The PCB layout plays an important role for achieving passing EMI results, and this is especially true for RE. The following layout guidelines are recommended:

• Due to the H-field coming from integrated transformer, the placement of ground planes on both sides of the device is crucial for passing RE. The ground planes provide low impedance at the return of the switching loop when used to connect the ground pins of the device on both sides. Moreover, these planes act as Faraday shields, mitigating the H-field coming from the transformer. PCB assembly images are shown in Figure 2-7 .
• High frequency decoupling capacitors must be placed as close as possible to the input and output pins. The 0402-capacitor case size offers the lowest ESL and highest self-resonant frequency (SRF) for achieving low impedance at the 64MHz, fundamental switching frequency. 15nF values were selected to achieve the lowest impedance. The impedance vs. frequency plot of is shown in Figure 2-9.
• Placing copper beneath the DM inductor is not recommended. The high frequency currents can find a sneak path to bypass the inductor if copper is placed underneath which creates a parasitic capacitance.
• A keep out zone (KOZ) across the EMI filter area is recommended to avoid any bypass of the HF currents that travel in the switching loop. In this case a 1mm KOZ is used.
• The DM inductor is recommended to be placed away from the noise source to avoid any coupling from the H-field of the transformer. Many times, a DM inductor, such as the LC filter of a buck converter, is already present at the output of the pre-regulator.