Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com

202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013

11

AAT2138

High Current Step-Down Converter

with High-Precision Programmable Current Limited Load Switch

DATA SHEET

Functional Description

The AAT2138 is a high performance, 2.8MHz, synchro-

nous step-down converter with a high-precision, pro-

grammable current limited P-channel load switch up to

3A.

The P-channel load switch is adopted to limit the system

input current. The current limit value is programmed by

external resistor between RSET and GND. Its fast tran-

sient response time makes it ideal to protect the system

from input power surges.

The AAT2138 employs internal error amplifier and com-

pensation. It provides excellent transient response, load

and line regulation. Its output voltage is programmed by

an external resistor divider from 3.0V to converter input

voltage. Soft start eliminates any output voltage over-

shoot when the enable or input voltage is applied.

Dropout mode makes the converter increase the switch

duty cycle to 100% and the output voltage tracks the

high-side MOSFET of the converter.

The AAT2138's input voltage range is 2.7V to 5.5V. Two

independent enable pins control the load switch and

step-down converter separately. The converter efficiency

has been optimized for a 1μH inductor.

Control Scheme

The AAT2138 uses a peak current-mode step-down con-

trol scheme. The converter senses the current through

the high-side P-channel MOSFET for current loop control

as well as overload protection. A fixed slope compensa-

tion signal is added to the sensed current to maintain

stability for duty cycles greater than 50%. The input

current of the step-down converter is limited by the load

switch.

The feedback amplifier compares the FB voltage against

the 0.6V reference voltage. The error amplifier’s trans-

conductance output is internally compensated, and pro-

grams the current-mode loop for the necessary peak

switch current to force a constant output voltage over all

load and line conditions.

Enable/Soft Start

AAT2138 has two independent enable pins: ENSYS and

ENREG. When ENSYS is pulled high, the current limit

load switch is turned off and REGIN drops to zero. When

ENREG is pulled low, the step-down converter is forced

into the low-power, no-switching, high impedance state.

The total input current during shutdown is less than 1uA.

When ENSYS is pulled low, the system will turn on the

load switch in a soft start process. To avoid a big inrush

current during the startup, the REGIN voltage will be

charged from SYSIN with limited input current under

500mA. The startup time depends on the capacitance

between REGIN and GND.

When ENREG is pulled high, the step-down converter is

Functional Block Diagram

Step-Down

Converter

600mV

Reference

FB

SW

PGND

ENREG

REGIN

EN

VP

Load Switch

Control

SYSIN

EN

VP

ENSYS

1.0V

Ref.

Current

Limit

RSET

Current

Limit

MODE

MODE

AGND

Discharge

Control