AAT1154 Advanced Analogic Technologies, AAT1154 Datasheet - Page 12

• Manufacturer Part Number: AAT1154
• Description: 1MHz 3A Buck DC/DC Converter
• Manufacturer: Advanced Analogic Technologies

Thermal
The losses associated with the AAT1154 output
switching MOSFET are due to switching losses
and conduction losses. The conduction losses are
associated with the R
output switching device. At the full load condition,
assuming continuous conduction mode (CCM), an
accurate calculation of the R
derived from the following equations.
R
Internal switch RMS current
D is the duty cycle and V
Schottky diode.
A simplified form of calculating the R
switching losses is given by
where I
Figure 1. AAT1154 Fixed Output Top Side
Layout
12
I
P
D
I
Q
DS(ON)
RMS
ON
=
is the peak to peak inductor ripple current.
=
=
V
V
I
P
O
IN
RMS
Q
losses
+
+
=
I
is the AAT1154 quiescent current.
2
V
V
I
O
·
2
F
F
O
R
+
2
·
DS(ON)
R
12
I
DS(ON)
V
2
IN
·
D
V
DS(ON)
o
+
F
is the forward drop of the
t
SW
· ·
characteristics of the
DS(ON)
F I
O
+
losses can be
I V
Q
·
DS(ON)
IN
and
Once the total losses have been determined the
junction temperature can be derived. The thermal
resistance (
an FR4 printed circuit board in still air is 110°C/W.
T
is the resultant maximum junction temperature.
Design Example
I
I
V
V
F
T
Inductor Selection
Use standard value of 1.5 µH
OUT
RIPPLE
AMB
S
MAX
OUT
IN
1MHz 3A Buck DC/DC Converter
Figure 2. AAT1154 Fixed Output Bottom
Side Layout
1MHz
5V
3A
3.3V
= 70°C
is the maximum ambient temperature and T
30% of full load at max Vin
=
L
=
3
A
5%
I k F
O
·
V
0.3 1
· ·
3.3
JA
OUT
) for the SO-8 package mounted on
T
V
·
J
MHz
= P ·
·
1
-
·
V
V
1
OUT
IN
JA
-
3.3
+ T
5
V
V
AMB
AAT1154
=
1.25 H
1154.2003.08.0.91
J