AAT1156_07 datasheet(8/15 Pages) ANALOGICTECH

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AAT1156

1MHz 700mA Step-Down DC-DC Converter

1156.2007.01.1.4

Functional Block Diagram

VP = 2.7V to 5.5V

VCC

SGND

PGND

LOGIC

1.0V REF

Temp.

Sensing

OSC

OP. AMP

CMP

Operation

Control Loop

The AAT1156 is a peak current mode step-down

converter. The inner wide bandwidth loop controls

the inductor peak current. The inductor current is

sensed through the P-channel MOSFET (high

side) and is also used for short-circuit and overload

protection. A fixed slope compensation signal is

added to the sensed current to maintain stability for

duty cycles greater than 50%. The loop appears

as a voltage-programmed current source in paral-

lel with the output capacitor.

The voltage error amplifier output programs the

current loop for the necessary inductor current to

force a constant output voltage for all load and line

conditions. The external voltage feedback resistive

divider divides the output voltage to the error ampli-

fier reference voltage of 0.6V. The voltage error

amplifier DC gain is limited. This eliminates the

need for external compensation components, while

still providing sufficient DC loop gain for good load

regulation. The voltage loop crossover frequency

and phase margin are set by the output capacitor.

Soft Start/Enable

Soft start increases the inductor current limit point in

discrete steps once the input voltage or enable

input is applied. It limits the current surge seen at

the input and eliminates output voltage overshoot.

When pulled low, the enable input forces the

AAT1156 into a non-switching shutdown state. The

total input current during shutdown is less than 1μA.

Power and Signal Source

Separate small signal ground and power supply

pins isolate the internal control circuitry from the

noise associated with the output MOSFET switch-

ing. The low pass filter R1 and C2 (shown in the

schematic in Figure 1) filters the input noise asso-

ciated with the power switching.

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