8.1.12 Power Dissipation (P_D)

Circuit reliability demands that proper consideration be given to device power dissipation, location of the circuit on the printed circuit board (PCB), and correct sizing of the thermal plane. The PCB area around the regulator must be as free as possible of other heat-generating devices that cause added thermal stresses.

As a first-order approximation, power dissipation in the regulator depends on the input-to-output voltage difference and load conditions. Use Equation 5 to calculate P_D:

Equation 5.

The primary heat conduction path for the package is through the thermal pad to the PCB. Solder the thermal pad to a copper pad area under the device. This pad area contains an array of plated vias that conduct heat to any inner plane areas or to a bottom-side copper plane.

The maximum power dissipation determines the maximum allowable junction temperature (T_J) for the device. Power dissipation and junction temperature are most often related by the junction-to-ambient thermal resistance (R_θJA) of the combined PCB and device package and the temperature of the ambient air (T_A), according to Equation 6. The equation is rearranged for output current in Equation 7.

Unfortunately, this thermal resistance (R_θJA) is highly dependent on the heat-spreading capability built into the particular PCB design, and therefore varies according to the total copper area, copper weight, and location of the planes. The R_θJA recorded in the Electrical Characteristics table is determined by the JEDEC standard, PCB, and copper-spreading area, and is only used as a relative measure of package thermal performance. For a well-designed thermal layout, R_θJA is actually the sum of the VQFN package junction-to-case (bottom) thermal resistance (R_θJC(bot)) plus the thermal resistance contribution by the PCB copper.