PRODUCT DATASHEET
AAT1146
SwitchRegTM
FastTransient 400mA Step-Down Converter
1
2
3
VIN
Enable
CFF
U1
AAT1146
1
2
3
4
8
7
6
5
EN
PGND
OUT PGND
R1 118k
L1
VIN
LX
PGND
AGND
VOUT
4.7µH
C1
C2
4.7µF
R2
59k
10µF
GND
GND2
LX
U1 AAT1146 SC70JW-8
L1 CDRH3D16-4R7
C1 10µF 10V 0805 X5R
C2 4.7µF 10V 0805 X5R
Figure 1: Enhanced Transient Response Schematic.
Control Loop
Current Limit and
Over-Temperature Protection
The AAT1146 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.
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.
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.
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
conditions is removed, the output voltage automatically
recovers.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN input.
Under-voltage lockout (UVLO) guarantees sufficient VIN
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 AAT1146 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
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1146.2008.06.1.6
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
