wm8400 Wolfson Microelectronics plc, wm8400 Datasheet - Page 202

• Manufacturer Part Number: wm8400
• Description: Wolfson Audioplustm Hi-fi Audio Codec And Power Management Unit For Mobile Multimedia
• Manufacturer: Wolfson Microelectronics plc

WM8400
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Figure 104 Class D Output Filter Components
A simple equivalent circuit of a loudspeaker consists of a serially connected resistor and inductor, as
shown in Figure 105. 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 105 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:
It should be noted, however, 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 WM8400 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.
PP, April 2009, Rev 3.0
Pre-Production
202