WM8281ECSSLASHR datasheet(377/392 Pages) CIRRUS

377 / 392 page

WM8281

Rev 4.0

377

SPEAKER DRIVER OUTPUT PATH

The WM8281 incorporates two Class D speaker drivers, offering high amplifier efficiency at large

signal levels. As the Class D output is a pulse width modulated signal, the choice of speakers and

tracking of signals is critical for ensuring good performance and reducing EMI in this mode.

The efficiency of the speaker drivers is affected by the series resistance between the WM8281 and

the speaker (e.g. PCB track loss and inductor ESR) as shown in Figure 89. This resistance should be

as low as possible to maximise efficiency.

GND

Switching

Losses

Class D

output

SPKVDD

SPKVDD/2

Losses due to resistance between output driver and

speaker (e.g. inductor ESR). This resistance must be

minimised in order to maximise efficiency.

Figure 89 Speaker Connection Losses

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 90.

WM8281

C = 3

F

L = 22

H

SPKOUTxP

SPKOUTxN

Fc = low pass 3dB

cut-off frequency

Fc =

p LC

Figure 90 Class D Output Filter Components

A simple equivalent circuit of a loudspeaker consists of a serially connected resistor and inductor, as

shown in Figure 91. 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

des

cribed earlier. This is known as ‘filterless’ operation.

WM8281ECS/R Datenblatt(PDF) 377 Page - Cirrus Logic

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