AAT1149 Datasheet, PDF (14/20 Pages) Advanced Analogic Technologies – 3MHz Fast Transient 400mA Step-Down Converter

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.
R1
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R2
=
⎛ ^1.5V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
88.5kΩ
AAT1149
3MHz Fast Transient
400mA Step-Down Converter
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 (V)
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 = 59kΩ
R1 (kΩ)
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R2 = 221kΩ
R1
113K
150K
187K
221K
261K
301K
332K
442K
464K
523K
715K
1.00M
Table 2: Feedback Resistor Values.
V_OUT
GND
GND
V_IN
C3
R1
C1
4.7μF
L1
R2
59k
U1
AAT1149
1 _EN
PGND ⁸
2 _{OUT PGND} 7
3 _IN
PGND ⁶
4 _LX
AGND ⁵
C2
4.7μF
1
2
3
Enable
LX
Figure 4: AAT1149 Evaluation Board Schematic.
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1149.2006.11.1.0