Datasheet

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
500
RT
JA
P
DISS
50
100
150
200
250
300
350
400
450
:
=
TJA
R
P
nDissipatio
-
A
T
J
T
=
<JC
R
P
nDissipatio
-
Case-Top
T
J
T
LM2735
SNVS485F –JUNE 2007–REVISED APRIL 2013 www.ti.com
Example from previous calculations:
Pdiss = 475 mW
Ta @ Shutdown = 139°C
Tc @ Shutdown = 155°C
(48)
R
θJA
WSON = 55°C/W
R
ψJC
WSON = 21°C/W
WSON & MSOP-PowerPAD typical applications will produce R
θJA
numbers in the range of 50°C/W to 65°C/W,
and R
ψJC
will vary between 18°C/W and 28°C/W. These values are for PCB’s with two and four layer boards with
0.5 oz copper, and four to six thermal vias to bottom side ground plane under the DAP.
For 5-pin SOT-23 package typical applications, R
θJA
numbers will range from 80°C/W to 110°C/W, and R
ψJC
will
vary between 50°C/W and 65°C/W. These values are for PCB’s with two & four layer boards with 0.5 oz copper,
with two to four thermal vias from GND pin to bottom layer.
Here is a good rule of thumb for typical thermal impedances, and an ambient temperature maximum of 75°C: If
your design requires that you dissipate more than 400mW internal to the LM2735, or there is 750mW of total
power loss in the application, it is recommended that you use the 6 pin WSON or the 8 pin MSOP-PowerPAD
package.
NOTE
NOTE: To use these procedures it is important to dissipate an amount of power within the
device that will indicate a true thermal impedance value. If one uses a very small internal
dissipated value, one can see that the thermal impedance calculated is abnormally high,
and subject to error. The graph below shows the nonlinear relationship of internal power
dissipation vs R
θJA
.
Figure 31. R
θJA
vs Internal Dissipation for the WSON
and MSOP-PowerPAD Package
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