AAT2500M
400mA Step-Down Converter and 300mA LDO
is 59kΩ. 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 2 summarizes
the resistor values for various output voltages with
R2 set to either 59kΩ for good noise immunity or
221kΩ for reduced no load input current.
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ
R1 (kΩ)
VOUT (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
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
75
113
150
187
221
261
301
332
442
464
523
715
V
V
1.5V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
⎠
R1 =
OUT -1 · R2 =
- 1 · 59kΩ = 88.5kΩ
⎠
REF
The adjustable version of the AAT2500M, com-
bined with an external feedforward capacitor (C8 in
Figures 2 and 5), delivers enhanced transient
response for extreme pulsed load applications. The
addition of the feedforward capacitor typically
requires a larger output capacitor C1 for stability.
124
137
187
Table 2: Adjustable Resistor Values For Use
With 0.6V Step-Down Converter.
VIN1
3
2
1
3
2
1
LX1
LDO Input
LDO Enable
VOUT BUCK
L1
4.7µH
3
2
C1
4.7µF
1
C7
0.01µF
U1
AAT2500M
Buck Enable
1
2
3
4
5
6
12
11
10
9
LX
IN_BUCK
AGND
AGND
R1
Table 2
C8
n/a
PGND
C2
10µF
EN_BUCK
EN_LDO
FB_BUCK
AGND
8
AGND
C9
n/a
R2
59k
7
OUT_LDO IN_LDO
C4
4.7µF
C3
10µF
GND
GND
VOUT LDO
Figure 5: AAT2500M Evaluation Board Schematic.
16
2500M.2007.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
