LM117, LM317-N
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SNVS774L –MAY 2004–REVISED FEBRUARY 2011
As shown in Figure 31, 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.
Figure 31. θ(J−A) vs Copper (1 ounce) Area for the TO-263 Package
As a design aid, Figure 32 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 maximum junction temperature is 125°C).
Figure 32. Maximum Power Dissipation vs TAMB for the TO-263 Package
Heatsinking the PFM Package
If the maximum allowable value for θJA is found to be ≥103°C/W (Typical Rated Value) for PFM 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 PFM for different heatsink area. The copper patterns that
we used to measure these θJAs are shown in Figure 37. Figure 33 reflects the same test results as what are in
Table 1.
Figure 34 shows the maximum allowable power dissipation vs. ambient temperature for the PFM device.
Figure 35 shows the maximum allowable power dissipation vs. copper area (in2) for the PFM device. Please see
AN-1028 (literature number SNVA036) for thermal enhancement techniques to be used with SOT-223 and PFM
packages.
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