Datasheet

R
T
JC
=
T
J
- T
C
Power
LM27341, LM27342, LM27341-Q1, LM27342-Q1
SNVS497E –NOVEMBER 2008–REVISED APRIL 2013 www.ti.com
R
θJA
values for the MSOP-PowerPad @ 1Watt dissipation:
Number of Board Size of Bottom Layer Copper Size of Top Layer Copper Number of 10 mil R
θJA
Layers Connected to DAP Connected to Dap Thermal Vias
2 0.25 in
2
0.05 in
2
8 80.6 °C/W
2 0.5625 in
2
0.05 in
2
8 70.9 °C/W
2 1 in
2
0.05 in
2
8 62.1 °C/W
2 1.3225 in
2
0.05 in
2
8 54.6 °C/W
4 (Eval Board) 3.25 in
2
2.25 in
2
14 35.3 °C/W
R
θJA
values for the SON @ 1Watt dissipation:
Number of Board Size of Bottom Layer Copper Size of Top Layer Copper Number of 8 mil R
θJA
Layers Connected to DAP Connected to Dap Thermal Vias
2 0.25 in
2
0.05 in
2
8 78 °C/W
2 0.5625 in
2
0.05 in
2
8 65.6 °C/W
2 1 in
2
0.05 in
2
8 58.6 °C/W
2 1.3225 in
2
0.05 in
2
8 50 °C/W
4 (Eval Board) 3.25 in
2
2.25 in
2
15 30.7 °C/W
Figure 38. Estimate of Thermal Resistance vs. Ground Copper Area
Eight Thermal Vias and Natural Convection
Method 2:
The second method requires the user to know the thermal impedance of the silicon junction to case. (R
θJC
) is
approximately 9.5°C/W for the MSOP-PowerPad package or 9.1°C/W for the SON. The case temperature should
be measured on the bottom of the PCB at a thermal via directly under the DAP of the LM27341/LM27342. The
solder resist should be removed from this area for temperature testing. The reading will be more accurate if it is
taken midway between pins 2 and 9, where the NMOS switch is located. Knowing the internal dissipation from
the efficiency calculation given previously, and the case temperature (T
C
) we have:
(50)
Therefore:
T
J
= (R
θJC
x P
LOSS
) + T
C
(51)
22 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM27341 LM27342 LM27341-Q1 LM27342-Q1