45 50 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 85 c 25 c -40 c frequency variation vs. input voltage (v out = 1.8v) input voltage (v) frequency variation (%) -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 switching frequency variation vs. temperature (v in = 3.6v; v out = 1.8v) temperature ( c) variation (%) -10.0 -8.0 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 10.0 -40 -20 0 20 40 60 80 100 output voltage error vs. temperature (v in = 3.6v; v out = 1.8v; i out = 250ma) temperature ( c) output error (%) -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 -40 -20 0 20 40 60 80 100
typical characteristics ? step-down converter aat2556 battery charger and step-down converter for portable applications 8 2556.2006.05.1.0 load transient response (10ma to 250ma; v in = 3.6v; v out = 1.8v; c out = 4.7f) output voltage (top) (v) load and inductor current (bottom) (200ma/div) time (25s/div) 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 v o i lx i o load transient response (10ma to 250ma; v in = 3.6v; v out = 1.8v; c out = 4.7f; c ff = 100pf) output voltage (top) (v) load and inductor current (bottom) (200ma/div) time (25s/div) 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 v o i lx i o output ripple (v in = 3.6v; v out = 1.8v; i out = 250ma) output voltage (ac coupled) (top) (v) inductor current (bottom) (a) time (200ns/div) -120 -100 -80 -60 -40 -20 0 20 40 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 v o i l output ripple (v in = 3.6v; v out = 1.8v; i out = 1ma) output voltage (ac coupled) (top) (mv) inductor current (bottom) (a) time (2s/div) -120 -100 -80 -60 -40 -20 0 20 40 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 v o i l line response (v out = 1.8v @ 250ma; c ff = 100pf) output voltage (top) (v) input voltage (bottom) (v) time (25s/div) 1.50 1.55 1.60 1.65 1.70 1.75 1.80 1.85 1.90 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 v o v in
typical characteristics ? battery charger aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 9 constant charging current vs. temperature (r set = 8.06k ) temperature ( 190 193 195 198 200 203 205 208 210 -50 -25 0 25 50 75 100 constant charging current vs. supply voltage (r set = 8.06k ) v adp (v) i ch (ma) 170 180 190 200 210 220 4 4.25 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 v bat = 3.6v v bat = 4v v bat = 3.3v end of charge voltage regulation vs. temperature (r set = 8.06k ) temperature ( 4.17 4.18 4.19 4.20 4.21 4.22 4.23 -50 -25 0 25 50 75 100 end of charge battery voltage vs. supply voltage v adp (v) v bat_eoc (v) 4.194 4.196 4.198 4.200 4.202 4.204 4.206 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 r set = 8.06k r set = 31.6k charging current vs. battery voltage (v adp = 5v) v bat (v) i ch (ma) 0 100 200 300 400 500 600 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4. 3 r set = 8.06k r set = 5.62k r set = 3.24k r set = 16.2k r set = 31.6k r set (k ) i ch (ma) constant charging current vs. set resistor values 1 10 100 1000 1 10 100 1000
typical characteristics ? battery charger aat2556 battery charger and step-down converter for portable applications 10 2556.2006.05.1.0 sleep mode current vs. supply voltage (r set = 8.06k ) v adp (v) i sleep (na) 0 100 200 300 400 500 600 700 800 4 4.25 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 85 c 25 c -40 c recharging threshold voltage vs. temperature (r set = 8.06k ) temperature ( 4.02 4.04 4.06 4.08 4.10 4.12 4.14 4.16 4.18 -50 -25 0 25 50 75 100 preconditioning charge current vs. supply voltage v adp (v) i trickle (ma) 0 10 20 30 40 50 60 4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6 6.2 6.4 r set = 8.06k r set = 5.62k r set = 3.24k r set = 16.2k r set = 31.6k preconditioning charge current vs. temperature (r set = 8.06k ) temperature ( 19.2 19.4 19.6 19.8 20.0 20.2 20.4 20.6 20.8 -50 -25 0 25 50 75 100 preconditioning threshold voltage vs. temperature (r set = 8.06k ) temperature ( 2.97 2.98 2.99 3 3.01 3.02 3.03 -50 -25 0 25 50 75 100 operating current vs. temperature (r set = 8.06k ) temperature ( 300 350 400 450 500 550 -50 -25 0 25 50 75 100
typical characteristics ? battery charger aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 11 v en(l) vs. supply voltage (r set = 8.06k ) v adp (v) v en(l) (v) 0.6 0.7 0.8 0.9 1 1.1 4 4.25 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 -40 c 25 c 85 c v en(h) vs. supply voltage (r set = 8.06k ) v adp (v) v en(h) (v) 0.7 0.8 0.9 1 1.1 1.2 4 4.25 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 -40 c 25 c 85 c
