AAT1138 Datasheet, PDF (13/21 Pages) Skyworks Solutions Inc. – 2A Step-Down Converter with Adjustable Input Current Limit

DATA SHEET
AAT1138
2A Step-Down Converter with Adjustable Input Current Limit
appears as a voltage-programmed current source in par-
allel with the output capacitor. The output of the voltage
error amplifier programs the current mode loop for the
necessary peak switch current to force a constant output
voltage for all load and line conditions. Internal loop
compensation terminates the transconductance voltage
error amplifier output. For fixed voltage versions, the
error amplifier reference voltage is internally set to pro-
gram the converter output voltage. For the adjustable
output, the error amplifier reference is fixed at 0.6V.
Current Limit
The AAT1138 includes two kinds of current limit. One is
input current limit by the load switch; the other is induc-
tor current limit by the high-side MOSFET current sense
loop of the step-down converter. For overload conditions,
the input current is limited by R_SET and the peak inductor
current is limited to 3.5A. To minimize power dissipation
and stresses under current limit and short-circuit condi-
tions, step-down converter switching is terminated after
entering current limit for a series of pulses. The termina-
tion lasts for seven consecutive clock cycles after a cur-
rent limit has been sensed during a series of four con-
secutive clock cycles.
Over-Temperature Protection
Thermal protection completely disables load switch and
step-down converter switching when internal dissipation
becomes excessive. The junction over-temperature
threshold is 125°C with 10°C of hysteresis. Once an
over-temperature or over-current fault condition is
removed, the output voltage automatically recovers.
Enable/Soft Start
AAT1138 has two independent enable pins: ENSYS and
ENBUCK. When ENSYS is pulled high, the current limit
load switch is turned off and SYSOUT drops to zero.
When ENBUCK is pulled low, the step-down converter is
forced into the low-power, no-switching state. Soft start
of the step-down converter limits the current surge seen
at the input and eliminates output voltage overshoot.
Dropout Operation
When the step-down converter input voltage V_PVIN is
close to the value of the output voltage, the main switch
is allowed to remain on for more than one switching cycle
and increases the duty cycle until it reaches 100%. The
duty cycle D of a step-down converter is defined as:
D = T_ON · f_OSC · 100% =
V_{OUT · 100%}
V_IN
Where T_ON is the main switch on time and f_OSC is the oscil-
lator frequency 1.2MHz. The output voltage then is the
input voltage minus the voltage drop across the main
switch and the inductor. At low input supply voltage, the
R_DS(ON) of the P-channel MOSFET increases, and the effi-
ciency of the converter decreases. Caution must be exer-
cised to ensure the heat dissipated does not exceed the
maximum junction temperature of the IC.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
201978B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013
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