AAT1149 Analogic Corporation, AAT1149 Datasheet - Page 14

• Manufacturer Part Number: AAT1149
• Manufacturer: Analogic Corporation

Dissipation due to the RMS current in the ceramic
output capacitor ESR is typically minimal, resulting in
less than a few degrees rise in hot-spot temperature.
Feedback Resistor Selection
Resistors R1 and R2 of Figure 4 program the output
to regulate at a voltage higher than 0.6V. To limit the
bias current required for the external feedback resis-
tor 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 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.
14
R1 =
⎛
⎝
V
V
OUT
REF
-1 · R2 =
GND
GND
V
⎞
⎠
OUT
⎛
⎝
1.5V
0.6V
V
4.7μF
IN
C1
Figure 4: AAT1149 Evaluation Board Schematic.
- 1 · 59kΩ = 88.5kΩ
⎞
⎠
L1
C3
R1
R2
59k
LX
4.7μF
C2
The AAT1149, combined with an external feedfor-
ward capacitor (C3 in Figure 4), delivers enhanced
transient response for extreme pulsed load appli-
cations. The addition of the feedforward capacitor
typically requires a larger output capacitor C1 for
stability.
V
OUT
1.85
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
2.0
2.5
3.3
1
2
3
4
Table 2: Feedback Resistor Values.
400mA Step-Down Converter
U1
AAT1149
(V)
EN
OUT
IN
LX
PGND
PGND
PGND
AGND
R2 = 59kΩ Ω
R1 (kΩ Ω )
3MHz Fast Transient
29.4
39.2
49.9
59.0
68.1
78.7
88.7
124
137
187
267
118
8
7
6
5
Enable
1
2
3
R2 = 221kΩ Ω
1.00M
1149.2006.11.1.0
113K
150K
187K
221K
261K
301K
332K
442K
464K
523K
715K
R1