functional description the aat2556 is a high performance power system comprised of a 500ma lithium-ion/polymer battery charger and a 250ma step-down converter. the battery charger is designed for single-cell lithium-ion/polymer batteries using a constant cur- rent and constant voltage algorithm. the battery charger operates from the adapter/usb input volt- age range from 4v to 6.5v. the adapter/usb charging current level can be programmed up to 500ma for rapid charging applications. a status monitor output pin is provided to indicate the bat- tery charge state by directly driving one external led. internal device temperature and charging state are fully monitored for fault conditions. in the event of an over-voltage or over-temperature fail- ure, the device will automatically shut down, pro- tecting the charging device, control system, and the battery under charge. other features include an integrated reverse blocking diode and sense resistor. the step-down converter operates with an input volt- age of 2.7v to 5.5v. the switching frequency is 1.5mhz, minimizing the size of the inductor. under light load conditions, the device enters power-saving mode; the switching frequency is reduced, and the converter consumes 30a of current, making it ideal for battery-operated applications. the output volt- age is programmable from v in to as low as 0.6v. aat2556 battery charger and step-down converter for portable applications 12 2556.2006.05.1.0 functional block diagram charge control reverse blocking constant current bat uvlo over- temperature protection stat gnd + - + - iset adp en_ba t vin lx logic dh dl + - v ref v ref fb input en_buck
power devices are sized for 250ma current capabil- ity while maintaining over 90% efficiency at full load. light load efficiency is maintained at greater than 80% down to 1ma of load current. a high-dc gain error amplifier with internal compensation controls the output. it provides excellent transient response and load/line regulation. under-voltage lockout the aat2556 has internal circuits for uvlo and power on reset features. if the adp supply voltage drops below the uvlo threshold, the battery charger will suspend charging and shut down. when power is reapplied to the adp pin or the uvlo condition recovers, the system charge con- trol will automatically resume charging in the appropriate mode for the condition of the battery. if the input voltage of the step-down converter drops below uvlo, the internal circuit will shut down. protection circuitry over-voltage protection an over-voltage protection event is defined as a condition where the voltage on the bat pin exceeds the over-voltage protection threshold (v ovp ). if this over-voltage condition occurs, the charger control circuitry will shut down the device. the charger will resume normal charging operation after the over-voltage condition is removed. current limit, over-temperature protection for overload conditions, the peak input current is limited at the step-down converter. as load imped- ance decreases and the output voltage falls closer to zero, more power is dissipated internally, which causes the internal die temperature to rise. in this case, the thermal protection circuit completely dis- ables switching, which protects the device from damage. the battery charger has a thermal protection circuit which will shut down charging functions when the internal die temperature exceeds the preset ther- mal limit threshold. once the internal die tempera- ture falls below the thermal limit, normal charging operation will resume. control loop the aat2556 contains a compact, current mode step-down dc/dc controller. the current through the p-channel mosfet (high side) is sensed for current loop control, as well as short-circuit and overload protection. a fixed slope compensation signal is added to the sensed current to maintain stability for duty cycles greater than 50%. the peak current mode loop appears as a voltage-pro- grammed current source in parallel with the output capacitor. the output of the voltage error amplifier programs the current mode loop for the necessary peak switch current to force a constant output volt- age for all load and line conditions. internal loop compensation