WM8959
Pre-Production
The register fields associated with soft start control are described in Table 74.
REGISTER
ADDRESS
BIT
LABEL
SOFTST
DEFAULT
DESCRIPTION
R57 (39h)
6
0b
Enables VMID soft start
Anti-Pop (2)
0 = Disabled
1 = Enabled
2
1
BUFDCOPEN
POBCTRL
0b
0b
Enables the Start-Up bias current
generator
0 = Disabled
1 = Enabled
Selects the bias current source for output
amplifiers and VMID buffer
0 = Default bias
1 = Start-Up bias
Table 74 Soft Start Control
DISABLED INPUT/OUTPUT CONTROL
After start-up, it may be desirable to disable an output stage, in order to reduce power consumption
on an unused output. In order to avoid audible pops caused by a disabled output dropping to AGND,
the WM8959 can maintain the output at VMID even when the output driver is disabled. This is
achieved by connecting a buffered VMID reference to the output. The buffered VMID is enabled by
setting BUFIOEN. When BUFIOEN is enabled, it will be connected to any disabled output driver. It is
recommended that BUFIOEN is enabled prior to disabling the output driver.
The buffered VMID, enabled by BUFIOEN, also maintains the charge on the input capacitors
connected to any disabled input amplifier. Buffered VMID is connected to each input through 1kΩ
resistors. This suppresses the audible artefacts that would otherwise arise when an input amplifier is
disabled or enabled. In some applications, a pop generated at an input stage can be entirely
suppressed by correctly managing the output stages. However, it may be desirable to use the
buffered VMID feature in order to eliminate the input PGA start-up delay (the input capacitor charging
time) in addition to suppressing any mute/un-mute pops. In applications where frequent enabling and
configuration of signal paths is used, it is recommended to enable BUFIOEN at all times.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R57 (39h)
3
BUFIOEN
0b
Enables the Buffered VMID reference at
disabled inputs/outputs
Anti-Pop (2)
0 = Disabled
1 = Enabled
Table 75 Disabled Input/Output Control
OUTPUT DISCHARGE CONTROL
The output paths may also be actively discharged to AGND through internal resistors if desired. This
is desirable at start-up in order to achieve a known output stage condition prior to enabling the soft-
start VMID reference voltage. This is also desirable in shut-down in order to eliminate pops arising
from memory effects in the output capacitors on completion of the controlled shut-down of the VMID
reference. Note that, for any signal paths that do not use output capacitors (eg. capless headphone
drive), the discharge control is not normally required.
It is recommended that the output paths should be actively discharged prior to commencing a start-
up sequence. The active discharging should then be disabled prior to enabling the output drivers.
In shut-down, it is recommended that the output paths should be actively discharged after the VMID
reference has settled to AGND and the output drivers have been disabled.
The line and headphone output pins are discharged by setting DIS_LLINE, DIS_RLINE, DIS_OUT3,
DIS_OUT4, DIS_LOUT and DIS_ROUT, as described in Table 76. Note that the buffered VMID
reference is not applied to an actively discharged output, regardless of BUFIOEN.
PP, May 2008, Rev 3.1
120
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
