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.
Figure 3
shows for the TO-263 the measured values of
θ
(J−A)
for different copper area sizes using a typical PCB with 1
ounce copper
and no solder mask over the copper area used
for heatsinking.
DS100113-22
FIGURE 5.
θ
(J−A)
vs Copper (2 ounce) Area for the
SOT-223 Package
DS100113-20
FIGURE 3.
θ
(J−A)
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
(J−A)
for the TO-263
package mounted to a PCB is 32˚C/W.
As a design aid,
Figure 4
shows the maximum allowable
power dissipation compared to ambient temperature for the
TO-263 device (assuming
θ
(J−A)
is 35˚C/W and the maxi-
mum junction temperature is 125˚C).
DS100113-23
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.
SOT-223 SOLDERING RECOMMENDATIONS
It is not recommended to use hand soldering or wave solder-
ing to attach the small SOT-223 package to a printed circuit
board. The excessive temperatures involved may cause
package cracking.
Either vapor phase or infrared reflow techniques are pre-
ferred soldering attachment methods for the SOT-223 pack-
age.
DS100113-21
FIGURE 4. Maximum Power Dissipation vs T
AMB
for
the TO-263 Package
Figure 5
and
Figure 6
show the information for the SOT-223
package.
Figure 6
assumes a
θ
(J−A)
of 74˚C/W for 1 ounce
copper and 51˚C/W for 2 ounce copper and a maximum
junction temperature of 125˚C.
7
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