AAT1149 View Datasheet(PDF) - Advanced Analogic Technologies
Part Name
Description
MFG CO.
'AAT1149' PDF : 21 Pages View PDF
SwitchRegTM
PRODUCT DATASHEET
AAT1149
3MHz Fast Transient 400mA Step-Down Converter
VIN
U1
L1
4
5
IN
LX
EN
AAT1149IUV
3
22
EN
1
FB
C3
1
C2
4.7μF
3
GND
VOUT
R1
adj
C1
4.7μF
R2
59k
WLCSP-5
Figure 7: AAT1149IUV Evaluation Board Schematic.
The maximum output capacitor RMS ripple current is
given by:
I = RMS(MAX)
1
2·
·
3
VOUT · (VIN(MAX) - VOUT)
L · FS · VIN(MAX)
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 resistor 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 volt-
ages with R2 set to either 59kΩ for good noise immunity
or 221kΩ for reduced no load input current.
R1
=
⎛ VOUT
⎝ VREF
-1⎞⎠
·
R2
=
⎛ 1.5V
⎝ 0.6V
-
1 ⎞⎠
·
59kΩ
=
88.5kΩ
The AAT1149, combined with an external feedforward
capacitor (C3 in Figure 4), delivers enhanced transient
response for extreme pulsed load applications. The addi-
tion of the feedforward capacitor typically requires a
larger output capacitor C1 for stability.
VOUT (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 = 221kW
R1 (kΩ)
113K
150K
187K
221K
261K
301K
332K
442K
464K
523K
715K
1.00M
Table 2: Feedback Resistor Values.
1149.2008.08.1.3
www.analogictech.com
15
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