10.2.2.3 When to Use an Output Filter for EMI Suppression

The TPA3137D2 device has been tested with a simple ferrite bead filter for a variety of applications including long speaker wires up to 100 cm and high power. The TPA3137D2 EVM passes FCC Class B specifications under these conditions using twisted speaker wires. The size and type of ferrite bead can be selected to meet application requirements. Also, the filter capacitor can be increased if necessary with some impact on efficiency.

There may be a few circuit instances where it is necessary to add a complete LC reconstruction filter. These circumstances might occur if there are nearby circuits which are sensitive to noise. In these cases, a classic second order Butterworth filter similar to those shown in the following figures can be used.

Some systems have little power supply decoupling from the AC line, but are also subject to line conducted interference (LCI) regulations. These include systems powered by "wall warts" and "power bricks." In these cases, LC reconstruction filters can be the lowest cost means to pass LCI tests. Common mode chokes using low frequency ferrite material can also be effective at preventing line conducted interference.

Figure 20. Typical Ferrite Chip Bead Filter (Chip Bead Example: NFZ2MSM series from Murata)

Figure 21. Typical LC Output Filter, Cutoff Frequency of 27 kHz, Speaker Impedance = 8 Ω

Figure 22. Typical LC Output Filter, Cutoff Frequency of 27 kHz, Speaker Impedance = 6 Ω