WM1811A datasheet(319/328 Pages) WOLFSON

319 / 328 page

Production Data

WM1811A

PD, November 2013, Rev 4.1

319

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

shown in Figure 90. 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 90 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:

 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 WM1811A 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.

RECOMMENDED EXTERNAL COMPONENTS DIAGRAM

Figure 91provides a summary of recommended external components for WM1811A. Note that this

diagram does not include any components that are specific to the end application e.g. it doesnot

include filtering on the speaker outputs (assume filterless Class D operation), RF decoupling, or RF

filtering for pins which connect to the external world i.e. headphone or speaker outputs.

WM1811A Datasheet(PDF) 319 Page - Wolfson Microelectronics plc