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Heatsinking TO-220 Package Parts

Heatsinking TO-263 and SOT-223 Package Parts

Copper Foil Area – in

2

0

1

2

3

30

40

50

60

70

80

LM2940

1-A LOW-DROPOUT VOLTAGE REGULATOR

SLVS634 – MAY 2006

APPLICATION INFORMATION (continued)

The SOT-223 can be attached to a typical heatsink or secured to a copper plane on a PC board. If a copper

plane is use, the values of

θ

JA are the same as shown in under Heatsinking TO-263 and SOT-223 Package

Parts.

If a manufactured heatsink is selected, the value of heatsink-to-ambient thermal resistance,

θ

HA, must be

calculated:

θ

HA = θJA – θCH – θJC

Where

θ

JC is defined as the thermal resistance from the junction to the surface of the case. A value of 3°C/W can

be assumed for

θ

JC for this calculation.

θ

CH is defined as the thermal resistance between the case and the surface of the heatsink. The value of θCH

varies from about 1.5

°C/W to about 2.5°C/W, depending on the method of attachment, insulator, etc. If the

exact value is unknown, 2

°C/W should be assumed for θ

CH.

Both the TO-263 and SOT-223 packages use a copper plane on the PCB and the PCB itself as a heatsink. To

optimize the heatsinking ability of the plane and PCB, solder the tab of the package to the plane.

Figure 4 shows the measured values of

θ

JA for the TO-263 for different copper area sizes using a typical PCB

with 1-oz copper and no solder mask over the copper area used for heatsinking.

Figure 4.

θ

JA vs Copper (1 oz) Area for TO-263 Package

As shown in Figure 4, increasing the copper area beyond 1 in2 produces very little improvement. It should also

be observed that the minimum value of

θ

JA 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

JA is 35°C/W and the maximum junction temperature is 125°C.

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