The PCB copper planes act as heat sinks for the regulator. The area and thickness (weight) of the copper is important. Most of the heat will be dissipated to ambient through the layer that is on the same side as the converter. Figure 3-3 demonstrates a typical thermal performance curve for LMR36015S across different copper board area and a typical start-up waveform is shown in Figure 3-4.

Figure 3-3 R_θJA versus Copper Board Area for the VQFN (RNX) Package

Figure 3-4 Start-Up Behavior at V_IN = 24 V, V_OUT = 5 V, I_OUT = 1.5 A at -55°C