WM9093ECSSLASHR datasheet(79/84 Pages) WOLFSON

79 / 84 page

Production Data

WM9093

PD, March 2012, Rev 4.2

Figure 17 Speaker Connection Losses

The Class D output requires external filtering in order to recreate the audio signal. This may be

implemented using a 2

nd order LC or 1st order RC filter, or else may be achieved by using a

loudspeaker whose internal inductance provides the required filter response. An LC or RC filter

should be used if the loudspeaker characteristics are unknown or unsuitable, or if the length of the

loudspeaker connection is likely to lead to EMI problems.

In applications where it is necessary to provide Class D filter components, a 2

nd order LC filter is the

recommended solution as it provides more attenuation at higher frequencies and minimises power

dissipated in the filter when compared to a first order RC filter (lower ESR). This maximises both

rejection of unwanted switching frequencies and overall speaker efficiency. A suitable implementation

is illustrated in Figure 18.

Figure 18 Class D Output Filter Components

A simple equivalent circuit of a loudspeaker consists of a serially connected resistor and inductor, as

shown in Figure 19. This circuit provides a low pass filter for the speaker output. If the loudspeaker

characteristics are suitable, then the loudspeaker itself can be used in place of the filter components

described earlier. This is known as ‘filterless’ operation.

Figure 19 Speaker Equivalent Circuit for Filterless Operation

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