WM5102 datasheet(318/334 Pages) WOLFSON

318 / 334 page

WM5102

Production Data

PD, June 2014, Rev 4.2

318

SPEAKER DRIVER OUTPUT PATH

The WM5102 incorporates two Class D speaker drivers, offering high amplifier efficiency at large

signal levels. As the Class D output is a pulse width modulated 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 WM5102 and

the speaker (e.g. PCB track loss and inductor ESR) as shown in Figure 88. This resistance should be

as low as possible to maximise efficiency.

Figure 88 Speaker Connection Losses

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

implemented using a 2nd 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 89.

Figure 89 Class D Output Filter Components

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

shown in Figure 90. 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.

WM5102 Datasheet(PDF) 318 Page - Wolfson Microelectronics plc