9.3 Power Dissipation

The ability to remove heat from the die is different for each package type, which presents different considerations in the PCB layout. The PCB area around the device that is free of other components moves the heat from the device to the ambient air. Performance data for JEDEC low- and high-K boards are shown in the Thermal Information section. Heavier copper increases the effectiveness in removing heat from the device. The addition of plated through-holes to heat-dissipating layers improves the heat sink effectiveness.

Power dissipation depends on input voltage and load conditions. Power dissipation can be approximated by the product of the output current and the voltage drop across the output pass element, as Equation 2 shows.

Equation 2.

Equation 3 is an example calculation for the power dissipation (P_D) of the DRB package.

Equation 3.

Power dissipation can be minimized and greater efficiency can be achieved by using the lowest possible input voltage necessary to achieve the required output performance.

On the DRB package, the primary conduction path for heat is through the exposed thermal pad to the PCB. The pad can be connected to ground or left floating. The pad must be attached to an appropriate amount of copper PCB area to ensure that the device does not overheat. The maximum allowable junction-to-ambient thermal resistance depends on the maximum ambient temperature, maximum device junction temperature, and power dissipation of the device. Equation 4 calculates the maximum junction-to-ambient thermal resistance.

Equation 4.

Figure 23 estimates the maximum R_θJA and the minimum amount of PCB copper area required to heat sink.

NOTE:

θ_JA value at board size of 9 in² (that is, 3 in × 3 in) is a JEDEC standard.

Figure 23. θ_JA vs Board Size

Figure 23 shows the variation of θ_JA as a function of ground plane copper area in the board. It is intended only as a guideline to demonstrate the effects of heat spreading in the ground plane and must not be used to estimate actual thermal performance in real application environments.