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201999B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013

AAT1171

600mA Voltage-Scaling Step-Down Converter

for RF Power Amplifiers with Bypass Switch

DATA SHEET

footprint and height. The output voltage of the converter

is regulated to within 0.5% and will settle in less than

30μs (according to WCDMA specifications) in response to

any step change in the DAC input.

Under-voltage lockout, internal compensation, soft-start,

over-current, and over-temperature protection are also

included.

DAC Output Voltage Control

The output voltage is programmed by way of the DAC

input voltage. The DAC to output gain for the AAT1171

is 3.

The DAC input voltage range is 0.2V to 1.2V, which cor-

responds to an output voltage range of 0.6V to 3.6V (see

Figure 1). For a 1.3V DAC level, the bypass switch is

activated and the output voltage level is equivalent to

drop.

Bypass Mode

In bypass mode, the AAT1171 bypasses the output

inductor, connecting the input directly to the output

initiated by applying 1.3V to the DAC input or by apply-

ing a logic high to the bypass input. When not activated,

a logic level low must be applied to the bypass input pin.

The bypass MOSFET current is limited to 600mA.

LL/PWM Control

Two control modes are available with the AAT1171: LL

mode and PWM mode. PWM mode maintains a fixed

switching frequency regardless of load. The fixed switch-

ing frequency gives the advantage of lower output ripple

and simplified output and input noise filtering.

PWM

mode also provides a faster output voltage response to

changes in the DAC voltage.

In LL mode, the converter transitions to a variable

switching frequency as the load decreases below 100mA.

Above 100mA, where switching losses no longer domi-

nate, the switching frequency is fixed. The LL mode’s

effect on the DAC to output voltage response time is

most notable when transitioning from a high output volt-

age to a low voltage. When the converter is in PWM

mode, the inductor current can be reversed and the out-

put voltage actively discharged by the synchronous

MOSFET. While in LL mode, the output voltage is dis-

charged by the load only, resulting in a slower response

to a DAC transition from a high to a low voltage.

For PWM mode, apply a logic level high to the MODE/

SYNC pin; for LL mode, apply a logic level low to the

MODE/SYNC pin.

Soft Start/Enable

The AAT1171 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 less than 1μA bias current.

1V

2V

3V

4V

3.6V

1V

1.2V

DAC Output

0.6V

0.2V

V

IN

1.3V