AAT1146
Fast Transient 400mA Step-Down Converter
The adjustable version of the AAT1146, combined
Thermal Calculations
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.
There are three types of losses associated with the
AAT1146 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 = 59k
R2 = 221k
R1
Ω
VOUT (V)
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
75K
113K
150K
187K
221K
261K
301K
332K
442K
464K
523K
715K
1.00M
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
PTOTAL
=
VIN
+ (tsw · F · 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: Adjustable Resistor Values For Use
With 0.6V Step-Down Converter.
1
2
3
Enable
VIN
U1
AAT1146
1
2
3
4
8
7
6
5
EN
PGND
R1
OUT PGND
118k
VIN
LX
PGND
AGND
VOUT
L1
4.7μH
C1
C2
4.7μF
R2
59k
10μF
GND
GND2
LX
U1 AAT1146 SC70JW-8
L1 CDRH3D16-4R7
C1 10μF 10V 0805 X5R
C2 4.7μF 10V 0805 X5R
Figure 5: AAT1146 Adjustable Evaluation Board Schematic.
14
1146.2006.04.1.3
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
