WM8959 Datasheet, PDF (151/155 Pages) Wolfson Microelectronics plc – Mobile Multimedia DAC with Dual-Mode Class AB/D Speaker Driver

Pre-Production
WM8959
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 / 2πL
e.g. for an 8Ω speaker and required cut-off frequency of 20kHz, the speaker should be chosen to
have an inductance of:
L = RL / 2πfc = 8Ω / 2π * 20kHz = 64µH
8Ω 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
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PP, May 2008, Rev 3.1
151