AAT1147
High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
The AAT1147, combined with an external feedfor-
Thermal Calculations
ward capacitor (C4 in Figure 1), 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.
There are three types of losses associated with the
AAT1147 step-down converter: switching losses,
conduction losses, and quiescent current losses.
Conduction losses are associated with the RDS(ON)
characteristics of the power output switching
devices. Switching losses are dominated by the
gate charge of the power output switching devices.
At full load, assuming continuous conduction mode
(CCM), a simplified form of the losses is given by:
Ω
Ω
R2 = 221k
R2 = 59k
Ω
Ω
VOUT (V)
R1 (k )
R1 (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
IO2 · (RDSON(H) · VO + RDSON(L) · [VIN - VO])
PTOTAL
=
VIN
+ (tsw · FS · IO + IQ) · VIN
IQ is the step-down converter quiescent current.
The term tsw is used to estimate the full load step-
down converter switching losses.
124
137
187
267
Table 2: Resistor Values For Use With 0.6V
Step-Down Converter.
1
2
3
Enable
VIN
U1
AAT1147
1
2
3
4
8
7
6
5
EN
PGND
R1
OUT PGND
118k
L1
VIN
LX
PGND
AGND
VOUT
4.7μH
C1
10μF
C2
4.7μF
R2
59k
GND
GND2
LX
U1 AAT1147 SC70JW-8
L1 CDRH3D16-4R7
C1 10μF 10V 0805 X5R
C2 4.7μF 10V 0805 X5R
Figure 5: AAT1147 Evaluation Board Schematic.
1147.2006.05.1.0
13
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