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THA d TJA - (TCH + TJC)

TJA

'TJ = TJ(MAX) - TA(MAX)

LP38859

SNVS337D – JUNE 2006 – REVISED OCTOBER 2011

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The first part is the power that is dissipated in the NMOS pass element, and can be determined with the formula:

PD(PASS) = (VIN - VOUT) × IOUT

(2)

The second part is the power that is dissipated in the bias and control circuitry, and can be determined with the

formula:

PD(BIAS) = VBIAS × IGND(BIAS)

where

•

IGND(BIAS) is the portion of the operating ground current of the device that is related to VBIAS.

(3)

The third part is the power that is dissipated in portions of the output stage circuitry, and can be determined with

the formula:

PD(IN) = VIN × IGND(IN)

where

•

IGND(IN) is the portion of the operating ground current of the device that is related to VIN.

(4)

The total power dissipation is then:

PD = PD(PASS) + PD(BIAS) + PD(IN)

(5)

The maximum allowable junction temperature rise (

ΔTJ) depends on the maximum anticipated ambient

temperature (TA) for the application, and the maximum allowable operating junction temperature (TJ(MAX)) .

(6)

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

(7)

Heat-Sinking The TO-220 Package

The TO-220 package has a

θJA rating of 60°C/W and a θJC rating of 3°C/W. These ratings are for the package

only, no additional heat-sinking, and with no airflow. If the needed

θJA, as calculated above, is greater than or

equal to 60°C/W then no additional heat-sinking is required since the package can safely dissipate the heat and

not exceed the operating TJ(MAX). If the needed θJA is less than 60°C/W then additional heat-sinking is needed.

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 heat-sink to be used in the application should have a heat-sink to ambient thermal resistance,

θHA:

where

•

θJA is the required total thermal resistance from the junction to the ambient air

•

θCH is the thermal resistance from the case to the surface of the heart-sink

•

θJC is the thermal resistance from the junction to the surface of the case.

(8)

For this equation,

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. Consult the heat-sink manufacturer

datasheet for details and recommendations.

Heat-Sinking The DDPAK/TO-263 Package

The DDPAK/TO-263 package has a

θJA rating of 60°C/W, and a θJC rating of 3°C/W. These ratings are for the

package only, no additional heat-sinking, and with no airflow.

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

soldered to the copper plane for heat sinking. 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.

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