One important aspect of the ferrite bead selection is the type of material used in the ferrite bead. Not all ferrite material is alike, selecting a material that is effective in the 10 to 100MHz range is important and key to the operation of the Class-D amplifier. Many of the specifications regulating consumer electronics have emissions limits as low as 30Hz. Using the ferrite bead filter to block radiation in the 30MHz and above range from appearing on the speaker wires and the power supply lines which are good antennas for these signals is important. The impedance of the ferrite bead can be used along with a small capacitor with a value in the range of 1000pF to reduce the frequency spectrum of the signal to an acceptable level. For best performance, the resonant frequency of the ferrite bead and capacitor filter is less than 10MHz.

Establish that the ferrite bead is large enough to maintain impedance at the peak currents expected for the amplifier. Some ferrite bead manufacturers specify the bead impedance at a variety of current levels. Whenever possible, make sure the ferrite bead maintains an adequate amount of impedance at the peak current that the amplifier detects. If these specifications are not available, estimating the bead current handling capability by measuring the resonant frequency of the filter output at low power and at maximum power is possible. A change of resonant frequency of less than fifty percent under this condition is desirable.

A high-quality ceramic capacitor is also required for the ferrite bead filter. A low ESR capacitor with good temperature and voltage characteristics works best.

Additional EMC improvements is obtained by adding snubber networks from each of the Class-D output to ground. Suggested values for a simple RC series snubber network is 68Ω in series with a 100pF capacitor although design of the snubber network is specific to every application and must be designed taking into account the parasitic reactance of the printed circuit board and the audio amplifier. Take care to evaluate the stress on the components in the snubber network especially if the amp is running at high PVCC. Also, make sure the layout of the snubber network is tight and returns directly to the GND or the thermal pad beneath the chip.

Figure 8-1 Typical Ferrite Chip Bead Filter (Chip Bead Example: NFZ2MSM series from Murata)

Figure 8-2 Typical LC Output Filter, Cutoff Frequency of 27kHz, Speaker Impedance = 8Ω

Figure 8-3 Typical LC Output Filter, Cutoff Frequency of 27kHz, Speaker Impedance = 6Ω