WM8994ECSSLASHR datasheet(355/363 Pages) CIRRUS

355 / 363 page

WM8994

Rev 4.6

355

The Class D output requires external filtering in order to recreate the audio signal. This may be

implemented using a 2nd 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 2nd 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.

WM8994

C = 3

F

L = 22

H

SPKP

SPKN

Fc = low pass 3dB

cut-off frequency

Fc =

p LC

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.

Fc =

p L

WM8994

SPKP

SPKN

Fc = low pass 3dB cut-off frequency

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 8

Ω and the desired cut-off frequency is

20kHz, then the optimum speaker inductance may be calculated as:

L =

= 64

H

p Fc



p * 20kHz

 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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