WM9090ECSSLASHR datasheet(77/81 Pages) WOLFSON

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Production Data

WM9090

PD, November 2010, Rev 4.1

CLASS D SPEAKER CONNECTIONS

The WM9090 incorporates a Class D speaker driver. As the Class D output is a pulse width

modulated (PWM) signal, the choice of speakers and tracking of signals is critical for ensuring good

performance and reducing EMI in this mode.

The efficiency of the speaker drivers is affected by the series resistance between the WM9090 and

the speaker (e.g. PCB track loss and inductor ESR) as shown in Figure 18. This resistance should

be as low as possible to maximise efficiency.

Figure 18 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 2nd 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 19.

Figure 19 Class D Output Filter Components

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