8.1.1 Power Dissipation

Circuit reliability demands that proper consideration is 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. P_D can be approximated using Equation 1:

An important note is that power dissipation can be minimized, and thus greater efficiency achieved, by proper selection of the system voltage rails. Proper selection allows the minimum input-to-output voltage differential to be obtained. The low dropout of the device allows for maximum efficiency across a wide range of output voltages.

The main heat conduction path for the device is through the thermal pad on the package. As such, the thermal pad must be soldered 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, device package, and the temperature of the ambient air (T_A), according to Equation 2. The equation is rearranged for output current in Equation 3.

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 v table is determined by the JEDEC standard, PCB, and copper-spreading area, and is only used as a relative measure of package thermal performance. Note that for a well-designed thermal layout, R_θJA is actually the sum of the package junction-to-case (bottom) thermal resistance (R_θJCbot) plus the thermal resistance contribution by the PCB copper.

Figure 25 through Figure 30 show the functions of R_θJA and ψ_JB vs. copper area and thickness. These plots are generated with a 101.6 mm x 101.6 mm x 1.6mm PCB of two and four layers. For the four layer board, inner planes use 1 oz copper thickness. Outer layers are simulated with both 1 oz and 2 oz copper thickness. A 2 x 1 array of thermal vias of 300 µm drill diameter and 25 µm Cu plating is located beneath the thermal pad of the device. The thermal vias connect the top layer, the bottom layer and, in the case of the 4-layer board, the first inner GND plane. Each of the layers has a copper plane of equal area.

Figure 25. R_θJA versus Cu Area for the WSON (DRV) Package

Figure 26. ψ_JB versus Cu Area for the WSON (DRV) Package

Figure 27. R_θJA versus Cu Area for the HVSSOP (DGN) Package

Figure 28. ψ_JB versus Cu Area for the HVSSOP (DGN) Package

Figure 29. R_θJA versus Cu Area for the TO-252 (KVU) Package

Figure 30. ψ_JB versus Cu Area for the TO-252 (KVU) Package