AAT2556
Battery Charger and Step-Down
Converter for Portable Applications
Adjustable Output Resistor Selection
Ω
Ω
R4 = 221k
R4 = 59k
Resistors R3 and R4 of Figure 5 program the out-
put to regulate at a voltage higher than 0.6V. To
limit the bias current required for the external feed-
back resistor string while maintaining good noise
immunity, the suggested value for R4 is 59kΩ.
Decreased resistor values are necessary to main-
tain noise immunity on the FB pin, resulting in
increased quiescent current. Table 4 summarizes
the resistor values for various output voltages.
Ω
Ω
VOUT (V)
R3 (k )
R3 (k )
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
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
1000
V
V
3.3V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
- 1 ·
R3 =
OUT -1
·
R4 =
59kΩ = 267kΩ
124
137
187
267
⎠
⎠
REF
With enhanced transient response for extreme
pulsed load application, an external feed-forward
capacitor (C5 in Figure 5) can be added.
Table 4: Adjustable Resistor Values For
Step-Down Converter.
JP4
1
2 3
Buck Input
C3
BAT
ADP
VIN
R4
59k
R3
118k
VOUT
4.7µF
L1
3.3µH
C4
C5
100pF
U1
4.7µF
1
12
11
10
9
FB
GND
VIN
2
3
4
5
6
LX
GND
ADP
EN_BUCK
EN_BAT
ISET
8
GND
STAT
C1
10µF
7
BAT
JP1
0Ω
C2
R2
AAT2556
D1
R1
1K
2.2µF
8.06K
RED LED
1
2
3
1
2
JP3
Enable_Bat
Enable_Buck
JP2
Figure 5: AAT2556 Evaluation Board Schematic.
2556.2006.05.1.0
21
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