AAT3603 datasheet(27/35 Pages) ANALOGICTECH

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AAT3603178

Total Power Solution for Portable Applications

PRODUCT DATASHEET

3603.2008.06.1.0

www .analogictech.com

Next, the power dissipation for the charger can be cal-

culated by the following equation:

D = (VCHGIN - VBAT) · ICH_CC + (VCHGIN · IOP) + (VCHGIN - VBAT) · IBAT

+ (V

BAT - VOUT1) · IOUT1 + (VBAT - VOUT2) · IOUT2

+ (V

BAT - VOUT3) · IOUT3 + (VBAT - VOUT4) · IOUT4

+ (V

BAT - VOUT5) · IOUT5

+ I

OUTBUCK

· R

DS(ON)L ·

DS(ON)H · [VBAT - VOUTBUCK]

BAT

OUTBUCK

BAT

Where:

PD = Total Power Dissipation by the Device

VCHGIN = CHGIN Input Voltage

VBAT = Battery Voltage at the BAT Pin

ICH_CC = Constant Charge Current Programmed for the

Application

IOP = Quiescent Current Consumed by the IC for Normal

Operation [0.5mA]

VBAT = Load current from the BAT pin for the system

LDOs and step-down converter

RDS(ON)H and RDS(ON)L = On-resistance of step-down high

and low side MOSFETs [0.8Ω each]

VOUTX and IOUTX = Output voltage and load currents for

the LDOs and step-down converter [3V out for each

LDO]

By substitution, we can derive the maximum charge cur-

rent before reaching the thermal limit condition (TREG =

100°C, Thermal Loop Regulation). The maximum charge

current is the key factor when designing battery charger

applications.

CH_CC(MAX) =

- [(V

BAT - VOUT1) · IOUT1] - (VBAT - VOUT2) · IOUT2

- [(V

BAT - VOUT3) · IOUT3] - (VBAT - VOUT4) · IOUT4

- (V

BAT - VOUT5) · IOUT5

CHGIN - VBAT

- I

OUTBUCK

2 · R

DS(ON)L ·

DS(ON)H · (VBAT - VOUTBUCK)

BAT

OUTBUCK

BAT

REG - TA)

- (V

CHGIN · IOP)

- (V

CHGIN - VBAT) · IBAT

In general, the worst condition is when there is the

greatest voltage drop across the charger, when battery

voltage is charged up to just past the preconditioning

voltage threshold and the LDOs and step-down con-

verter are sourcing full output current.

For example, if 977mA is being sourced from the BAT pin

to the LDOs and Buck channels (300mA to LDO1, 100mA

to LDO2-5, and 277mA to the Buck; see buck efficiency

graph for 300mA output current) with a CHGIN supply of

5V, and the battery is being charged at 3.0V with 800mA

charge current, then the power dissipated will be 3.64W.

A reduction in the charge current (through I2C) may be

necessary in addition to the reduction provided by the

internal thermal loop of the charger itself.

For the above example at TA = 30°C, the ICH_CC(MAX) =

386mA.

Thermal Overload Protection

The AAT3603 integrates thermal overload protection

circuitry to prevent damage resulting from excessive

thermal stress that may be encountered under fault con-

ditions, for example. This circuitry disables all regulators

if the AAT3603 die temperature exceeds 140°C, and

prevents the regulators from being enable until the die

temperature drops by 15°C (typ).

Synchronous Step-Down

Converter (Buck)

The AAT3603 contains a high performance 300mA,

1.5MHz synchronous step-down converter. The step-

down converter operates to ensure high efficiency per-

formance over all load conditions. It requires only three

external power components (CIN, COUT, and L). A high DC

gain error amplifier with internal compensation controls

the output. It provides excellent transient response and

load/line regulation. Transient response time is typically

less than 20μs. The converter has soft start control to

limit inrush current and transitions to 100% duty cycle

at drop out.

The step-down converter input pin PVIN should be con-

nected to the BAT output pin. The output voltage is

internally fixed at 1.8V. Power devices are sized for

300mA current capability while maintaining over 90%

efficiency at full load.

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