Datasheet

=
<JC
R
P
nDissipatio
-
CASE
T
J
T
and
=
TJA
R
P
nDissipatio
-
A
T
J
T
R
T
JA
R
<
JC
LM2735
www.ti.com SNVS485F –JUNE 2007–REVISED APRIL 2013
Total Power Losses are:
Table 3. Power Loss Tabulation
V
IN
5V
V
OUT
12V
I
OUT
500mA POUT 6W
V
D
0.4V PDIODE 236mW
F
SW
1.6MHz
T
RISE
6nS PSWR 80mW
T
FALL
5nS PSWF 70mW
I
Q
4mA PQ 20mW
R
DSon
250mΩ PCOND 305mW
R
DCR
75mΩ PIND 145mW
D 0.623
η 86% PLOSS 856mW
P
INTERNAL
= P
COND
+ P
SW
= 475 mW (46)
Calculating and
(47)
We now know the internal power dissipation, and we are trying to keep the junction temperature at or below
125°C. The next step is to calculate the value for R
θJA
and/or R
ψJC
. This is actually very simple to accomplish,
and necessary if you think you may be marginal with regards to thermals or determining what package option is
correct.
The LM2735 has a thermal shutdown comparator. When the silicon reaches a temperature of 160°C, the device
shuts down until the temperature reduces to 150°C. Knowing this, one can calculate the R
θJA
or the R
ψJC
of a
specific application. Because the junction to top case thermal impedance is much lower than the thermal
impedance of junction to ambient air, the error in calculating R
ψJC
is lower than for R
θJA
. However, you will need
to attach a small thermocouple onto the top case of the LM2735 to obtain the R
ψJC
value.
Knowing the temperature of the silicon when the device shuts down allows us to know three of the four variables.
Once we calculate the thermal impedance, we then can work backwards with the junction temperature set to
125°C to see what maximum ambient air temperature keeps the silicon below the 125°C temperature.
Procedure:
Place your application into a thermal chamber. You will need to dissipate enough power in the device so you can
obtain a good thermal impedance value.
Raise the ambient air temperature until the device goes into thermal shutdown. Record the temperatures of the
ambient air and/or the top case temperature of the LM2735. Calculate the thermal impedances.
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