Datasheet

LM2940-N, LM2940C
www.ti.com SNVS769H –MAY 2004–REVISED MARCH 2007
θ
(H−A)
= θ
(JA)
− θ
(C−H)
− θ
(J−C)
(3)
Where: θ
(J−C)
is defined as the thermal resistance from the junction to the surface of the case. A value of 3°C/W
can be assumed for θ
(J−C)
for this calculation.
θ
(C−H)
is defined as the thermal resistance between the case and the surface of the heatsink. The value of
θ
(C−H)
will vary from about 1.5°C/W to about 2.5°C/W (depending on method of attachment, insulator, etc.).
If the exact value is unknown, 2°C/W should be assumed for θ
(C−H)
.
When a value for θ
(H−A)
is found using the equation shown, a heatsink must be selected that has a value that is
less than or equal to this number.
θ
(H−A)
is specified numerically by the heatsink manufacturer in the catalog, or shown in a curve that plots
temperature rise vs power dissipation for the heatsink.
HEATSINKING TO-263 PACKAGE PARTS
The TO-263 (“S”) package uses a copper plane on the PCB and the PCB itself as a heatsink. To optimize the
heat sinking ability of the plane and PCB, solder the tab of the package to the plane.
Figure 10 shows for the TO-263 the measured values of θ
(JA)
for different copper area sizes using a typical PCB
with 1 ounce copper and no solder mask over the copper area used for heatsinking.
Figure 10. θ
(JA)
vs. Copper (1 ounce) Area for the TO-263 Package
As shown in the figure, increasing the copper area beyond 1 square inch produces very little improvement. It
should also be observed that the minimum value of θ
(JA)
for the TO-263 package mounted to a PCB is 32°C/W.
As a design aid, Figure 11 shows the maximum allowable power dissipation compared to ambient temperature
for the TO-263 device. This assumes a θ
(JA)
of 35°C/W for 1 square inch of 1 ounce copper and a maximum
junction temperature (T
J
) of 125°C.
Figure 11. Maximum Power Dissipation vs. T
A
for the TO-263 Package
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