LM340 datasheet(12/17 Pages) NSC

12 / 17 page

Application Hints (Continued)

HEATSINKING TO-263 AND SOT-223 PACKAGE PARTS

Both the TO-263 (“S”) and SOT-223 (“MP”) 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 plane.

shows for the TO-263 the measured values of

(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.

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

(J–A) for the TO-263

package mounted to a PCB is 32˚C/W.

As a design aid, Figure 5 shows the maximum allowable

power dissipation compared to ambient temperature for the

TO-263 device (assuming

(J–A) is 35˚C/W and the maxi-

mum junction temperature is 125˚C).

Figures 6, 7 show the information for the SOT-223 package.

Figure 6 assumes a

(J–A) of 74˚C/W for 1 ounce copper and

51˚C/W for 2 ounce copper and a maximum junction tem-

perature of 125˚C.

Please see AN-1028 for power enhancement techniques to

be used with the SOT-223 package.

00778139

FIGURE 4.

(J–A) vs Copper (1 ounce)

Area for the TO-263 Package

00778140

FIGURE 5. Maximum Power Dissipation vs

AMB for the TO-263 Package

00778141

FIGURE 6.

(J–A) vs Copper (2 ounce) Area

for the SOT-223 Package

00778142

FIGURE 7. Maximum Power Dissipation vs

AMB for the SOT-223 Package

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