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.
Ω
R2 = 59kΩ
V
OUT
(V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
Thermal Calculations
There are three types of losses associated with
the AAT1143 step-down converter: switching loss-
es, conduction losses, and quiescent current loss-
es. Conduction losses are associated with the
R
DS(ON)
characteristics of the power output switch-
ing devices. Switching losses are dominated by
the gate charge of the power output switching
devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the LDO
losses is given by:
I
O2
· (R
DSON(HS)
· V
O
+ R
DSON(LS)
· [V
IN
- V
O
])
V
IN
Ω
R2 = 221kΩ
Ω
R1 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
Ω
R1 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
P
TOTAL
=
+ (t
sw
· F · I
O
+ I
Q
) · V
IN
I
Q
is the step-down converter quiescent current.
The term t
sw
is used to estimate the full load step-
down converter switching losses.
Table 2: Adjustable Resistor Values For Use
With 0.6V Step-Down Converter.
1
2
3
V
IN
Enable
1
U1
AAT1143
EN
OUT
VIN
LX
PGND
PGND
PGND
AGND
8
7
6
5
2
3
4
R1
118k
V
OUT
C1
4.7µF
L1
4.7µH
R2
59k
C2
4.7µF
GND
GND2
LX
U1 AAT1143 SC70JW-8
L1 CDRH3D16-4R7
C1, C2 4.7µF 10V 0805 X5R
Figure 5: AAT1143 Adjustable Evaluation Board Schematic.
1143.2005.09.1.7
13
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