If no other method is available, Equation 5 can be used to estimate the thermal resistance for a package with a DAP. Alternatively, the equation can be re-arranged to give the PCB copper area for a given required thermal resistance. This equation assumes an unbroken plane, using 1-oz copper, a perfect connection between top and bottom layers, and 1 W of power dissipation.

Here the copper heatsink area is in square centimeters and θ_JC can be found in the regulator data sheet. Let us use the LMR33630 in the HSOIC package as an example. Assuming a copper area of 20 cm², and
θ_JC = 4.3°C/W, yields θ_JA ≈ 29°C/W. This compares favorably with the value in the data sheet curve. It is also close to the data shown in Figure 5-1 for a similar package. Keep in mind that this equation is only a very rough estimate and should not be relied upon for values closer than ±50% or so.