Production Data

WM9001

w

PD, March 2010, Rev 4.1

21

SPEAKER SELECTION

In Class D driver mode, the WM9001 output contains high frequency signals resulting from the

switched PWM operation. To avoid the need for specific filter components, it is important to make an

appropriate choice of loudspeaker. Note that, for Class AB mode usage, the choice of speaker is not

so important as there are no high frequency harmonics in the WM9001 output.

The speaker inductance and load resistance create a low-pass filter which, ideally, will attenuate the

high frequency Class D switching harmonics whilst passing the desired audio frequencies. The 3dB

cut-off frequency of the speaker inductance and resistance may be calculated as follows:

πL

Therefore, for an 8

Ω speaker and a desired 3dB cut-off frequency of 20kHz, the speaker should be

chosen to have an inductance of:

8

Ω speakers for portable applications typically have an inductance in the range 20μH to 100μH. If the

inductance is higher than value calculated above, then the cut-off frequency will be reduced, limiting

the audio bandwidth. Lower values of inductance will result in a higher cut-off frequency. The Class D

outputs contain harmonics at much higher frequencies than is recommended for most speakers, and

the cut-off frequency of the filter must therefore be low enough to protect the speaker.

Figure 11 Speaker Equivalent Circuit