WM8903 datasheet(77/178 Pages) WOLFSON

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WM8903

PD, Rev 4.5, June 2012

REFERENCE VOLTAGES AND MASTER BIAS

This section describes the analogue reference voltage and bias current controls. It also describes the

VMID soft-start circuit for pop-free start-up and shut-down. Note that, under the recommended usage

conditions of the WM8903, these features will be configured by running the default Start-Up and Shut-

Down sequences as described in the “Control Write Sequencer” section. In these cases, the user

does not need to set these register fields directly.

The analogue circuits in the WM8903 require a mid-rail analogue reference voltage, VMID. This

reference is generated from AVDD via a programmable resistor chain. Together with the external

VMID decoupling capacitor, the programmable resistor chain results in a slow, normal or fast

charging characteristic on VMID. This is controlled by VMID_RES [1:0], and can be used to optimise

the reference for normal operation, low power standby or for fast start-up as described in Table 49.

For normal operation, the VMID_RES field should be set to 01.

The analogue circuits in the WM8903 require a bias current. The normal bias current is enabled by

setting BIAS_ENA. Note that the normal bias current source requires VMID to be enabled also. The

normal bias current can also be controlled using the ISEL field as described in Table 49. This can be

used to reduce power consumption, but may have a detrimental impact on audio performance in

some usage modes. The default setting is recommended.

Note that the DAC and Output PGA bias circuits may also be adjusted in order to reduce power

consumption. For details, see “Digital-to-Analogue Converter (DAC)“ or “Output Signal Path”.

An alternate bias current source (Start-Up Bias) is provided for pop-free start-up; this is selected

using POBCTRL (see Table 50). Note that the default setting of POBCTRL selects the Start-Up Bias.

The normal bias is only selected when POBCTRL is set to logic 0.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R4 (04h)

Bias Control

3:2

ISEL [1:0]

Master Bias control

00 = Normal bias x 0.5

01 = Normal bias x 0.75

10 = Normal bias

11 = Normal bias x 1.5

BIAS_ENA

Enables the Normal bias current

generator (for all analogue functions)

0 = Disabled

1 = Enabled

R5 (05h)

VMID

Control 0

2:1

VMID_RES [1:0]

VMID Divider Enable and Select

00 = VMID disabled (for OFF mode)

01 = 2 x 50k

 divider (for normal

operation)

10 = 2 x 250k

 divider (for low power

standby)

11 = 2 x 5k

 divider (for fast start-up)

Table 49 Reference Voltages and Master Bias Enable

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