terminates the transconductance voltage error amplifier output. the error amplifier reference is fixed at 0.6v. battery charging operation battery charging commences only after checking several conditions in order to maintain a safe charging environment. the input supply (adp) must be above the minimum operating voltage (uvlo) and the enable pin must be high (internal- ly pulled down). when the battery is connected to the bat pin, the charger checks the condition of the battery and determines which charging mode to apply. if the battery voltage is below v min , the charger begins battery pre-conditioning by charg- ing at 10% of the programmed constant current; e.g., if the programmed current is 150ma, then the pre-conditioning current (trickle charge) is 15ma. pre-conditioning is purely a safety precaution for a deeply discharged cell and will also reduce the power dissipation in the internal series pass mos- fet when the input-output voltage differential is at its highest. aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 13
pre-conditioning continues until the battery voltage reaches v min . at this point, the charger begins con- stant-current charging. the current level for this mode is programmed using a single resistor from the iset pin to ground. programmed current can be set from a minimum 15ma up to a maximum of 500ma. constant current charging will continue until the battery voltage reaches the voltage regu- lation point, v bat . when the battery voltage reach- es v bat , the battery charger begins constant volt- age mode. the regulation voltage is factory pro- grammed to a nominal 4.2v (0.5%) and will con- tinue charging until the charging current has reduced to 10% of the programmed current. after the charge cycle is complete, the pass device turns off and the device automatically goes into a power-saving sleep mode. during this time, the series pass device will block current in both direc- tions, preventing the battery from discharging through the ic. the battery charger will remain in sleep mode, even if the charger source is disconnected, until one of the following events occurs: the battery ter- minal voltage drops below the v rch threshold; the charger en pin is recycled; or the charging source is reconnected. in all cases, the charger will mon- itor all parameters and resume charging in the most appropriate mode. aat2556 battery charger and step-down converter for portable applications 14 2556.2006.05.1.0 figure 1: current vs. voltage profile during charging phases. constant current charge phase constant voltage charge phase preconditioning trickle charge phase charge complete voltage constant current mode voltage threshold regulated current trickle charge and termination threshold i = cc / 10 i = max cc
battery charging system operation flow chart aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 15 power on reset power input voltage v adp > v uvlo fault conditions monitoring ov, ot preconditioning test v min > v bat current phase test v adp > v bat voltage phase test i bat > i term no no yes no preconditioning (trickle charge) constant current charge mode constant voltage charge mode yes yes yes charge completed charge control no recharge test v rch > v bat yes no shut down yes enable yes no
application information soft start / enable the en_bat pin is internally pulled down. when pulled to a logic high level, the battery charger is enabled. when left open or pulled to a logic low level, the battery charger is shut down and forced into the sleep state. charging will be halted regard- less of the battery voltage or charging state. when it is re-enabled, the charge control circuit will auto- matically reset and resume charging functions with the appropriate charging mode based on the bat- tery charge state and measured cell voltage from the bat pin. the step-down converter features a soft start that limits the inrush current and eliminates output volt- age overshoot during startup. the circuit is designed to increase the inductor current limit in discrete steps when the input voltage or enable input is applied. typical start up time is 100s. pulling en_buck to logic low forces the converter in a low power, non-switching state, and it con- sumes less than 1a of quiescent current. connecting it to logic high enables the converter and resumes normal operation. adapter or usb power input constant current