AAT2556 Datasheet, PDF (21/29 Pages) Advanced Analogic Technologies – Battery Charger and Step-Down Converter for Portable Applications

AAT2556
Battery Charger and Step-Down
Converter for Portable Applications
Adjustable Output Resistor Selection
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.
R3
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R4
=
⎛ ^3.3V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
267kΩ
With enhanced transient response for extreme
pulsed load application, an external feed-forward
capacitor (C5 in Figure 5) can be added.
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
R4 = 59kΩ
R3 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R4 = 221kΩ
R3 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 4: Adjustable Resistor Values For
Step-Down Converter.
BAT
C2
2.2µF
JP4 _{1 2 3 Buck Input}
ADP
R4
59k
R2
8.06K
R3
VIN
118k
C5
100pF
U1 AAT2556
1
2
3
4
5
6
FB
VIN
GND
LX
EN_BUCK GND
EN_BAT ADP
ISET
GND
BAT
STAT
12
11
10
9
8
7
C3
4.7uF
L1
3µH
VOUT
VOUT
C4
4.7µF
JP1
R1
D1 _0Ω
1K
RED LED
C1
10µF
123
JP3
Enable_Buck
12
JP2
Enable_Bat
Figure 5: AAT2556 Evaluation Board Schematic.
2556.2006.09.1.2
21