LM340-N Datasheet, PDF (13/19 Pages) Texas Instruments – LM340/LM78XX Series 3-Terminal Positive Regulators

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 θ_(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.
Figures 6, 7 show the information for the SOT-223 package.
Figure 6 assumes a θ_(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.
00778141
FIGURE 6. θ_(J–A) vs Copper (2 ounce) Area
for the SOT-223 Package
00778139
FIGURE 4. θ_(J–A) vs Copper (1 ounce)
Area for the TO-263 Package
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 θ_(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 θ_(J–A) is 35˚C/W and the maxi-
mum junction temperature is 125˚C).
00778142
FIGURE 7. Maximum Power Dissipation vs
T_AMB for the SOT-223 Package
Please see AN-1028 for power enhancement techniques to
be used with the SOT-223 package.
00778140
FIGURE 5. Maximum Power Dissipation vs
T_AMB for the TO-263 Package
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