CS47L85 datasheet(353/370 Pages) CIRRUS

353 / 370 page

CS47L85

Rev 4.3

353

SPEAKER DRIVER OUTPUT PATH

The CS47L85 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 CS47L85 and the speaker (e.g. PCB

track loss and inductor ESR) as shown in Figure 93. 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 93 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 94.

CS47L85

C = 3

F

L = 22

H

SPKOUTxP

SPKOUTxN

Fc = low pass 3dB

cut-off frequency

Fc =

p LC

Figure 94 Class D Output Filter Components

A simple equivalent circuit of a loudspeaker consists of a serially connected resistor and inductor, as shown in Figure 95.

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.

CS47L85 Datasheet(PDF) 353 Page - Cirrus Logic

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