AAT2556 datasheet(13/29 Pages) AAT

13 / 29 page

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

(VOVP). 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 VMIN, 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

2556.2006.09.1.2

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