AAT1120
500mA Step-Down Converter
For the condition where the step-down converter is
in dropout at 100% duty cycle, the total device dis-
sipation reduces to:
Thermal Calculations
There are three types of losses associated with
the AAT1120 step-down converter: switching loss-
es, conduction losses, and quiescent current loss-
es. Conduction losses are associated with the
RDS(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 conduc-
tion mode (CCM), a simplified form of the losses is
given by:
PTOTAL = IO2 · RDS(ON)H + IQ · VIN
Since RDS(ON), quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
Given the total losses, the maximum junction tem-
perature can be derived from the θJA for the
STDFN22-8 package which is 50°C/W.
IO2 · (RDS(ON)H · VO + RDS(ON)L · [VIN - VO])
PTOTAL
=
VIN
TJ(MAX)
=
PTOTAL
·
Θ
JA + TAMB
+ (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.
U1
1
8
7
6
5
VIN
VP
PGND
LX
L1
2
3
4
LX
VIN
+VOUT
GND
FB
EN
C3
(optional)
100pF
R1
Adj.
N/C
C2
4.7μF
AAT1120
C1
4.7μF
R2
59kΩ
GND
GND
Figure 1: AAT1120 Schematic.
1120.2007.01.1.0
13
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