906357
906355
FIGURE 5. θ(J−A) vs Copper (2 ounce) Area for the
FIGURE 7. θ(J−A) vs Copper (1 ounce) Area for the TO-263
SOT-223 Package
Package
As a design aid, Figure 8 shows the maximum allowable pow-
er dissipation compared to ambient temperature for the
TO-263 device (assuming θ(J−A) is 35°C/W and the maximum
junction temperature is 125°C).
906358
FIGURE 6. Maximum Power Dissipation vs TAMB for the
SOT-223 Package
HEATSINKING THE TO-263 PACKAGE
906356
Figure 7 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.
FIGURE 8. Maximum Power Dissipation vs TAMB for the
TO-263 Package
HEATSINKING THE TO-252 PACKAGE
As shown in Figure 7, 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.
If the maximum allowable value for θJA is found to be ≥103°
C/W (Typical Rated Value) for TO-252 package, no heatsink
is needed since the package alone will dissipate enough heat
to satisfy these requirements. If the calculated value for θJA
falls below these limits, a heatsink is required.
As a design aid, Table 1 shows the value of the θJA of TO-252
for different heatsink area. The copper patterns that we used
to measure these θJAs are shown at the end of the Application
Notes Section. Figure 9 reflects the same test results as what
are in Table 1.
Figure 10 shows the maximum allowable power dissipation
vs. ambient temperature for the TO-252 device. Figure 11
shows the maximum allowable power dissipation vs. copper
area (in2) for the TO-252 device. Please see AN-1028 for
thermal enhancement techniques to be used with SOT-223
and TO-252 packages.
www.national.com
10
NSC [ National Semiconductor ]
