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
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
Figure 3 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
FIGURE 4. Maximum Power Dissipation vs. TA for the
HEATSINKING SOT-223 PACKAGE PARTS
The 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 5 and Figure 6 show the information for the SOT-223
package. Figure 6 assumes a θ(JA) of 74°C/W for 1 square
inch of 1 ounce copper and 51°C/W for 1 square inch of 2
ounce copper, with a maximum ambient temperature (TA) of
85°C and a maximum junction temperature (TJ) of 125°C.
For techniques for improving the thermal resistance and pow-
er dissipation for the SOT-223 package, please refer to Ap-
plication Note AN-1028.
FIGURE 3. θ(JA) vs. Copper (1 ounce) Area for the TO-263
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 4 shows the maximum allowable pow-
er 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
(TJ) of 125°C.
FIGURE 5. θ(JA) vs. Copper (2 ounce) Area for the SOT-223