AAT1171
600mA Voltage-Scaling Step-Down Converter
for RF Power Amplifiers with Bypass Switch
Soft Start/Enable
Current Limit and Short-Circuit
Protection
The AAT1171 soft-start control prevents output volt-
age overshoot and limits inrush current when either
the input power or the enable input is applied.
When pulled low, the enable input forces the con-
verter into a low-power, non-switching state with
less than 1µA bias current.
The high-side P-channel MOSFET current limit
comparator limits the peak inductor current to 1.6A.
In PWM mode, the synchronous MOSFET current
limit comparator limits the peak negative inductor
current, and output capacitor discharge current is
limited to 1A. In bypass mode, the bypass MOS-
FET current is limited to 600mA. In the event of an
overload or short-circuit condition, the current limit
protects the load and the AAT1171 power devices.
Upon removal of the short-circuit or fault condition,
the AAT1171 output automatically recovers to the
regulated level.
Low Dropout Operation
For conditions where the input voltage drops to the
output voltage level, the converter duty cycle
increases to 100%. As 100% duty cycle is
approached, the minimum off-time initially forces
the high-side on-time to exceed the 2MHz clock
period, reducing the converter switching frequency.
Once the input drops to the level where the output
can no longer be regulated, the high-side P-chan-
nel MOSFET is enabled continuously for 100%
duty cycle. The output voltage then tracks the input
voltage minus the IR drop of the high side P-chan-
Thermal Overload Protection
The maximum junction temperature is limited by
the AAT1171 over-temperature shutdown protec-
tion circuitry. Both the step-down converter and the
bypass MOSFET are disabled when the junction
temperature reaches 140°C. Normal operation
resumes once the junction temperature drops to
125°C.
nel MOSFET RDS(ON)
.
UVLO Shutdown
Under-voltage lockout (UVLO) circuitry monitors
the input voltage and disables the converter when
the input voltage drops to 2.4V, guaranteeing suffi-
cient operating input voltage to maintain output
voltage regulation and control. For a rising input
voltage, the UVLO circuitry enables the converter
200mV above the shutdown level at 2.6V.
External Synchronization
The AAT1171 switching frequency can be synchro-
nized to an external square wave clock via the
MODE/SYNC input. The external clock frequency
range and logic levels for which the AAT1171 will
remain synchronized are listed in the Electrical
Characteristics table of this datasheet.
14
1171.2006.06.1.0
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