charge levels up to 500ma may be programmed by the user when powered from a sufficient input power source. the battery charger will operate from the adapter input over a 4.0v to 6.5v range. the constant current fast charge cur- rent for the adapter input is set by the r set resistor connected between iset and ground. refer to table 1 for recommended r set values for a desired constant current charge level. programming charge current the fast charge constant current charge level is user programmed with a set resistor placed between the iset pin and ground. the accuracy of the fast charge, as well as the preconditioning trick- le charge current, is dominated by the tolerance of the set resistor used. for this reason, a 1% toler- ance metal film resistor is recommended for the set resistor function. fast charge constant current lev- els from 15ma to 500ma may be set by selecting the appropriate resistor value from table 1. table 1: r set values. figure 2: constant charging current vs. set resistor values. charge status output the aat2556 provides battery charge status via a status pin. this pin is internally connected to an n- channel open drain mosfet, which can be used to drive an external led. the status pin can indicate several conditions, as shown in table 2. table 2: led status indicator. event description status no battery charging activity off battery charging via adapter on or usb port charging completed off normal set resistor i charge (ma) value r2 (k ) 500 3.24 400 4.12 300 5.62 250 6.49 200 8.06 150 10.7 100 16.2 50 31.6 40 38.3 30 53.6 20 78.7 15 105 aat2556 battery charger and step-down converter for portable applications 16 2556.2006.05.1.0 r set (k ) i ch (ma) 1 10 100 1000 1 10 100 1000
the led should be biased with as little current as necessary to create reasonable illumination; there- fore, a ballast resistor should be placed between the led cathode and the stat pin. led current consumption will add to the overall thermal power budget for the device package, hence it is good to keep the led drive current to a minimum. 2ma should be sufficient to drive most low-cost green or red leds. it is not recommended to exceed 8ma for driving an individual status led. the required ballast resistor values can be esti- mated using the following formulas: example: note: red led forward voltage (v f ) is typically 2.0v @ 2ma. thermal considerations the aat2556 is offered in a tdfn33-12 package which can provide up to 2w of power dissipation when it is properly bonded to a printed circuit board and has a maximum thermal resistance of 50c/w. many considerations should be taken into account when designing the printed circuit board layout, as well as the placement of the charger ic package in proximity to other heat generating devices in a given application design. the ambient temperature around the ic will also have an effect on the ther- mal limits of a battery charging application. the maximum limits that can be expected for a given ambient condition can be estimated by the follow- ing discussion. first, the maximum power dissipation for a given situation should be calculated: where: p d(max) = maximum power dissipation (w) ja = package thermal resistance (c/w) t j(max) = maximum device junction temperature (c) [135c] t a = ambient temperature (c) figure 3 shows the relationship of maximum power dissipation and ambient temperature of the aat2556. figure 3: maximum power dissipation. next, the power dissipation of the battery charger can be calculated by the following equation: where: p d = total power dissipation by the device v adp = adp/usb voltage v bat = battery voltage as seen at the bat pin i ch = constant charge current programmed for the application i op = quiescent current consumed by the charger ic for normal operation [0.5ma] by substitution, we can derive the maximum charge current before reaching the thermal limit condition (thermal cycling). the maximum charge current is the key factor when designing battery charger applications. aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 17 t a ( c) p d(max) (mw) 0 500 1000 1500 2000 2500 3000 0 20 40 60 80 100 120 (t j(max) - t a ) p d(max) = ja (5.5v - 2.0 v) r 1 = = 1.75k 2ma (v adp - v f(led) ) r 1 = i led p d = [(v adp - v bat ) i ch + (v adp i op )]
