High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
AAT1147
1
2
3
Enable
V
IN
C4
100pF
1
U1
AAT1147
R1
EN
OUT
VIN
LX
PGND
PGND
PGND
AGND
8
7
6
5
2
3
4
V
OUT
=1.8V
L1 118k
4.7μH
C1
10μF
C3
n/a
R2
59k
C2
4.7μF
GND
LX
GND2
U1 AAT1147 SC70JW-8
L1 CDRH3D16-4R7
C2 4.7μF 10V 0805 X5R
C1 10μF 6.3V 0805 X5R
Figure 1: Enhanced Transient Response Schematic.
Control Loop
The AAT1147 is a peak current mode step-down
converter. The current through the P-channel
MOSFET (high side) is sensed for current loop
control, as well as short circuit and overload pro-
tection. A fixed slope compensation signal is added
to the sensed current to maintain stability for duty
cycles greater than 50%. The peak current mode
loop appears as a voltage-programmed current
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 compen-
sation terminates the transconductance voltage
error amplifier output. The error amplifier reference
is 0.6V.
Current Limit and Over-Temperature
Protection
For overload conditions, the peak input current is
limited. 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 hysteresis. Once an over-temperature or
over-current fault conditions is removed, the output
voltage automatically recovers.
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 AAT1147
into a low-power, non-switching state. The total
input current during shutdown is less than 1µA.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN
input. Under-voltage lockout (UVLO) guarantees
sufficient V
IN
bias and proper operation of all inter-
nal circuitry prior to activation.
1147.2006.05.1.0
9
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
