AAT1143
1MHz 400mA Step-Down Converter
The adjustable version of the AAT1143, combined
with an external feedforward capacitor (C4 in
Figure 1), delivers enhanced transient response for
extreme pulsed load applications. The addition of
the feedforward capacitor typically requires a larg-
er output capacitor C1 for stability.
Thermal Calculations
There are three types of losses associated with the
AAT1143 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
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
IO2 · (RDS(ON)H · VO + RDS(ON)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
Table 2: Adjustable Resistor Values For Use
With 0.6V Step-Down Converter.
1
2
3
Enable
VIN
U1
AAT1143
1
2
3
4
8
7
6
5
EN
PGND
R1
OUT PGND
118k
L1
VIN
LX
PGND
AGND
VOUT
4.7μH
C1
4.7μF
C2
4.7μF
R2
59k
GND
GND2
LX
U1 AAT1143 SC70JW-8
L1 CDRH3D16-4R7
C1, C2 4.7μF 10V 0805 X5R
Figure 5: AAT1143 Adjustable Evaluation Board Schematic.
1143.2006.07.1.10
13
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