AAT1171 Datasheet, PDF (13/22 Pages) Advanced Analogic Technologies – 600mA Voltage-Scaling Step-Down Converter for RF Power Amplifiers with Bypass Switch

DATA SHEET
AAT1171
600mA Voltage-Scaling Step-Down Converter
for RF Power Amplifiers with Bypass Switch
Low Dropout Operation
For conditions where the input voltage drops to the out-
put 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-channel
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-channel MOSFET R_DS(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 sufficient 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.
Current Limit and
Short-Circuit Protection
The high-side P-channel MOSFET current limit compara-
tor limits the peak inductor current to 1.6A. In PWM
mode, the synchronous MOSFET current limit compara-
tor limits the peak negative inductor current, and output
capacitor discharge current is limited to 1A. In bypass
mode, the bypass MOSFET 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 condi-
tion, the AAT1171 output automatically recovers to the
regulated level.
Thermal Overload Protection
The maximum junction temperature is limited by the
AAT1171 over-temperature shutdown protection circuit-
ry. 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.
External Synchronization
The AAT1171 switching frequency can be synchronized 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.
Applications Information
Inductor Selection
The step-down converter uses peak current mode con-
trol with slope compensation to maintain stability for
duty cycles greater than 50%. Because the required
slope compensation varies with output voltage, the
AAT1171 varies the slope compensation to match the
output voltage. This allows the use of a single inductor
value for all output voltage levels. The inductor value is
2.2μH for the AAT1171-1/AAT1171-4 and 4.7μH for the
AAT1171-5.
Manufacturer’s specifications list both the inductor DC
current rating, which is a thermal limitation, and the
peak current rating, which is determined by the satura-
tion characteristics.
The inductor should not show any appreciable saturation
under normal load conditions. The inductor ripple cur-
rent varies with both the input voltage and the output
voltage and peaks at the maximum input voltage with
the output at one half of the input voltage. For the typi-
cal AAT1171, this corresponds to a 4.2V input voltage
and a 2.1V output voltage. With the suggested 2.2μH
inductor, this corresponds to 239mA peak-to-peak ripple
current. For a 600mA DC load current, the peak inductor
current would be 718mA. In order to prevent saturation
under normal load conditions, the peak inductor current
should be less than the inductor saturation current.
I_PK(MAX) = I_O +
V_IN(MAX)
8 ∙ L ∙ F_S
=
0.6A
+
8
∙
4.2V
2.2μH ∙
2MHz
= 0.6A + 0.12A
= 0.72A
Some inductors may meet peak and average current
requirements yet result in excessive losses due to a high
DCR. Always consider the losses associated with the
DCR and its effect on the total converter efficiency when
selecting an inductor. The inductor losses can be esti-
mated by using the full load output current. The output
inductor losses can then be calculated to estimate their
effect on overall device efficiency.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
201999B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013
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