10.1.3 Thermal Considerations

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 ensuring reliable operation.

Power dissipation of the device depends on input voltage and load conditions and can be calculated using Equation 4:

Equation 4.

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 VSON (DRB) package, the primary conduction path for heat is through the exposed pad to the printed-circuit-board (PCB). The pad can be connected to ground or be left floating; however, it should be attached to an appropriate amount of copper PCB area to ensure the device does not overheat. On the SOT-223 (DCQ) package, the primary conduction path for heat is through the tab to the PCB. The tab should be connected to ground. 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 5:

Equation 5.

Knowing the maximum R_θJA, the minimum amount of PCB copper area needed for appropriate heatsinking can be estimated using Figure 29.

NOTE:

θ_JA value at board size of 9 in.² (that is, 3 in. × 3 in.) is a JEDEC standard.

Figure 29. Θ_JA vs Board Size

Figure 29 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 effect of heat spreading in the ground plane and should not be used to estimate the thermal performance in real application environments.