AAT1171 Datasheet, PDF (11/22 Pages) Advanced Analogic Technologies – 600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch

Functional Block Diagram
DATA SHEET
AAT1171
600mA Voltage-Scaling Step-Down Converter
for RF Power Amplifiers with Bypass Switch
VOUT
VCC
VIN
DAC
EN
BYPASS
MODE/SYNC
Comp
DH
Error
Amp
Logic
LX
DL
MODE/SYNC
Interface
AGND PGND
Functional Description
The AAT1171 is a 600mA 2MHz peak current mode syn-
chronous step-down (buck) converter designed to oper-
ate from a single-cell lithium-ion battery with a 2.7V to
4.2V input range. The output voltage is dynamically
programmed by the DAC input voltage.
To maximize converter efficiency over all load conditions,
the converter automatically transitions to a variable fre-
quency light load (LL) mode when the load is less than
100mA. When combined with the very low quiescent
current, the LL mode maintains a high efficiency over the
complete load range. For noise sensitive applications,
the converter can be forced into a fixed frequency PWM
mode. Provisions are also made for synchronization of
the converter to an external system clock.
The synchronous buck converter power output devices
are sized at 230mΩ for a 600mA full load output current.
In addition to the converter output, an additional low
resistance bypass MOSFET (85mΩ) can be connected
between the battery input and the converter output (V_IN
to V_OUT), bypassing the converter and output inductor to
improve headroom and extend the WCDMA PA full power
range. This reduces the battery voltage necessary for a
WCDMA RF power amplifier to meet linearity require-
ments, thus extending operating time. In dual mode
systems, the bypass mode may also be used when the
WCDMA RF power amplifier is in GSM mode. Bypass
mode is activated by setting the bypass input high or by
forcing the baseband DAC output voltage to 1.3V.
The AAT1171 requires only three external components
for operation (C_IN, C_OUT, L_X). The high 2MHz switching
frequency reduces the inductor size required to 2.2μH
for the AAT1171-1/AAT1171-4 and 4.7μH for the
AAT1171-5. This reduces the DC resistance and improves
the converter efficiency while minimizing the inductor
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201999B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013
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