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AAT1153

2A Step-Down Converter

PRODUCT DATASHEET

1153.2008.02.1.2

www .analogictech.com

Control Loop

The AAT1153 is a peak current mode step-down con-

verter. The current through the P-channel MOSFET (high

side) is sensed for current loop control, as well as short

circuit and overload protection. A slope compensation

signal is added to the sensed current to maintain stabil-

ity 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 programs 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 transconduc-

tance voltage error amplifier output. For fixed voltage

versions, the error amplifier reference voltage is inter-

nally set to program the converter output voltage. For

the adjustable output, the error amplifier reference is

fixed at 0.6V.

Soft Start / Enable

Soft start limits the current surge seen at the input and

eliminates output voltage overshoot. The enable pin is

active high. When pulled low, the enable input (EN)

forces the AAT1153 into a low-power, non-switching

state. The total input current during shutdown is less

than 1μA.

Current Limit and

Over-Temperature Protection

For overload conditions, the peak input current is limited

to 3.5A. To minimize power dissipation and stresses

under current limit and short-circuit conditions, switch-

ing is terminated after entering current limit for a series

of pulses. The termination lasts for seven consecutive

clock cycles after a current limit has been sensed during

a series of four consecutive clock cycles.

Thermal protection completely disables switching when

internal dissipation becomes excessive. The junction

over-temperature threshold is 170°C with 10°C of hys-

teresis. Once an over-temperature or over-current fault

conditions is removed, the output voltage automatically

recovers.

Dropout Operation

When the battery input voltage decreases near the value

of the output voltage, the AAT1153 allows the main

switch to remain on for more than one switching cycle

and increases the duty cycle until it reaches 100%. The

duty cycle D of a step-down converter is defined as:

· 100%

D = T

ON · FOSC · 100%

≈

OUT

Where TON is the main switch on time and FOSC is the

oscillator frequency. The output voltage then is the input

voltage minus the voltage drop across the main switch

and the inductor. At low input supply voltage, the RDS(ON)

of the P-channel MOSFET increases, and the efficiency of

the converter decreases. Caution must be exercised to

ensure the heat dissipated does not exceed the maxi-

mum junction temperature of the IC.

Maximum Load Current

The AAT1153 will operate with an input supply voltage

as low as 2.5V, however, the maximum load current

decreases at lower input voltages due to a large IR drop

on the main switch and synchronous rectifier. The slope

compensation signal reduces the peak inductor current

as a function of the duty cycle to prevent sub-harmonic

oscillations at duty cycles greater than 50%. Conversely

the current limit increases as the duty cycle decreases.

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