PRODUCT DATASHEET
AAT1145
SwitchReg
TM
Applications Information
VIN 2.5V-5.5V
1
2
C1
10μF
3
EN
IN
AIN
LX
LX
8
7
5
10
9
L1
2.2μH
C3
22pF
VOUT
1.8V,1.2A
R1
634kΩ
R2
316kΩ
1.2A Step-Down Converter
delivers enhanced transient response for extreme pulsed
load applications. The addition of the feed forward
capacitor typically requires a larger output capacitor C2
for stability. The external resistor sets the output voltage
according to the following equation:
AAT1145-0.6
FB
C2
22μF
6
AGND
4
AGND
PGND
PGND
⎛
R1
⎞
V
OUT
= 0.6V · 1 +
⎝
R2
⎠
Figure 1: Basic Application Circuit for the
Adjustable Output Version.
L1
2.2μH
VOUT
1.8V,1.2A
R1
=
⎛
V
OUT
⎞
- 1 · R2
⎝
0.6V
⎠
Table 1 shows the resistor selection for different output
voltage settings.
R2 = 59kΩ
R1 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
VIN 2.5V-5.5V
1
2
3
EN
IN
AIN
LX
LX
8
7
C1
10μF
AAT1145-1.8
OUT
5
PGND
PGND
10
9
C2
22μF
V
OUT
(V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
R2 = 316kΩ
R1 (kΩ)
105
158
210
261
316
365
422
475
634
655
732
1000
1430
6
AGND
4
AGND
Figure 2: Basic Application Circuit for the Fixed
Output Versions.
Setting the Output Voltage
Figure 1 shows the basic application circuit with the
AAT1145 adjustable output version while Figure 2 shows
the application circuit with the AAT1145 fixed output
version. For applications requiring an adjustable output
voltage, the AAT1145-0.6 adjustable version can be
externally programmed. Resistors R1 and R2 in Figure 1
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 R2 is 59kΩ.
Although a larger value will further reduce quiescent
current, it will also increase the impedance of the feed-
back node, making it more sensitive to external noise
and interference. Table 1 summarizes the resistor values
for various output voltages with R2 set to either 59kΩ
for good noise immunity or 316kΩ for reduced no load
input current.
The adjustable version of the AAT1145, combined with
an external feed forward capacitor (C3 in Figure 1),
Table 1: Resistor Selections for Different Output
Voltage Settings (Standard 1% Resistors
Substituted For Calculated Values).
Inductor Selection
For most designs, the AAT1145 operates with inductor
values of 1μH to 4.7μH. Low inductance values are
physically smaller but require faster switching, which
results in some efficiency loss. The inductor value can
be derived from the following equation:
L=
V
OUT
· (V
IN
- V
OUT
)
V
IN
·
ΔI
L
· f
OSC
1145.2007.11.1.1
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