LP38855 datasheet(12/20 Pages) TI

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THA d TJA - (TCH + TJC)

LP38855

SNVS461D – OCTOBER 2006 – REVISED APRIL 2013

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The heat-sink to be used in the application should have a heat-sink to ambient thermal resistance,

θHA:

where

•

θJA is the required total thermal resistance from the junction to the ambient air

•

θCH is the thermal resistance from the case to the surface of the heat sink

•

θJC is the thermal resistance from the junction to the surface of the case

(7)

For this equation,

θJC is about 3°C/W for a TO-220 package. The value for θCH depends on method of

attachment, insulator, etc.

θCH varies between 1.5°C/W to 2.5°C/W. Consult the heat-sink manufacturer

datasheet for details and recommendations.

Heat-Sinking the DDPAK/TO-263 Package

The DDPAK/TO-263 package has a

θJA rating of 60°C/W, and a θJC rating of 3°C/W. These ratings are for the

package only, no additional heat-sinking, and with no airflow.

The DDPAK/TO-263 package uses the copper plane on the PCB as a heat-sink. The tab of this package is

soldered to the copper plane for heat-sinking. The graph below shows a curve for the

θJA 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 27.

θJA vs Copper (1 Ounce) Area for the DDPAK/TO-263 package

As shown in Figure 27, increasing the copper area beyond 1 square inch produces very little improvement. The

minimum value for

θJA for the DDPAK/TO-263 package mounted to a PCB is 32°C/W.

Figure 28 shows the maximum allowable power dissipation for DDPAK/TO-263 packages for different ambient

temperatures, assuming

θJA is 35°C/W and the maximum junction temperature is 125°C.

Figure 28. Maximum Power Dissipation vs Ambient Temperature for DDPAK/TO-263 Package

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