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LP38856

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SNVS336D – JUNE 2006 – REVISED OCTOBER 2011

POWER DISSIPATION AND HEAT-SINKING

A heat-sink may be required depending on the maximum power dissipation and maximum ambient temperature

of the application. Under all possible conditions, the junction temperature must be within the range specified

under operating conditions.

The total power dissipation of the device is the sum of three different points of dissipation in the device.

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

(1)

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

formula:

where

•

(2)

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

the formula:

where

•

(3)

The total power dissipation is then:

(4)

The maximum allowable junction temperature rise (

(5)

The maximum allowable value for junction to ambient Thermal Resistance,

formula:

(6)

The LP38856 is available in TO-220 and DDPAK/TO-263 packages. The thermal resistance in the application

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

calculated above is

≥ 60 °C/W for TO-220 package and ≥ 60 °C/W for DDPAK/TO-263 package no heat-sink is

needed since the package alone can dissipate enough heat to satisfy these requirements. If the value needed for

allowable

Heat-Sinking The TO-220 Package

The TO-220 package has a

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

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

DDPAK/TO-263 package.

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

where

•

•

•

(7)

For this equation,

attachment, insulator, etc.

datasheet for details and recommendations.

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