8.1.4 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.

To first-order approximation, power dissipation in the regulator depends on the input-to-output voltage difference and load conditions. P_D can be calculated using Equation 6:

An important note is that power dissipation can be minimized, and thus greater efficiency achieved, by proper selection of the system voltage rails. For the lowest power dissipation use the minimum input voltage necessary for proper output regulation.

The primary heat conduction path for the DSK package is through the thermal pad to the PCB. Solder the thermal pad to a copper pad area under the device. This pad area should contain 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 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 7.

Unfortunately, the thermal resistance (θ_JA) 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 Thermal Information table is determined by the JEDEC standard, PCB, and copper-spreading area and is only used as a relative measure of package thermal performance.