AAT2500
1MHz Step-Down Converter/LDO Regulator
bination of these two internal protection circuits
gives a comprehensive safety system to guard
against extreme adverse operating conditions.
Functional Description
The AAT2500 is a high performance power man-
agement IC comprised of a buck converter and a
linear regulator. The buck converter is a high effi-
ciency converter capable of delivering up to
400mA. Designed to operate at 1.0MHz, the con-
verter requires only three external components
(CIN, COUT, and LX) and is stable with a ceramic
output capacitor. The linear regulator delivers
300mA and is also stable with ceramic capacitors.
The regulator features an enable/disable function.
This pin (ENLDO) is active high and is compatible
with CMOS logic. To assure the LDO regulator will
switch on, the ENLDO turn-on control level must be
greater than 1.5V. The LDO regulator will go into
the disable shutdown mode when the voltage on
the EN pin falls below 0.6V. If the enable function is
not needed in a specific application, it may be tied
to VIN to keep the LDO regulator in a continuously
on state.
When the regulator is in shutdown mode, an inter-
nal 1.5kΩ resistor is connected between OUT and
GND. This is intended to discharge COUT when the
LDO regulator is disabled. The internal 1.5KΩ
resistor has no adverse impact on device turn-on
time.
Linear Regulator
The advanced circuit design of the linear regulator
has been specifically optimized for very fast start-
up and shutdown timing. This proprietary CMOS
LDO has also been tailored for superior transient
response characteristics. These traits are particu-
larly important for applications that require fast
power supply timing.
Step-Down Converter
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. Fast turn-off time response is
achieved by an active output pull-down circuit,
which is enabled when the LDO regulator is
placed in shutdown mode. This active fast shut-
down circuit has no adverse effect on normal
device operation. 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 AAT2500 buck is a constant frequency peak
current mode PWM converter with internal com-
pensation. It is designed to operate with an input
voltage range of 2.7V to 5.5V. The output voltage
ranges from 0.6V to the input voltage for the inter-
nally fixed version, and up to 2.5V for the external-
ly adjustable version. The 0.6V fixed model shown
in Figure 1 is also the adjustable version and is
externally programmable with a resistive divider, as
shown in Figure 2. The converter MOSFET power
stage is sized for 400mA load capability with up to
96% efficiency. Light load efficiency exceeds 80%
at a 500µA load.
A bypass pin has been provided to allow the addi-
tion of an optional voltage reference bypass capac-
itor to reduce output self noise and increase power
supply ripple rejection. Device self noise and
PSRR will be improved by the addition of a small
ceramic capacitor in this pin. However, increased
values of CBYPASS may slow down the LDO regula-
tor turn-on time. The regulator comes with com-
plete short-circuit and thermal protection. The com-
Soft Start
The AAT2500 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 1µA.
2500.2006.05.1.16
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