AAT2138 Datasheet, PDF (11/18 Pages) Skyworks Solutions Inc. – High Current Step-Down Converter with High-Precision Programmable Current Limited Load Switch

DATA SHEET
AAT2138
High Current Step-Down Converter
with High-Precision Programmable Current Limited Load Switch
Functional Block Diagram
SYSIN
RSET
ENSYS
FB
AGND
ENREG
MODE
1.0V
Ref.
Current
Limit
VP
Load Switch
Control
EN
Discharge
Control
600mV
Reference
Current
VP
Limit
Step-Down
Converter
EN MODE
REGIN
SW
PGND
Functional Description
The AAT2138 is a high performance, 2.8MHz, synchro-
nous step-down converter with a high-precision, pro-
grammable current limited P-channel load switch up to
3A.
The P-channel load switch is adopted to limit the system
input current. The current limit value is programmed by
external resistor between RSET and GND. Its fast tran-
sient response time makes it ideal to protect the system
from input power surges.
The AAT2138 employs internal error amplifier and com-
pensation. It provides excellent transient response, load
and line regulation. Its output voltage is programmed by
an external resistor divider from 3.0V to converter input
voltage. Soft start eliminates any output voltage over-
shoot when the enable or input voltage is applied.
Dropout mode makes the converter increase the switch
duty cycle to 100% and the output voltage tracks the
input voltage minus the R_DS(ON) drop of the P-channel
high-side MOSFET of the converter.
The AAT2138's input voltage range is 2.7V to 5.5V. Two
independent enable pins control the load switch and
step-down converter separately. The converter efficiency
has been optimized for a 1μH inductor.
Control Scheme
The AAT2138 uses a peak current-mode step-down con-
trol scheme. The converter senses the current through
the high-side P-channel MOSFET for current loop control
as well as overload protection. A fixed slope compensa-
tion signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The input
current of the step-down converter is limited by the load
switch.
The feedback amplifier compares the FB voltage against
the 0.6V reference voltage. The error amplifier’s trans-
conductance output is internally compensated, and pro-
grams the current-mode loop for the necessary peak
switch current to force a constant output voltage over all
load and line conditions.
Enable/Soft Start
AAT2138 has two independent enable pins: ENSYS and
ENREG. When ENSYS is pulled high, the current limit
load switch is turned off and REGIN drops to zero. When
ENREG is pulled low, the step-down converter is forced
into the low-power, no-switching, high impedance state.
The total input current during shutdown is less than 1uA.
When ENSYS is pulled low, the system will turn on the
load switch in a soft start process. To avoid a big inrush
current during the startup, the REGIN voltage will be
charged from SYSIN with limited input current under
500mA. The startup time depends on the capacitance
between REGIN and GND.
When ENREG is pulled high, the step-down converter is
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202006C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 18, 2013
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