AAT2500M
400mA Step-Down Converter and 300mA LDO
Soft Start
Applications Information
The AAT2500M soft-start control prevents output
voltage overshoot and limits inrush current when
either the input power or the enable input is
applied. When pulled low, the enable input forces
the converter into a low-power, non-switching state
with a bias current of less than 1ꢀA.
LDO Regulator
Input and Output Capacitors: An input capacitor
is not required for basic operation of the linear reg-
ulator. However, if the AAT2500M is physically
located at a reasonable distance from an input
power source, an input capacitor (C3) will be need-
ed for stable operation. Typically, a 1ꢀF or larger
capacitor is recommended for C3 in most applica-
tions. C3 should be located as closely to the input
voltage (IN_LDO) pin as practically possible.
Low Dropout Operation
For conditions where the input voltage drops to the
output voltage level, the converter duty cycle
increases to 100%. As 100% duty cycle is
approached, the minimum off-time initially forces
the high side on-time to exceed the 1.8MHz clock
cycle and reduce the effective switching frequency.
Once the input drops below the level where the out-
put can be regulated, the high side P-channel
MOSFET is turned on continuously for 100% duty
cycle. At 100% duty cycle, the output voltage tracks
the input voltage minus the IR drop of the high side
An input capacitor greater than 1ꢀF will offer supe-
rior input line transient response and maximize
power supply ripple rejection. Ceramic, tantalum,
or aluminum electrolytic capacitors may be select-
ed for C3. There is no specific capacitor ESR
requirement for C3. However, for 300mA LDO reg-
ulator output operation, ceramic capacitors are rec-
ommended for C3 due to their inherent capability
over tantalum capacitors to withstand input current
surges from low impedance sources such as bat-
teries in portable devices.
P-channel MOSFET RDS(ON)
.
Low Supply
For proper load voltage regulation and operational
stability, a capacitor is required between the
OUT_LDO and AGND pins. The output capacitor
(C4) connection to the LDO regulator ground pin
should be made as directly as practically possible
for maximum device performance. Since the regu-
lator has been designed to function with very low
ESR capacitors, ceramic capacitors in the 1.0ꢀF to
10ꢀF range are recommended for best perform-
ance. Applications utilizing the exceptionally low
output noise and optimum power supply ripple
rejection should use 2.2ꢀF or greater for C4. In low
output current applications, where output load is
less than 10mA, the minimum value for C4 can be
as low as 0.47ꢀF.
The under-voltage lockout (UVLO) guarantees suf-
ficient VIN bias and proper operation of all internal
circuitry prior to activation.
Fault Protection
For overload conditions, the peak inductor current is
limited. Thermal protection disables switching when
the internal dissipation or ambient temperature
becomes excessive. The junction over-temperature
threshold is 150°C with 15°C of hysteresis.
12
2500M.2007.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
