LP3852 datasheet(20/32 Pages) TI

20 / 32 page

LP3852, LP3855

SNVS174I – FEBRUARY 2003 – REVISED FEBRUARY 2015

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Power Dissipation/Heatsinking (continued)

In this equation, RθCH is the thermal resistance from the case to the surface of the heat sink, and RθJC is the

thermal resistance from the junction to the surface of the case. RθJC is about 3°C/W for a TO-220 package. The

value for RθCH depends on method of attachment, insulator, etc. RθCH varies between 1.5°C/W to 2.5°C/W. If the

exact value is unknown, 2°C/W can be assumed.

12.4.2 Heatsinking TO-263 Package

The TO-263 package uses the copper plane on the PCB as a heatsink. The tab of these packages are soldered

to the copper plane for heat sinking. Figure 26 shows a curve for the RθJA of TO-263 package for different copper

area sizes, using a typical PCB with 1 ounce copper and no solder mask over the copper area for heat sinking.

Figure 26. RθJA vs Copper (1 Ounce) Area for TO-263 package

As shown in the figure, increasing the copper area beyond 1 square inch produces very little improvement. The

minimum value for RθJA for the TO-263 package mounted to a PCB is 32°C/W.

Figure 27 shows the maximum allowable power dissipation for TO-263 packages for different ambient

temperatures, assuming RθJA is 35°C/W and the maximum junction temperature is 125°C.

Figure 27. Maximum Power Dissipation vs Ambient Temperature for TO-263 Package

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