WM8994 Datasheet, PDF (351/359 Pages) Wolfson Microelectronics plc – Multi-Channel Audio Hub CODEC for Smartphones

Production Data
WM8994
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 1^st 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 92.
Figure 92 Class D Output Filter Components
A simple equivalent circuit of a loudspeaker consists of a serially connected resistor and inductor, as
shown in Figure 93. 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.
Figure 93 Speaker Equivalent Circuit for Filterless Operation
For filterless Class D operation, it is important to ensure that a speaker with suitable inductance is
chosen. For example, if we know the speaker impedance is 8Ω and the desired cut-off frequency is
20kHz, then the optimum speaker inductance may be calculated as:
8 loudspeakers typically have an inductance in the range 20H to 100H, however, it should be
noted that a loudspeaker inductance will not be constant across the relevant frequencies for Class D
operation (up to and beyond the Class D switching frequency). Care should be taken to ensure that
the cut-off frequency of the loudspeaker’s filtering is low enough to suppress the high frequency
energy of the Class D switching and, in so doing, to prevent speaker damage. The Class D outputs of
the WM8994 operate at much higher frequencies than is recommended for most speakers and it must
be ensured that the cut-off frequency is low enough to protect the speaker.
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PD, April 2012, Rev 4.4
351