X
X
R
R
P
P
7
7
7
7
1
1
4
4
 
 
Q
Q
u
u
a
a
d
d
 
 
C
C
h
h
a
a
n
n
n
n
e
e
l
l
 
 
D
D
i
i
g
g
i
i
t
t
a
a
l
l
 
 
P
P
W
W
M
M
 
 
S
S
t
t
e
e
p
p
 
 
D
D
o
o
w
w
n
n
 
 
C
C
o
o
n
n
t
t
r
r
o
o
l
l
l
l
e
e
r
r
 
 
 
?2011 Exar Corporation
27/29
Rev. 1.1.6
Power MOSFETs Selection
Selecting MOSFETs with lower Rdson reduces conduction losses at the expense of increased
switching losses. Conduction losses are expressed by the two following equations.
High Side MOSFET Conducted Loss:
out
dson
out
cond
V
V
R
I
?/DIV>
?/DIV>
=
2
 
Low Side MOSFET Conducted Loss:
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>

?/DIV>
?/DIV>
=
in
out
dson
out
cond
V
V
R
I
1
2
 
The MOSFETs junction temperature can be estimated from:
ambient
thja
cond
j
T
R
P
+
= 2
 
This assumes that the switching loss is the same as the conduction loss.
th a
 is the total MOSFET
thermal resistance from junction to ambient.
LAYOUT GUIDELINES
Refer to application note ANP-32 Practical Layout Guidelines for Power
XR
 Designs.
 
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