Knowing the device power dissipation and proper sizing of the thermal plane that is connected to the tab or pad is critical to avoiding thermal shutdown and providing reliable operation.

Power dissipation of the device depends on input voltage and load conditions and can be calculated using Equation 1:

Equation 1.

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

On the VQFN (RGW ) package, the primary conduction path for heat is through the exposed pad to the PCB. The pad can be connected to ground or be left floating; however, the pad must be attached to an appropriate amount of copper PCB area to make sure the device does not overheat. The maximum junction-to-ambient thermal resistance depends on the maximum ambient temperature, maximum device junction temperature, and power dissipation of the device and can be calculated using Equation 2:

Equation 2.

Knowing the maximum R_θJA, the minimum amount of PCB copper area needed for appropriate heat sinking can be estimated using Figure 7-13.

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

Figure 7-13 R_θJA vs Board Size

Figure 7-13 shows the variation of R_θJA as a function of ground plane copper area in the board. This figure 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.