AAT2556
Battery Charger and Step-Down
Converter for Portable Applications
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.
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.
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
Battery Charging Operation
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.
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.
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-
2556.2006.05.1.0
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