Production Data

WM9082

w

PD, August 2012, Rev 4.1

19

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:

8

 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).

The Class D outputs of the WM9082 operate at much higher frequencies than is recommended for

most speakers; 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.

A simple test can be used to confirm if the loudspeaker is compatible with filterless operation. Under

quiescent input conditions (idle digital audio input while in the ON state), the SPKVDD current is

measured with the speaker disconnected, and measured again with the speaker connected. If the

SPKVDD current increases by more than 10mA when the speaker is connected, then the speaker

alone is not effective as a filter, and it is recommended to consider changing the speaker or adding

LC filter components.

RECOMMENDED EXTERNAL COMPONENTS DIAGRAM

Figure 7 provides a summary of recommended external components for WM9082. Note that the

actual requirements may differ according to the specific target application.

Figure 7 WM9082 Recommended External Components Diagram