8.2.2.1 Power Dissipation and Device Operation

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, 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 allowable power dissipation for the device in a given package can be calculated using Equation 1:

The actual power being dissipated in the device can be represented by Equation 2:

Equation 1 and Equation 2 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 derated. 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 3:

Alternately, if T_A-MAX can not be derated, the P_D value must be reduced. This 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.