WM8959 datasheet(119/155 Pages) WOLFSON

119 / 155 page

Pre-Production

WM8959

PP, May 2008, Rev 3.1

119

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.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

2:1

VMID_MODE

[1:0]

(rw)

00b

VMID Divider Enable and Select

00 = VMID disabled (for OFF mode)

01 = 2 x 50k

Ω divider (Normal mode)

10 = 2 x 250k

Ω divider (Standby mode)

11 = 2 x 5k

Ω divider (for fast start-up)

R1 (01h)

VREF_ENA

(rw)

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.

WM8959 Datasheet(PDF) 119 Page - Wolfson Microelectronics plc

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