WM8991 datasheet(135/174 Pages) WOLFSON

135 / 174 page

Pre-Production

WM8991

PP, May 2008, Rev 3.1

135

The register fields associated with soft start control are described in Table 83.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

SOFTST

Enables VMID soft start

0 = Disabled

1 = Enabled

BUFDCOPEN

Enables the Start-Up bias current

generator

0 = Disabled

1 = Enabled

R57 (39h)

Anti-Pop (2)

POBCTRL

Selects the bias current source for output

amplifiers and VMID buffer

0 = Default bias

1 = Start-Up bias

Table 83 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 WM8991 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 1k

Ω

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.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R57 (39h)

Anti-Pop (2)

BUFIOEN

Enables the Buffered VMID reference at

disabled inputs/outputs

0 = Disabled

1 = Enabled

Table 84 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 85. Note that the buffered VMID

reference is not applied to an actively discharged output, regardless of BUFIOEN.

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