LP3881

SNVS224F – MARCH 2003 – REVISED APRIL 2013

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The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

θJA = TRmax / PD

These parts are available in TO-220 and TO-263 packages. The thermal resistance depends on amount of

copper area or heat sink, and on air flow. If the maximum allowable value of

θJA calculated above is ≥ 60 °C/W

for TO-220 package and

≥ 60 °C/W for TO-263 package no heatsink is needed since the package can dissipate

enough heat to satisfy these requirements. If the value for allowable

θJA falls below these limits, a heat sink is

required.

HEATSINKING TO-220 PACKAGE

The thermal resistance of a TO220 package can be reduced by attaching it to a heat sink or a copper plane on a

PC board. If a copper plane is to be used, the values of

θJA will be same as shown in next section for TO263

package.

The heatsink to be used in the application should have a heatsink to ambient thermal resistance,

θHA≤ θJA − θCH − θJC.

In this equation,

θCH is the thermal resistance from the case to the surface of the heat sink and θJC is the thermal

resistance from the junction to the surface of the case.

θJC is about 3°C/W for a TO220 package. The value for

θCH depends on method of attachment, insulator, etc. θCH varies between 1.5°C/W to 2.5°C/W. If the exact value

is unknown, 2°C/W can be assumed.

HEATSINKING TO-263 PACKAGE

The TO-263 package uses the copper plane on the PCB as a heatsink. The tab of these packages are soldered

to the copper plane for heat sinking. The graph below shows a curve for the

θJA of TO-263 package for different

copper area sizes, using a typical PCB with 1 ounce copper and no solder mask over the copper area for heat

sinking.

Figure 20.

θJA vs Copper (1 Ounce) Area for TO-263 package

As shown in the graph below, increasing the copper area beyond 1 square inch produces very little improvement.

The minimum value for

θJA for the TO-263 package mounted to a PCB is 32°C/W.

Figure 21 shows the maximum allowable power dissipation for TO-263 packages for different ambient

temperatures, assuming

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

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