Datasheet

D
2
xx
=
R
DSON
¸
¸
¹
·
¨
¨
©
§
D
'
I
O
P
NFET-COND
Isw-rms = I
IND
D
1 +
'i
I
IND
1
3
2
x
I
IND
|
P
IND
= I
IN
2
x R
IND-DCR
D
t
'i
I
I
( )
t
sw
LM2735
www.ti.com SNVS485F –JUNE 2007–REVISED APRIL 2013
Figure 30. LM2735 Switch Current
(small ripple approximation) (28)
P
COND-NFET
= I
IN
2
x R
DSON
x D (29)
(30)
The value for should be equal to the resistance at the junction temperature you wish to analyze. As an example,
at 125°C and V
IN
= 5V, R
DSON
= 250 mΩ (See typical graphs for value).
Switching losses are also associated with the internal NMOS switch. They occur during the switch on and off
transition periods, where voltages and currents overlap resulting in power loss.
The simplest means to determine this loss is to empirically measuring the rise and fall times (10% to 90%) of the
switch at the switch node:
P
SWR
= 1/2(V
OUT
x I
IN
x F
SW
x T
RISE
) (31)
P
SWF
= 1/2(V
OUT
x I
IN
x F
SW
x T
FALL
) (32)
P
SW
= P
SWR
+ P
SWF
(33)
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