LM3940 datasheet(7/11 Pages) NSC

7 / 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) =

(JA) −

(C−H) −

(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 between

the case and the surface of the heatsink. The

value of

(C−H) will vary from about 1.5˚C/W to

about 2.5˚C/W (depending on method of at-

tachment, 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

(JA)

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

(JA) 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

(JA) is 35˚C/W and the maximum

junction temperature is 125˚C).

Figure 5 and Figure 6 show the information for the SOT-223

package. Figure 6 assumes a

(JA) 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.

01208007

FIGURE 3.

(JA) vs. Copper (1 ounce) Area for the

TO-263 Package

01208008

FIGURE 4. Maximum Power Dissipation vs. T

AMB for

the TO-263 Package

01208011

FIGURE 5.

(JA) vs. Copper (2 ounce) Area for the

SOT-223 Package

01208012

FIGURE 6. Maximum Power Dissipation vs. T

AMB for

the SOT-223 Package

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