Radiated EMI generated by high di/dt components relates to pulsing currents in switching converters. The larger area covered by the path of a pulsing current, the more electromagnetic emission is generated. The key to minimize radiated EMI is to identify the pulsing current path and minimize the area of that path.

The critical switching loop of the power stage in terms of EMI is denoted in Figure 8-36. The topological architecture of a buck converter means that a particularly high di/dt current effective path exists in the loop comprising the input capacitor and the LM5165 integrated MOSFETs, and it becomes mandatory to reduce the parasitic inductance of this loop by minimizing the effective loop area.

Figure 8-36 Synchronous Buck Converter With Power Stage Critical Switching Loop

The input capacitor provides the primary path for the high di/dt components of the high-side MOSFET current. Placing a ceramic capacitor as close as possible to the VIN and GND pins is the key to EMI reduction. Keep the trace connecting SW to the inductor as short as possible and just wide enough to carry the load current without excessive heating. Use short, thick traces or copper pours (shapes) for current conduction path to minimize parasitic resistance. Place the output capacitor close to the V_OUT side of the inductor, and connect the capacitor return terminal to the LM5165 GND pin and exposed PAD.