LP38843

SNVS290C – DECEMBER 2004 – REVISED APRIL 2013

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The maximum allowable temperature rise (TRmax) depends on the maximum ambient temperature (TAmax) of the

application, and the maximum allowable junction temperature (TJmax):

TRmax = TJmax− TAmax

(2)

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

θJA = TRmax / PD

(3)

These parts are available in TO-220 and DDPAK/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 DDPAK/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 TO-220 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

DDPAK/TO-263 package.

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

θHA≤ θJA − θCH − θJC.

(4)

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 TO-220 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 DDPAK/TO-263 PACKAGE

The DDPAK/TO-263 package uses the copper plane on the PCB as a heatsink. The tab of this package is

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

θJA of DDPAK/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 13.

θJA vs Copper (1 Ounce) Area for DDPAK/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 DDPAK/TO-263 package mounted to a PCB is 32°C/W.

Figure 14 shows the maximum allowable power dissipation for DDPAK/TO-263 packages for different ambient

temperatures, assuming

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

8

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