LM3940
Application Hints
(Continued)
If a manufactured heatsink is to be selected, the value of
heatsink-to-ambient thermal resistance,
θ
(H−A)
, must first be
calculated:
θ
(H−A)
=
θ
(JA)
−
θ
(C−H)
−
θ
(J−C)
Where:
θ
(J−C)
is defined as the thermal resistance from the
junction to the surface of the case. A value of
4˚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 at-
tachment, 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 AND SOT-223 PACKAGE PARTS
Both the TO-263 (“S”) and SOT-223 (“MP”) packages use 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.
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.
01208008
FIGURE 4. Maximum Power Dissipation vs. T
AMB
for
the TO-263 Package
and
show the information for the SOT-223
package.
assumes a
θ
(JA)
of 74˚C/W for 1 ounce
copper and 51˚C/W for 2 ounce copper and a maximum
junction temperature of 125˚C.
01208011
FIGURE 5.
θ
(JA)
vs. Copper (2 ounce) Area for the
SOT-223 Package
01208007
FIGURE 3.
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,
shows the maximum allowable
power dissipation compared to ambient temperature for the
TO-263 device (assuming
θ
(JA)
is 35˚C/W and the maximum
junction temperature is 125˚C).
01208012
FIGURE 6. Maximum Power Dissipation vs. T
AMB
for
the SOT-223 Package
Please see AN1028 for power enhancement techniques to
be used with the SOT-223 package.
7
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