LM3940 datasheet(6/11 Pages) NSC

6 / 11 page

Application Hints (Continued)

If a manufactured heatsink is to be selected, the value of

heatsink-to-ambient thermal resistance,

(H−A), must first be

calculated:

(H−A) = θ(J−A) − θ(C−H) − θ(J−C)

Where:

(J−C)

is defined as the thermal resistance from

the junction to the surface of the case. A

value of 4˚C/W can be assumed for

(J−C)

for this calculation.

(C−H)

is defined as the thermal resistance be-

tween the case and the surface of the heat-

sink. The value of

(C−H)

will vary from

about 1.5˚C/W to about 2.5˚C/W (depend-

ing on method of attachment, insulator,

etc.). If the exact value is unknown, 2˚C/W

should be assumed for

(C−H).

When a value for

(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.

(H−A) 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 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 package to the plane.

Figure 3 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 4 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).

Figure 5 and Figure 6 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 temperature of 125˚C.

Please see AN1028 for power enhancement techniques to

be used with the SOT-223 package.

DS012080-7

FIGURE 3.

(J−A) vs Copper (1 ounce) Area for the

TO-263 Package

DS012080-8

FIGURE 4. Maximum Power Dissipation vs T

AMB for

the TO-263 Package

DS012080-11

FIGURE 5.

(J−A) vs Copper (2 ounce) Area for the

SOT-223 Package

DS012080-12

FIGURE 6. Maximum Power Dissipation vs T

AMB for

the SOT-223 Package

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