AAT2552_08 Datasheet, PDF (15/31 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

SystemPower^TM
PRODUCT DATASHEET
AAT2552178
Total Power Solution for Portable Applications
sumes 45μA of current, making it ideal for battery-
operated applications. The output voltage is program-
mable from V_IN 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
compensation controls the output. It provides excellent
transient response and load/line regulation.
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 tailored for supe-
rior transient response characteristics. These traits are
particularly important for applications that require fast
power supply timing.
The high-speed turn-on capability is enabled through
implementation of a fast-start control circuit which
accelerates the power-up behavior of fundamental con-
trol 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 regulator comes with complete short-circuit and ther-
mal protection. The combination of these two internal
protection circuits gives a comprehensive safety system
to guard against extreme adverse operating conditions.
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 disable shut-
down 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 regu-
lator in a continuously on state.
Under-Voltage Lockout
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 control 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-volt-
age protection threshold (V_OVP). If this over-voltage condi-
tion 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 impedance decreas-
es 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 protec-
tion circuit completely disables 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 thermal limit threshold.
Once the internal die temperature falls below the thermal
limit, normal charging operation will resume.
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 cur-
rent to maintain stability for duty cycles greater than
50%. The peak current mode loop appears as a voltage-
programmed current source in parallel with the output
capacitor. The output of the voltage error amplifier pro-
grams the current mode loop for the necessary peak
switch current to force a constant output voltage 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 environ-
ment. The input supply (ADP) must be above the mini-
mum operating voltage (UVLO) and the enable pin must
2552.2008.02.1.2
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