Radiated EMI generated by high slew-rate current edges relates to pulsating currents in switching converters. The larger area covered by the path of a pulsing current, the more electromagnetic emission is generated. The key to reducing radiated EMI is to identify the pulsing current path and minimize the area of that path.

The important high-frequency switching power loop (or hot loop) of the LM5005 power stage is denoted in blue in Figure 7-21. The topological architecture of a buck converter means that particularly high di/dt current exists in this loop as current commutates between the externally-connected Schottky diode and the integrated high-side MOSFET during switching transitions. It is mandatory to minimize this effective loop area, with an eye to reducing the layout-induced parasitic or stray inductances that cause excessive SW voltage overshoot and ringing, noise and ground bounce.

In general, MOSFET switching behavior and the consequences for waveform ringing, power dissipation, device stress and EMI correlate with the parasitic inductances of the power loop. As such, the cumulative benefits of reducing the switching loop area are increased reliability and robustness owing to lower power MOSFET voltage and current stress, increased margin for input voltage transients, and easier EMI filtering (particularly in the more challenging high-frequency band above 30MHz).

Figure 7-21 LM5005 Power Stage Circuit Switching Loops

High-frequency ceramic bypass capacitors at the input side provide the primary path for the high di/dt components of the pulsing current. Position low-ESL ceramic bypass capacitors with low-inductance, short trace routes to the VIN and PGND pins. Keep the SW trace connecting 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 polygon pours (shapes) for current conduction paths to minimize parasitic resistance. Place the output capacitors close to the VOUT side of the inductor and route the return connection using GND plane copper back to the PGND pins and the exposed pad of the LM5005.