PRODUCT DATASHEET
AAT1146
SwitchRegTM
FastTransient 400mA Step-Down Converter
switching devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the losses is
given by:
R2 = 59kΩ
R1 (kΩ)
R2 = 221kΩ
VOUT (V)
R1
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
124
137
187
267
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 con-
verter switching losses.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
Table 2: Adjustable Resistor Values For Use With
0.6V Step-Down Converter.
PTOTAL = IO2 · RDSON(HS) + IQ · VIN
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Thermal Calculations
There are three types of losses associated with the
AAT1146 step-down converter: switching losses, con-
duction 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
Given the total losses, the maximum junction tempera-
ture can be derived from the θJA for the SC70JW-8 pack-
age which is 160°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
1
2
3
Enable
VIN
C4
100pF
U1
AAT1146
1
2
3
4
8
7
6
5
EN
PGND
VOUT =1.8V
R1
118k
4.7μH
OUT PGND
L1
VIN
LX
PGND
AGND
C1
R2
10μF
C2
4.7μF
C3
n/a
59k
GND
LX
GND2
U1 AAT1146 SC70JW-8
L1 CDRH3D16-4R7
C2 4.7μF 10V 0805 X5R
C1 10μF 6.3V 0805 X5R
Figure 5: AAT1146 Adjustable Version Evaluation Board Schematic
(for Fixed Version, R1 is a short and C4, R2 are open).
w w w . a n a l o g i c t e c h . c o m
14
1146.2008.06.1.6
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
