AAT2500 datasheet(13/26 Pages) ANALOGICTECH

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AAT2500

1MHz Step-Down Converter/LDO Regulator

2500.2006.05.1.16

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.

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.

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.

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-

bination 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 (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 1.5k

Ω resistor is connected between OUT and

GND. This is intended to discharge COUT when the

LDO regulator is disabled. The internal 1.5K

resistor has no adverse impact on device turn-on

time.

Step-Down Converter

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.

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.

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