PRODUCT DATASHEET
AAT1
161
SwitchReg
TM
Low Dropout Operation
The AAT1161 operates with duty cycle up to 100% to
minimize the dropout voltage, increasing the available
input voltage range for a given output voltage. As the
input voltage decreases toward the output voltage, the
duty cycle increases until it reaches the maximum on-
time. Further reduction of the supply voltage forces the
PMOS on 100%; the output voltage is determined by the
p-channel MOSFET switch and inductor voltage drops.
13.2V Input, 3A Step-Down Converter
Applications Information
Setting the Output Voltage
Figure 1 shows the basic application circuit for the AAT1161
and output setting resistors. Resistors R3 and R6 program
the output to regulate at a voltage higher than 0.6V. To
limit the bias current required for the external feedback
resistor string while maintaining good noise immunity, the
minimum suggested value for R6 is 5.9kΩ. Although a
larger value will further reduce quiescent current, it will
also increase the impedance of the feedback node, making
it more sensitive to external noise and interference. Table
1 summarizes the resistor values for various output volt-
ages with R6 set to either 5.9kΩ for good noise immunity
or 59kΩ for reduced no load input current. The external
resistors set the output voltage according to the follow-
ing equation:
Short-Circuit Protection
The AAT1161 uses a cycle-by-cycle current limit to pro-
tect itself and the load from an external fault condition.
When the inductor current reaches the internally set
6.0A current limit, the P-channel MOSFET switch turns
off, limiting the inductor and the load current. During an
overload condition, when the output voltage drops below
25% of the regulation voltage (0.15V at FB), the
AAT1161 switching frequency drops by a factor of 4. This
gives the inductor current ample time to reset during the
off time to prevent the inductor current from rising
uncontrolled in a short-circuit condition.
⎛
R3
⎞
V
OUT
= 0.6V 1
+
⎝
R6
⎠
or
Thermal Protection
The AAT1161 includes thermal protection that disables
the regulator when the die temperature reaches 140ºC.
It automatically restarts when the temperature decreas-
es by 25ºC or more.
⎛
V
OUT
⎞
R3 = V
-1
·
R6
⎝
REF
⎠
The adjustable feedback resistors, combined with an
external feed forward capacitor (C1 in Figure 1), deliver
enhanced transient response for extreme pulsed load
applications. The addition of the feed forward capacitor
typically requires a larger output capacitor C3/C4/C5 for
stability. Larger C3/C4/C5 values reduce overshoot and
undershoot during startup and load changes. However,
do not exceed 470pF to maintain stable operation.
V
IN
4.5V- 13.2V
6
L
1
3.8µH
EN
IN
C
2
0.1µ F
11
LX
LX
8
V
OUT
5V, 3A
R
3
432 kΩ
R
6
59kΩ
R
5
51kΩ
C
6
10µF
R
4
10Ω
10
9
1
IN
13
C
1
D1 100pF
AAT1161
FB
PGND
COMP
AGND
C
3, 4, 5
66µF
C
8
1µ F
AIN
4, 5
12
2
DGND
N/C
PGND
EP1
7
3
14
LDO
C
9
1µ F
C
7
150pF
Figure 1: Typical Application Circuit.
1161.2008.03.1.0
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