1MHz Step-Down Converter
AAT1123
1
2
3
V
IN
Enable
C4 100pF
1
2
U1
AAT1123
EN
OUT
VIN
LX
PGND
PGND
PGND
AGND
8
7
6
5
R1 118k
V
OUT
C1
22μF
L1
4.7μH
R2
59k
3
4
C2
4.7μF
GND
GND2
LX
U1 AAT1123 SC70JW-8
L1 CDRH3D16-4R7
C1 22μF 6.3V 0805 X5R
C2 4.7μF 6.3V 0805 X5R
Figure 1: Enhanced Transient Response Schematic.
Control Loop
The AAT1123 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. For fixed voltage versions,
the error amplifier reference voltage is internally set
to program the converter output voltage. For the
adjustable output, the error amplifier reference is
fixed at 0.6V.
input current during shutdown is less than 1µA.
The AAT1123 provides turn-on within 100µs (typi-
cal) of the enable input transition.
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 AAT1123
into a low-power, non-switching state. The total
Under-Voltage Lockout
Internal bias of all circuits is controlled via the V
IN
input. Under-voltage lockout (UVLO) guarantees
sufficient V
IN
bias and proper operation of all inter-
nal circuitry prior to activation.
9
1123.2006.05.1.5
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
