WM1811A datasheet(318/328 Pages) WOLFSON

318 / 328 page

WM1811A

Production Data

PD, November 2013, Rev 4.1

318

CLASS D SPEAKER CONNECTIONS

The WM1811A incorporates two Class D speaker drivers, which offer 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 WM1811A 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 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 89.

Figure 89 Class D Output Filter Components

WM1811A Datasheet(PDF) 318 Page - Wolfson Microelectronics plc