LM117, LM317-N
SNVS774L –MAY 2004–REVISED FEBRUARY 2011
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where (θCH is the thermal resistance of the contact area between the device case and the heatsink surface, and
θJC is thermal resistance from the junction of the die to surface of the package case.
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.
The θ(H−A) rating 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 Surface Mount Packages
The TO-263 (KTT), SOT-223 (DCY) and PFM (MDT) 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.
Heatsinking the SOT-223 Package
Figure 29 and Figure 30 show the information for the SOT-223 package. Figure 30 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. Please see
AN-1028 (literature number SNVA036) for thermal enhancement techniques to be used with SOT-223 and PFM
packages.
Figure 29. θ(J−A) vs Copper (2 ounce) Area for the SOT-223 Package
Figure 30. Maximum Power Dissipation vs TAMB for the SOT-223 Package
Heatsinking the TO-263 Package
Figure 31 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.
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