Power dissipation depends on input voltage and load conditions. Power dissipation (P_D) is equal to the product of the output current and the voltage drop across the output pass transistor, as shown in Equation 1.

Equation 1.

where

• P_D is continuous power dissipation
• I_OUT is output current
• V_IN is input voltage
• V_OUT is output voltage

Because I_Q << I_OUT, the term I_Q × V_IN is always ignored.

For a device under operation at a given ambient air temperature (T_A), use Equation 2 to calculate the junction temperature (T_J).

Equation 2.

where:

• Z_θJA is the junction-to-ambient air temperature thermal impedance

Use Equation 3 to calculate the rise in junction temperature due to power dissipation.

Equation 3.

For a given maximum junction temperature (T_J(MAX), use Equation 4 to calculate the maximum ambient air temperature (T_A(MAX) at which the device can operate.

Equation 4.