AAT1171
600mA Voltage-Scaling Step-Down Converter
for RF Power Amplifiers with Bypass Switch
Functional Block Diagram
VOUT
VCC
VIN
DH
Comp
Error
Amp
DAC
EN
Logic
LX
DL
BYPASS
MODE/SYNC
Interface
MODE/SYNC
AGND PGND
bypassing the converter and output inductor to
improve headroom and extend the WCDMA PA full
power range. This reduces the battery voltage nec-
essary for a WCDMA RF power amplifier to meet
linearity requirements, thus extending operating
time. In dual mode systems, the bypass mode may
also be used when the WCDMA RF power amplifi-
er 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.
Functional Description
The AAT1171 is a 600mA 2MHz peak current mode
synchronous step-down (buck) converter designed
to operate 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 con-
ditions, the converter automatically transitions to a
variable frequency 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 AAT1171 requires only three external compo-
nents for operation (CIN, COUT, LX). The high 2MHz
switching frequency reduces the inductor size
required to 2.2µH. This reduces the DC resistance
and improves the converter efficiency while mini-
mizing the inductor 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.
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 (VIN to VOUT),
Under-voltage lockout, internal compensation, soft-
start, over-current, and over-temperature protec-
tion are also included.
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1171.2006.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