in general, the worst condition is the greatest volt- age drop across the ic, when battery voltage is charged up to the preconditioning voltage thresh- old. figure 4 shows the maximum charge current in different ambient temperatures. figure 4: maximum charging current before thermal cycling becomes active. there are three types of losses associated with the step-down converter: switching losses, conduction losses, and quiescent current losses. conduction losses are associated with the r ds(on) characteris- tics 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 losses is given by: i q is the step-down converter quiescent current. the term t sw is used to estimate the full load step- down converter switching losses. for the condition where the step-down converter is in dropout at 100% duty cycle, the total device dis- sipation 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 tdfn33-12 package which is 50c/w. capacitor selection battery charger input capacitor (c1) in general, it is good design practice to place a decoupling capacitor between the adp pin and gnd. an input capacitor in the range of 1f to 22f is recommended. if the source supply is unregulated, it may be necessary to increase the capacitance to keep the input voltage above the under-voltage lockout threshold during device enable and when battery charging is initiated. if the adapter input is to be used in a system with an external power supply source, such as a typical ac-to-dc wall adapter, then a c in capacitor in the range of 10f should be used. a larger input capacitor in this application will minimize switching or power transient effects when the power supply is "hot plugged" in. step-down converter input capacitor (c3) 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 in . the calculated value varies with input voltage and is a maximum when v in is double the output voltage. aat2556 battery charger and step-down converter for portable applications 18 2556.2006.05.1.0 t j(max) = p total ja + t amb p total = i o 2 r dson(h) + i q v in p total i o 2 (r dson(h) v o + r dson(l) [v in - v o ]) v in = + (t sw f s i o + i q ) v in v in (v) i cc(max) (ma) 0 100 200 300 400 500 4.25 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 6.75 t a = 85c t a = 60c (t j(max) - t a ) ja v in - v bat i ch(max) = - v in i op (p d(max) - v in i op ) v in - v bat i ch(max) =
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 current. for v in = 2 v o the term appears in both the input voltage ripple and input capacitor rms current equations and is a maximum 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 provides a low impedance loop for the edges of pulsed current drawn by the step- down converter. 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 (c3) can be seen in the evaluation board layout in figure 6. 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. 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 capacitor should be placed in parallel with the low esr, esl bypass ceramic capacitor. this dampens the high q net- work and stabilizes the system. battery charger output capacitor (c2) the aat2556 only requires a 1f ceramic capaci- tor on the bat pin to maintain circuit stability. this value should be increased to 10f or more if the battery connection is made any distance from the charger output. if the aat2556 is to be used in applications where the battery can be removed from the charger, such as with desktop charging cradles, an output capacitor greater than 10f may be required to prevent the device from cycling on and off when no battery is present. step-down converter output capacitor (c4) the output capacitor limits the output ripple and provides holdup during large load transitions. a 4.7f to 10f x5r or x7r ceramic capacitor typi- cally provides sufficient bulk capacitance to stabi- lize the output during large load transitions and has the esr and esl characteristics necessary for low output ripple. for enhanced transient response and aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 19 ?? 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 s ?? 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 s ?? v pp i o
