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 IC, to the ultimate heat sink, the ambient environment. Thus, the power dissipation is dependent on the ambient temperature and the thermal resistance across the various interfaces between the die junction and ambient air.

The maximum continuous allowable power dissipation for the device in a given package can be calculated using Equation 17:

The actual power being dissipated in the device can be represented by Equation 18:

p_RESET is calculated by Equation 19 or Equation 20

Equation 17 and Equation 18 establish the relationship between the maximum power dissipation allowed due to thermal consideration, the voltage drop across the device, and the continuous current capability of the device. These two equations should be used to determine the optimum operating conditions for the device in the application.

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

In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature (T_A-MAX) may have to be de-rated. 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 part/package in the application (R_θJA), as given by Equation 21: