11.1.1 Compact Layout for EMI Reduction

Radiated EMI is generated by the high di/dt components in 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 the path. In Buck converters,the pulsing current path is from the V_IN side of the input capacitors to HS switch, to the LS switch, and then return to the ground of the input capacitors, as shown in Figure 106.

Figure 106. Buck Converter High di / dt Path

High frequency ceramic bypass capacitors at the input side provide primary path for the high di/dt components of the pulsing current. Placing ceramic bypass capacitor(s) as close as possible to the VIN and PGND pins is the key to EMI reduction.

The SW pin connecting to the inductor should be as short as possible, and just wide enough to carry the load current without excessive heating. Short, thick traces or copper pours (shapes) should be used for high current condution path to minimize parasitic resistance. The output capacitors should be place close to the V_OUT end of the inductor and closely grounded to PGND pin and exposed PAD.

The bypass capacitors on VCC and BIAS pins should be placed as close as possible to the pins respectively and closely grounded to PGND and the exposed PAD.