LM2940-N, LM2940C
SNVS769I –MARCH 2000–REVISED APRIL 2013
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Figure 37. θ(JA) vs. Copper (1 ounce) Area for the DDPAK/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 DDPAK/TO-263 package mounted to a PCB is
32°C/W.
As a design aid, Figure 38 shows the maximum allowable power dissipation compared to ambient temperature
for the DDPAK/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 38. Maximum Power Dissipation vs. TA for the DDPAK/TO-263 Package
HEATSINKING SOT-223 PACKAGE PARTS
The SOT-223 (DCY) 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 39 and Figure 40 show the information for the SOT-223 package. Figure 40 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 power dissipation for the SOT-223 package, please
refer to Application Note AN-1028 (SNVA036).
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