WM8961
Pre-Production
To prevent pop noise, CLASSD_ACGAIN should not be modified while the speaker outputs are
enabled.
To avoid clipping at speaker supply, SPKVDD1 and SPKVDD2 must be high enough to support the
peak output voltage when using CLASSD_ACGAIN functions.
The maximum RMS output voltage is defined by the equation:
MAX. RMS OUTPUT VOLTAGE = AVDD/1.8* CLASSD_ACGAIN.
The above formula assumes there is 0.5dB loss (6%) due to PCB tracking. 0.5dB corresponds to 0.5Ω
total tracking impedance with an 8Ω speaker. If the tracking impedance is greater than or less than
6% of the speaker impedance, the output voltage will differ slightly to the above formula.
CLASS D SPEAKER OUTPUT DRIVER
The class D speaker outputs SPK_LN/SPK_LP and SPK_RN/SPK_RP can drive 1W into 8Ω BTL
speakers at SPKVDD=5v. The Class D output stage is shown in Figure 22.
Left Speaker Class D
Driver Output Stage
R51,
CLASSD_ACGAIN[2:0]
From Left Speaker PGA
Volume Control
-
SPK_LP
SPK_LN
R49, SPKL_ENA
+
SPKVDD1
SPKGND1
Right Speaker Class D
Driver Output Stage
R51,
CLASSD_ACGAIN[2:0]
From Right Speaker PGA
Volume Control
-
SPK_RP
SPK_RN
R49, SPKR_ENA
+
SPKVDD2
SPKGND2
Figure 22 Class D Speaker Output Stage
Class D outputs reduce power consumption and maximise efficiency by reducing power dissipated in
the output drivers, delivering most of the power directly to the load. This is achieved by pulse width
modulation (PWM) of a high frequency square wave, allowing the audio signal level to be set by
controlling the pulse width. The frequency of the output waveform is controlled by DCLKDIV, and is
derived from SYSCLK.
The speaker output enable bits SPKR_ENA and SPKL_ENA should not be enabled until there is a
valid switching clock to drive the Class D outputs. This means that SYSCLK must be active, and
DCLKDIV set to an appropriate value to produce a Class D clock which runs at a nominal frequency of
384kHz when SYSCLK=12.288 MHz, or 352.8kHz when SYSCLK=11.2896MHz. This clock should
not be altered or disabled while the Class D outputs are enabled.
PP, August 2009, Rev 3.1
w
50
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
