WM8994ECSSLASHR datasheet(136/363 Pages) CIRRUS

136 / 363 page

WM8994

136

Rev 4.6

The Charge Pump control fields are described in Table 70.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R76 (004Ch)

Charge Pump

(1)

CP_ENA

Enable charge-pump digits

0 = Disable

1 = Enable

R77 (004Dh)

Charge Pump

(2)

CP_DISCH

Charge Pump Discharge Select

0 = Charge Pump outputs floating

when disabled

1 = Charge Pump outputs

discharged when disabled

R81 (0051h)

Class W (1)

9:8

CP_DYN_SRC_SEL

Selects the digital audio source for

envelope tracking

00 = AIF1, DAC Timeslot 0

01 = AIF1, DAC Timeslot 1

10 = AIF2, DAC data

11 = Reserved

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)

Table 70 Charge Pump Control

DC SERVO

The WM8994 provides a DC servo circuit on the headphone outputs HPOUT1L and HPOUT1R in

order to remove DC offset from these ground-referenced outputs. When enabled, the DC servo

ensures 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, eg. 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 71.

DC SERVO ENABLE AND START-UP

The DC Servo circuit is enabled on HPOUT1L and HPOUT1R by setting DCS_ENA_CHAN_0 and

DCS_ENA_CHAN_1 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’ = 0 for Left channel, 1 for Right channel). On

completion, the headphone 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 71. Typically, this operation

takes 86ms per channel.

For correct operation of the DC Servo Start-Up mode, it is important that there is no active audio

signal present on the signal path while the mode is running. The DC Servo Start-Up mode should be

scheduled at the correct position within the Headphone Output Enable sequence, as described in the

“Analogue Output Signal Path” section. All other stages of the analogue signal path should be fully

enabled prior to commanding the Start-Up mode; the DAC Digital Mute function should be used,

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