8

AAT1151

850kHz 700mA Synchronous Buck DC/DC Converter

DATA SHEET

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

201991B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013

Operation

Control Loop

The AAT1151 is a peak current mode buck converter. The

inner wide bandwidth loop controls the peak current of

the output inductor. The output 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. The loop appears as a volt-

age programmed current source in parallel with the out-

put capacitor.

The voltage error amplifier output programs the current

loop for the necessary inductor current to force a con-

stant output voltage for all load and line conditions. The

voltage feedback resistive divider is internal, dividing the

output voltage to the error amplifier reference voltage of

1.0V. The voltage error amplifier does not have the large

DC gain typical of most error amplifiers. This eliminates

the need for external compensation components, while

still providing sufficient DC loop gain for load regulation.

The voltage loop crossover frequency and phase margin

are set by the output capacitor value only.

PFM/PWM Operation

Light load efficiency is maintained by way of PFM control.

The AAT1151 PFM control forces the peak inductor cur-

rent to a minimum level regardless of load demand. At

medium to high load demand, this has no effect on cir-

cuit operation and normal PWM controls take over. PFM

reduces the switching frequency at light loads, thus

reducing the associated switching losses.

Soft Start/Enable

Soft start increases the inductor current limit point in

discrete steps when 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 AAT1151 into a low-power,

non-switching state. The total input current during shut-

down is less than 1μA.

Functional Block Diagram

VP = 2.7V to 5.5V

VCC

EN

SGND

PGND

LOGIC

1.0V REF

Temp.

Sensing

OSC

OP. AMP

LX

FB

DH

DL

CMP

1M

Ω