AAT2552
Total Power Solution for Portable Applications
Switch-Mode Step-Down Converter
Under-Voltage Lockout
The step-down converter operates with an input
voltage 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-sav-
ing mode; the switching frequency is reduced, and
the converter consumes 45µA of current, making it
ideal for battery-operated applications. The output
voltage is programmable from VIN to as low as
0.6V. Power devices are sized for 300mA current
capability while maintaining over 96% 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 compen-
sation controls the output. It provides excellent
transient response and load/line regulation.
The AAT2552 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.
The AAT2552 synchronous step-down converter
can be synchronized to an external clock signal
applied to the MODE pin.
Linear Regulator
The advanced circuit design of the linear regulator
has been specifically optimized for very fast start-
up. This proprietary CMOS LDO has also been tai-
lored for superior transient response characteris-
tics. These traits are particularly important for appli-
cations that require fast power supply timing.
Current Limit, Over-Temperature Protection
For overload conditions, the peak input current is lim-
ited at the step-down converter. As load impedance
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 ther-
mal protection circuit completely disables switching,
which protects the device from damage.
The high-speed turn-on capability is enabled
through implementation of a fast-start control cir-
cuit which accelerates the power-up behavior of
fundamental control and feedback circuits within
the LDO regulator. The LDO regulator output has
been specifically optimized to function with low-
cost, low-ESR ceramic capacitors; however, the
design will allow for operation over a wide range
of capacitor types.
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.
The regulator comes with complete short-circuit
and thermal protection. The combination of these
two internal protection circuits gives a comprehen-
sive safety system to guard against extreme
adverse operating conditions.
Control Loop
The AAT2552 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
The regulator features an enable/disable function.
This pin (ENA) is active high and is compatible with
CMOS logic. The LDO regulator will go into the dis-
able shutdown mode when the voltage on the ENA
pin falls below 0.6V. If the enable function is not
needed in a specific application, it may be tied to INA
to keep the LDO regulator in a continuously on state.
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2552.2007.04.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
