WM8904ECSSLASHR datasheet(85/188 Pages) WOLFSON

85 / 188 page

Pre-Production

WM8904

PP, Rev 3.3, September 2012

The Charge Pump control fields are described in Table 51.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R98 (62h)

Charge

Pump 0

CP_ENA

Enable charge-pump digits

0 = disable

1 = enable

R104 (68h)

Class W (0)

CP_DYN_PWR

Enable dynamic charge pump power

control

0 = Charge pump controlled by

volume register settings (Class G)

1 = Charge pump controlled by real-

time audio level (Class W)

Class W is recommended for lowest

power consumption

Table 51 Charge Pump Control

DC SERVO

The WM8904 provides four DC servo circuits, two on the headphone outputs HPOUTL and HPOUTR

and two on the line outputs LINEOUTL and LINEOUTR, to remove DC offset from these ground-

referenced outputs. When enabled, the DC servos ensure that the DC level of these outputs remains

within 1mV of ground. Removal of the DC offset is important because any deviation from GND at the

output pin will cause current to flow through the load under quiescent conditions, resulting in

increased power consumption. Additionally, the presence of DC offsets can result in audible pops and

clicks at power up and power down.

The recommended usage of the DC Servo is initialised by running the default Start-Up sequence as

described in the “Control Write Sequencer” section. The default Start-Up sequence executes a series

of DC offset corrections, after which the measured offset correction is maintained on the headphone

output channels. If a different usage is required, e.g. if a periodic DC offset correction is required,

then the default Start-Up sequence may be modified according to specific requirements. The relevant

control fields are described in the following paragraphs and are defined in Table 52.

DC SERVO ENABLE AND START-UP

The DC Servo circuits are enabled on HPOUTL and HPOUTR by setting DCS_ENA_CHAN_0 and

DCS_ENA_CHAN_1 respectively. Similarly, the DC Servo circuits are enabled on LINEOUTL and

LINEOUTR by setting DCS_ENA_CHAN_2 and DCS_ENA_CHAN_3 respectively When the DC

Servo is enabled, the DC offset correction can be commanded in a number of different ways,

including single-shot and periodically recurring events.

Writing a logic 1 to DCS_TRIG_STARTUP_n initiates a series of DC offset measurements and

applies the necessary correction to the associated output; (‘n’ = 3 for LINEOUTR channel, 2 for

LINEOUTL channel, 1 for HPOUTR channel, 0 for HPOUTL channel). On completion, the output will

be within 1mV of AGND. This is the DC Servo mode selected by the default Start-Up sequence.

Completion of the DC offset correction triggered in this way is indicated by the

DCS_STARTUP_COMPLETE field, as described in Table 52. Typically, this operation takes 86ms

per channel.

Writing a logic 1 to DCS_TRIG_DAC_WR_n causes the DC offset correction to be set to the value

contained in the DCS_DAC_WR_VAL_n fields in Registers R73 to R76. This mode is useful if the

required offset correction has already been determined and stored; it is faster than the

DCS_TRIG_STARTUP_n mode, but relies on the accuracy of the stored settings. Completion of the

DC offset correction triggered in this way is indicated by the DCS_DAC_WR_COMPLETE field, as

described in Table 52. Typically, this operation takes 2ms per channel.

When using either of the DC Servo options above, the status of the DC offset correction process is

indicated

the

DCS_CAL_COMPLETE

field;

this

the

logical

the

DCS_STARTUP_COMPLETE and DCS_DAC_WR_COMPLETE fields.

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