low temperature operation applications, a 10f (x5r, x7r) ceramic capacitor is recommended to stabilize extreme pulsed load conditions. 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 ceram- ic output capacitor esr is typically minimal, resulting in less than a few degrees rise in hot- spot temperature. inductor selection the step-down converter uses peak current mode control with slope compensation to maintain stabil- ity for duty cycles greater than 50%. the output inductor value must be selected so the inductor current down slope meets the internal slope com- pensation requirements. the internal slope com- pensation for the aat2556 is 0.45a/sec. this equates to a slope compensation that is 75% of the inductor current down slope for a 1.8v output and 3.0h inductor. for most designs, the step-down converter operates with an inductor value of 1h to 4.7h. table 3 dis- plays inductor values for the aat2556 with different output voltage 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 3.0h cdrh2d09 series inductor selected from sumida has a 150m dcr and a 470ma dc current rating. at full load, the inductor dc loss is 9.375mw which gives a 2.08% loss in efficiency for a 250ma, 1.8v output. table 3: inductor values. output voltage (v) l1 (h) 1.0 1.5 1.2 2.2 1.5 2.7 1.8 3.0/3.3 2.5 3.9/4.2 3.0 4.7 3.3 5.6 aat2556 battery charger and step-down converter for portable applications 20 2556.2006.05.1.0 0.75 ? v o l = = 1.67 ? v o m 0.75 ? v o 0.45a sec a a sec 0.75 ? v o m = = = 0.45 l 0.75 ? 1.8v 3.0h a sec 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
adjustable output resistor selection resistors r3 and r4 of figure 5 program the out- put to regulate at a voltage higher than 0.6v. to limit the bias current required for the external feed- back resistor string while maintaining good noise immunity, the suggested value for r4 is 59k . decreased resistor values are necessary to main- tain noise immunity on the fb pin, resulting in increased quiescent current. table 4 summarizes the resistor values for various output voltages. with enhanced transient response for extreme pulsed load application, an external feed-forward capacitor (c5 in figure 5) can be added. table 4: adjustable resistor values for step-down converter. r4 = 59k r4 = 221k v out (v) r3 (k ) r3 (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 aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 21 figure 5: aat2556 evaluation board schematic. ?? ?? r3 = -1 r4 = - 1 59k = 267k v out v ref ?? ?? 3.3v 0.6v red led d1 1k r1 10f c1 2.2f c2 adp 8.06k r2 jp1 0 bat enable_buck jp3 12 12 3 3.3h l1 c5 100pf 4.7f c4 vout 118k r3 59k r4 buck input jp4 enable_bat jp2 4.7f c3 vin iset 5 fb 1 bat 6 gnd 8 gnd 2 en_buck 3 stat 7 en_bat 4 adp 9 gnd 10 lx 11 vin 12 aat2556 u1 12 123
printed circuit board layout considerations for the best results, it is recommended to physi- cally place the battery pack as close as possible to the aat2556 bat pin. to minimize voltage drops on the pcb, keep the high current carrying traces adequately wide. refer to the aat2556 evaluation board for a good layout example (see figures 6 and 7). the following guidelines should be used to help ensure a proper layout. 1. the input capacitors (c1, c3) should connect as closely as possible to adp (pin 9) and vin (pin 12). 2. c4 and l1 should be connected as closely as possible. the connection of l1 to the lx pin should be as short as possible. do not make the node small by using narrow trace. the trace should be kept wide, direct, and short. 3. the feedback pin (pin 1) 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 regula- tion. feedback resistors should be placed as closely as possible to the fb pin (pin 1) to mini- mize the length of the high impedance feedback trace. if possible, they should also be placed away from the lx (switching node) and inductor to improve noise immunity. 4. the resistance of the trace from the load return to pgnd (pin 10) and gnd (pin 2) 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. 5. a high density, small footprint layout can be achieved using an inexpensive, miniature, non- shielded, high dcr inductor. aat2556 battery charger and step-down converter for portable applications 22 2556.2006.05.1.0 figure 6: aat2556 evaluation board figure 7: aat2556 evaluation board top side layout. bottom side layout.
