PRODUCT DATASHEET
AAT1140
SwitchReg
TM
Fast Transient 600mA Step-Down Converter
SW
U1
AAT1140
4
V
IN
C1
4.7μF
2
IN
GND
EN
LX
3
L1
4.7μH
V
OUT
C3
100pF
R1
442k
R2
221k
C2
10μF
Enable
1
1
2
3
OUT
5
Figure 1: Enhanced Transient Response Schematic.
Control Loop
The AAT1140 is a peak current mode step-down con-
verter. The current through the P-channel MOSFET (high
side) is sensed for current loop control, as well as short
circuit and overload protection. A fixed slope compensa-
tion signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The peak cur-
rent mode loop appears as a voltage-programmed cur-
rent source in parallel 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 volt-
age is internally set to program the converter output
voltage. For the adjustable output, the error amplifier
reference is fixed at 0.6V.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limit-
ed. To minimize power dissipation and stresses under
current limit and short-circuit conditions, switching is
terminated after entering current limit for a series of
pulses. Switching is terminated for seven consecutive
clock cycles after a current limit has been sensed for a
series of four consecutive clock cycles.
Thermal protection completely disables switching when
internal dissipation becomes excessive. The junction
over-temperature threshold is 140°C with 15°C of hys-
teresis. Once an over-temperature or over-current fault
condition is removed, the output voltage automatically
recovers.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the IN input.
Under-voltage lockout (UVLO) guarantees sufficient V
IN
bias and proper operation of all internal circuitry prior to
activation.
Soft Start / Enable
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT1140 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
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