LP3855ES-ADJ datasheet(13/16 Pages) NSC

13 / 16 page

Application Hints (Continued)

HEATSINKING TO-220 PACKAGE

The thermal resistance of a TO220 package can be reduced

by attaching it to a heat sink or a copper plane on a PC

board. If a copper plane is to be used, the values of

JA will

be same as shown in next section for TO263 package.

The heatsink to be used in the application should have a

heatsink to ambient thermal resistance,

≤θ

JA −

CH −

JC.

In this equation,

CH is the thermal resistance from the case

to the surface of the heat sink and

JC is the thermal resis-

tance from the junction to the surface of the case.

JC is

about 3˚C/W for a TO220 package. The value for

CH de-

pends on method of attachment, insulator, etc.

CH varies

between 1.5˚C/W to 2.5˚C/W. If the exact value is unknown,

2˚C/W can be assumed.

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 3 shows a curve for the

JA 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.

As shown in the figure, increasing the copper area beyond 1

square inch produces very little improvement. The minimum

value for

JA for the TO-263 package mounted to a PCB is

32˚C/W.

Figure 4 shows the maximum allowable power dissipation

for TO-263 packages for different ambient temperatures,

assuming

JA is 35˚C/W and the maximum junction tempera-

ture is 125˚C.

20074332

FIGURE 3.

JA vs Copper (1 Ounce) Area for TO-263

package

20074333

FIGURE 4. Maximum power dissipation vs ambient

temperature for TO-263 package

LP3855ES-ADJ Datasheet(PDF) 13 Page - National Semiconductor (TI)

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