Circuit reliability requires consideration of the device power dissipation, location of the circuit on the PCB, and correct sizing of the thermal plane. Make sure the PCB area around the regulator has few or no other heat-generating devices that cause added thermal stress.

To first-order approximation, power dissipation in the regulator depends on the input-to-output voltage difference and load conditions. The following equation calculates power dissipation (P_D).

For devices with a thermal pad, the primary heat conduction path for the device package is through the thermal pad to the PCB. Solder the thermal pad to a copper pad area under the device. Make sure this pad area contains an array of plated vias that conduct heat to additional copper planes for increased heat dissipation.

The maximum power dissipation determines the maximum allowable ambient temperature (T_A) for the device. Power dissipation and junction temperature are most often related by the R_θJA of the combined PCB and device package and the T_A. R_θJA is the junction-to-ambient thermal resistance and T_A is the temperature of the ambient air. The following equation describes this relationship.

Thermal resistance (R_θJA) is highly dependent on the heat-spreading capability built into the particular PCB design. This resistance therefore varies according to the total copper area, copper weight, and location of the planes. The junction-to-ambient thermal resistance listed in the Thermal Information table is determined by the JEDEC standard PCB and copper-spreading area. R_θJA is used as a relative measure of package thermal performance.