WM8959 datasheet(151/155 Pages) WOLFSON

151 / 155 page

Pre-Production

WM8959

PP, May 2008, Rev 3.1

151

APPLICATIONS INFORMATION

SPEAKER SELECTION

For filterless operation, it is important to select a speaker with appropriate internal inductance. The

internal inductance and the speaker's load resistance create a low-pass filter with a cut-off frequency

of:

fc = RL / 2

πL

e.g. for an 8

Ω speaker and required cut-off frequency of 20kHz, the speaker should be chosen to

have an inductance of:

L = RL / 2

πfc = 8Ω / 2π * 20kHz = 64µH

Ω speakers typically have an inductance in the range 20µH to 100µH. Care should be taken to

ensure that the cut-off frequency of the speaker's internal filtering is low enough to prevent speaker

damage. The class D outputs of the WM8959 operate at much higher frequencies than is

recommended for most speakers, and the cut-off frequency of the filter should be low enough to

protect the speaker.

Figure 97 Speaker Equivalent Circuit

PCB LAYOUT CONSIDERATIONS

The efficiency of the speaker drivers is affected by the series resistance between the WM8959 and

the speaker (e.g. inductor ESR) as shown in Figure 98. This resistance should be as low as possible

to maximise efficiency.

Figure 98 Speaker Connection Losses

WM8959 Datasheet(PDF) 151 Page - Wolfson Microelectronics plc