The permissible power dissipation for any package is a measure of the capability of the device to pass heat from the power source (the junctions of the device) to the ultimate heat sink of the ambient environment. Thus, power dissipation is dependent on the ambient temperature and the thermal resistance across the various interfaces between the die junction and ambient air.

Equation 6 calculates the maximum allowable power dissipation for the device in a given package:

Equation 7 represents the actual power being dissipated in the device:

These two equations establish the relationship between the maximum power dissipation allowed resulting from thermal consideration, the voltage drop across the device, and the continuous current capability of the device. Use these two equations to determine the optimum operating conditions for the device in the application.

In applications where lower power dissipation (P_D) or excellent package thermal resistance (R_θJA) is present, the maximum ambient temperature (T_A-MAX) can be increased.

In applications where high power dissipation or poor package thermal resistance is present, the maximum ambient temperature (T_A-MAX) may have to be derated. As given by Equation 8, T_A-MAX is dependent on the maximum operating junction temperature (T_J-MAX-OP = 125°C), the maximum allowable power dissipation in the device package in the application (P_D-MAX), and the junction-to ambient thermal resistance of the device or package in the application (R_θJA):

Alternately, if T_A-MAX can not be derated, the P_D value must be reduced. This reduction can be accomplished by reducing V_IN in the V_IN–V_OUT term as long as the minimum V_IN is met, or by reducing the I_OUT term, or by some combination of the two.