table 5: aat2556 evaluation board component listing. component part number description manufacturer u1 aat2556iwp-t1 battery charger and step-down converter analogictech for portable applications; tdfn33-12 package c1 ecj-1vb0j106m cer 10f 10v 20% x5r 0603 panasonic - ecg c2 grm185b30j225ke25d cer 2.2f 6.3v 10% x7r 0603 murata c3, c4 grm188r60j475ke19b cer 4.7f 6.3v 10% x7r 0603 murata c5 grm1886r1h101jz01j cer 100pf 50v 5% r2h 0603 murata l1 cdrh2d09-3r0 shielded smd, 3.0h, 150m , 3x3x1mm sumida r1 chip resistor 1k , 5%, 1/4w; 0603 vishay r2 chip resistor 8.06k , 1%, 1/4w; 0603 vishay r3 chip resistor 118k , 1%, 1/4w; 0603 vishay r4 chip resistor 59k , 1%, 1/4w; 0603 vishay jp1 chip resistor 0 , 5%, 1/4w; 0603 vishay jp2, jp3, jp4 prpn401paen connecting header, 2mm zip sullins electronics d1 cmd15-21src/tr8 red led; 1206 chicago miniature lamp aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 23
step-down converter design example specifications v o = 1.8v @ 250ma, pulsed load i load = 200ma v in = 2.7v to 4.2v (3.6v nominal) f s = 1.5mhz t amb = 85c 1.8v output inductor (use 3.0h; see table 3) for sumida inductor cdrh2d09-3r0, 3.0h, dcr = 150m . 1.8v output capacitor v droop = 0.1v aat2556 battery charger and step-down converter for portable applications 24 2556.2006.05.1.0 1 23 1 1.8v (4.2v - 1.8v) 3.0h 1.5mhz 4.2v 23 rms i l1 f s v in(max) = 3 i load v droop f s 3 0.2a 0.1v 1.5mhz c out = = = 4f; use 4.7f = 66marms (v o ) (v in(max) - v o ) = p esr = esr i rms 2 = 5m (66ma) 2 = 21.8w v o v o 1.8 v 1.8v i l1 = ? 1 - = ? 1 - = 228m a l1 ? f s v in 3.0h ? 1.5mhz 4.2v i pkl1 = i o + i l1 = 250ma + 114ma = 364ma 2 p l1 = i o 2 ? dcr = 250ma 2 ? 150m = 9.375mw ? ? ? ? ? ? ? ? l1 = 1.67 ? v o2 = 1.67 ? 1.8v = 3h sec a sec a
input capacitor input ripple v pp = 25mv aat2556 losses aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 25 t j(max) = t amb + ja p loss = 85 c + (50 c/w) 34.5mw = 86.7 c p total + (t sw f s i o + i q ) v in i o 2 (r dson(h) v o + r dson(l) [v in -v o ] ) v in = = + (5ns 1.5mhz 0.2a + 30a) 4.2v = 34.5m w 0.2 2 (0.7 1.8v + 0.7 [4.2v - 1.8v]) 4.2v i o rms i p = esr i rms 2 = 5m (0.1a) 2 = 0.05mw 2 = = 0.1arms c in = = = 1.38f; use 4.7f 1 ?? - esr 4 f s ?? v pp i o 1 ?? - 5m 4 1.5mhz ?? 25mv 0.2a
aat2556 battery charger and step-down converter for portable applications 26 2556.2006.05.1.0 table 6: step-down converter component values. table 7: suggested inductors and suppliers. inductance max dc dcr size (mm) manufacturer part number (h) current (ma) (m ) lxwxh type sumida cdrh2d09-1r5 1.5 730 88 3.0x3.0x1.0 shielded sumida cdrh2d09-2r2 2.2 600 115 3.0x3.0x1.0 shielded sumida cdrh2d09-2r5 2.5 530 135 3.0x3.0x1.0 shielded sumida cdrh2d09-3r0 3 470 150 3.0x3.0x1.0 shielded sumida cdrh2d09-3r9 3.9 450 180 3.0x3.0x1.0 shielded sumida cdrh2d09-4r7 4.7 410 230 3.0x3.0x1.0 shielded sumida cdrh2d09-5r6 5.6 370 260 3.0x3.0x1.0 shielded sumida cdrh2d11-1r5 1.5 900 54 3.2x3.2x1.2 shielded sumida cdrh2d11-2r2 2.2 780 78 3.2x3.2x1.2 shielded sumida cdrh2d11-3r3 3.3 600 98 3.2x3.2x1.2 shielded sumida cdrh2d11-4r7 4.7 500 135 3.2x3.2x1.2 shielded taiyo yuden nr3010 1.5 1200 80 3.0x3.0x1.0 shielded taiyo yuden nr3010 2.2 1100 95 3.0x3.0x1.0 shielded taiyo yuden nr3010 3.3 870 140 3.0x3.0x1.0 shielded taiyo yuden nr3010 4.7 750 190 3.0x3.0x1.0 shielded fdk mipwt3226d-1r5 1.5 1200 90 3.2x2.6x0.8 chip shielded fdk mipwt3226d-2r2 2.2 1100 100 3.2x2.6x0.8 chip shielded fdk mipwt3226d-3r0 3 1000 120 3.2x2.6x0.8 chip shielded fdk mipwt3226d-4r2 4.2 900 140 3.2x2.6x0.8 chip shielded output voltage r4 = 59k r4 = 221k 1 v out (v) r3 (k ) r3 (k ) l1 (h) 0.6 2 ? ? 1.5 0.8 19.6 75 1.5 0.9 29.4 113 1.5 1.0 39.2 150 1.5 1.1 49.9 187 1.5 1.2 59.0 221 1.5 1.3 68.1 261 1.5 1.4 78.7 301 2.2 1.5 88.7 332 2.7 1.8 118 442 3.0/3.3 1.85 124 464 3.0/3.3 2.0 137 523 3.0/3.3 2.5 187 715 3.9/4.2 3.3 267 1000 5.6 1. for reduced quiescent current, r4 = 221k . 2. r4 is opened, r3 is shorted.
aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 27 table 8: surface mount capacitors. value voltage temp. case manufacturer part number (f) rating co. size murata grm118r60j475ke19b 4.7 6.3 x5r 0603 murata grm188r60j106me47d 10 6.3 x5r 0603
aat2556 battery charger and step-down converter for portable applications 28 2556.2006.05.1.0 ordering information package marking 1 part number (tape and reel) 2 tdfn33-12 spxyy AAT2556IWP-CA-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.
aat2556 battery charger and step-down converter for portable applications 2556.2006.05.1.0 29 package information tdfn33-12 all dimensions in millimeters 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. top view bottom view detail "b" detail "a" side view 3.00 2.40 +

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