wm8400 Wolfson Microelectronics plc, wm8400 Datasheet - Page 201

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

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Pre-Production
Due to the wide tolerance of many types of ceramic capacitors, care must be taken to ensure that the
selected components provide the required capacitance across the required temperature and voltage
ranges in the intended application.
MICROPHONE BIAS CIRCUIT
The WM8400 is designed to interface easily with up to four electret condenser microphones. These
may be connected in single-ended or pseudo-differential configurations. The single-ended method
allows greater capability for the connection of multiple audio sources simultaneously, whilst the
pseudo-differential method provides better performance due to its rejection of common-mode noise.
In either configuration, the electret microphone requires a bias current, which is provided via the
MICBIAS connection of the WM8400. The bias current is generated by an output-compensated
amplifier, which requires an external capacitor in order to ensure accuracy and stability. The
recommended capacitance is 4.7µF. A ceramic type is a suitable choice here, providing that care is
taken to choose a component that exhibits this capacitance at the intended MICBIAS voltage.
A current-limiting resistor is also required for the electret condenser microphone. The resistance
should be chosen according to the recommended operating voltage of the microphone and the
maximum bias current of the WM8400. The operating voltage of a typical electret microphone for
portable applications is around 1.3v. The recommended 2.2kΩ current limiting resistor is suggested
as it provides compatibility with a wide range of microphone models.
Note that the MICBIAS voltage may be adjusted using register control to suit the requirements of the
microphone. Also note the WM8400 supports a maximum MICBIAS current of 3mA in total; if more
than one microphone interface is connected, the combined current of these interfaces must not
exceed 3mA.
Figure 103 illustrates the recommended single-ended and pseudo-differential microphone
connections for the WM8400.
Figure 103 Single-Ended and Pseudo-Differential Microphone Connections
CLASS D SPEAKER CONNECTIONS
The WM8400 incorporates a Class D/AB 1W speaker driver. By default, the device operates in
Class D mode, which offers 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 Class D output requires external filtering in order to recreate the audio signal. This may be
implemented using an LC or 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, an LC filter is the
recommended solution, in order to minimise the power dissipated in the filter. A suitable
implementation is illustrated in Figure 104.
Electret
Microphone
4.7uF
R
C
MICBIAS
AGND
LIN1
VMID
Electret
Microphone
R
4.7uF
C
C
MICBIAS
AGND
LIN1
LIN2
PP, April 2009, Rev 3.0
WM8400
201