CS47L35 datasheet(233/300 Pages) CIRRUS

233 / 300 page

DS1085F3

233

CS47L35

5.1 Recommended External Components

Figure 5-8. Class D Output Filter Components

A simple equivalent circuit of a loudspeaker consists of a serially-connected resistor and inductor, as shown in Fig. 5-9.

This circuit provides a low-pass filter for the speaker output. If the loudspeaker characteristics are suitable, the

loudspeaker itself can be used in place of the filter components described earlier. This is known as filterless operation.

Figure 5-9. 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

 and the desired cut-off frequency is 20 kHz, the optimum speaker inductance

may be calculated as shown in Eq. 5-1.

Equation 5-1. Speaker Inductance Calculation

An 8-

 loudspeaker typically has an inductance in the range 20–100 H; however, it should be noted that a loudspeaker

inductance is not 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 CS47L35 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.

The Class D speaker outputs are designed to support monitoring of external loudspeakers, giving real-time feedback for

algorithms such as Cirrus Logic’s speaker protection software. This enables maximum audio output to be achieved, while

ensuring the loudspeakers are also fully protected from damage.

The external speaker connections, incorporating the output current monitoring requirements, are shown in Fig. 5-10. Note

that, if output current monitoring is not required on one or more speaker channels, the respective ground connections

should be tied directly to ground on the PCB.

C = 3

F

L = 22

H

SPKOUTP

SPKOUTN

= Low-pass 3 dB cut-off frequency

 LC

CS47L35

 L

SPKOUTP

SPKOUTN

= Low-pass 3 dB cut-off frequency

CS47L35

Fc

-----------





20kHz



-------------------------

H

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