WM8993 datasheet(153/229 Pages) WOLFSON

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WM8993

PD, November 2010, Rev 4.0

153

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 suppressed start-up and shut-down. Note that, under the

recommended usage conditions of the WM8993, 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 WM8993 require a mid-rail analogue reference voltage, VMID. This

reference is generated from AVDD1 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_SEL[1:0], and can be used to optimise

the reference for normal operation or low power standby as described in Table 106.

The analogue circuits in the WM8993 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.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R1 (01h)

Power

Management

(1)

2:1

VMID_SEL

[1:0]

VMID Divider Enable and Select

00 = VMID disabled (for OFF mode)

01 = 2 x 40k

Ω divider (for normal operation)

10 = 2 x 240k

Ω divider (for low power standby)

11 = Reserved

BIAS_ENA

Enables the Normal bias current generator (for

all analogue functions)

0 = Disabled

1 = Enabled

Table 106 Reference Voltages and Master Bias Enable

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 WM8993 incorporates pop-suppression circuits which address

these requirements.

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

the STARTUP_BIAS_ENA register bit. The start-up bias is selected (in place of the normal bias)

using the BIAS_SRC bit. It is recommended that the start-up bias is used during start-up, before

switching back to the higher quality, normal bias.

A soft-start circuit is provided in order to control the switch-on of the VMID reference. The soft-start

control circuit offers two slew rates for enabling the VMID reference; these are selected and enabled

by VMID_RAMP. When the soft-start circuit is enabled prior to enabling VMID_SEL, 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_SEL.

A soft shut-down is provided, using the soft-start control circuit and the start-up bias current

generator. The soft shut-down of VMID is achieved by setting VMID_RAMP, STARTUP_BIAS_ENA

and BIAS_SRC to select the start-up bias current and soft-start circuit prior to setting VMID_SEL=00.

The VMID soft-start register controls are defined in Table 107.

WM8993 Datasheet(PDF) 153 Page - Wolfson Microelectronics plc

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