Pre-Production
WM8959
POP SUPPRESSION CONTROL
In normal operation, the analogue circuits in the WM8959 are referenced to VMID (AVDD/2). When
this reference voltage is first enabled, it will ramp quickly from AGND to AVDD/2 and, if connected to
an active output, will result in an audible pop being heard. Enabling or disabling the output stage after
the internal reference has settled can also result in an audible pop as the output rises rapidly from
AGND.
The WM8959 provides a number of features which enable these pops to be suppressed. The
associated control bits are described in this section. Careful attention is required to the sequence
and timing of these controls in order to get maximum benefit. An outline of some generic control
sequences is provided in order to assist users in the definition of application-specific sequences.
REFERENCE VOLTAGES
VMID is generated from AVDD via a programmable resistor chain as shown in the audio signal paths
diagram on page 25. Together with the external decoupling capacitor on VMID, the programmable
resistor chain results in a slow, normal or fast charging characteristic on VMID. The VMID reference
is controlled by VMID_MODE[1:0].
The analogue circuits in the WM8959 require a bias current. The default bias current is enabled by
setting VREF_ENA. Note that the default bias current source requires VMID to be enabled also.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R1 (01h)
2:1
VMID_MODE
[1:0]
00b
VMID Divider Enable and Select
00 = VMID disabled (for OFF mode)
01 = 2 x 50kΩ divider (Normal mode)
10 = 2 x 250kΩ divider (Standby mode)
11 = 2 x 5kΩ divider (for fast start-up)
(rw)
0
VREF_ENA
(rw)
0b
VREF Enable (Bias for all analogue
functions)
0 = VREF bias disabled
1 = VREF bias enabled
Table 73 Reference Voltages
SOFT START CONTROL
A pop-suppressed start-up requires VMID to be enabled smoothly, without the step change normally
associated with the initial stage of the VMID capacitor charging. A pop-suppressed start-up also
requires the analogue bias current to be enabled throughout the signal path prior to the VMID
reference voltage being applied. The WM8959 incorporates pop-suppression circuits which address
these requirements.
The WM8959 provides an alternative start-up bias circuit which can be used in place of the default
bias current during start-up. The start-up bias current source is enabled by BUFDCOPEN. The start-
up bias source is selected (in place of the default bias source) by POBCTRL. It is recommended that
the start-up bias is used during start-up, before switching back to the higher quality, VREF-enabled
bias.
A soft-start circuit is provided in order to control the switch-on of the VMID reference. The soft-start
control circuit is enabled by setting SOFTST. When the soft-start circuit is enabled prior to enabling
VMID_MODE, the reference voltage rises smoothly, without the step change that would otherwise
occur. It is recommended that the soft-start circuit and the output signal path be enabled before
VMID is enabled by VMID_MODE.
Soft shut-down of VMID is also provided by the soft-start control circuit and the start-up bias current
generator. The soft shut-down of VMID is achieved by setting SOFTST = 1, BUFCOPEN = 1 and
POBCTRL = 1 prior to setting VMID_MODE = 00.
PP, May 2008, Rev 3.1
119
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
