WM8994ECSSLASHR datasheet(233/363 Pages) CIRRUS

233 / 363 page

WM8994

Rev 4.6

233

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.

The analogue circuits in the WM8994 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 determines the 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 136.

A buffered mid-rail reference voltage is provided. This is required for the single-ended configuration of

the Input PGAs, and also for direct signal paths from the input pins to the Input Mixers, Output Mixers

or Speaker Mixers. These requirements are note

d in the relevant “Analogue Input Signal Path” and

“Analogue Output Signal Path” sections. The buffered mid-rail reference is enabled by setting the

VMID_BUF_ENA register bit.

The analogue circuits in the WM8994 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 (0001h)

Power

Management

(1)

2:1

VMID_SEL

[1:0]

VMID Divider Enable and Select

00 = VMID disabled (for OFF mode)

01 = 2 x 40k

 divider (for normal operation)

10 = 2 x 240k

 divider (for low power standby)

11 = Reserved

BIAS_ENA

Enables the Normal bias current generator (for

all analogue functions)

0 = Disabled

1 = Enabled

R57 (0039h)

AntiPOP (2)

VMID_BUF_

ENA

VMID Buffer Enable

0 = Disabled

1 = Enabled (provided VMID_SEL > 00)

Table 136 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 WM8994 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.

Note that, if the VMID_RAMP function is enabled for soft start-up or soft shut-down then, after setting

VMID_SEL = 00 to disable VMID, the soft-start circuit must be reset before re-enabling VMID. The

soft-start circuit is reset by setting VMID_RAMP = 00. After resetting the soft-start circuit, the

VMID_RAMP register may be updated to the required setting for the next VMID transition.

The VMID soft-start register controls are defined in Table 137.

WM8994ECS/R 数据表(PDF) 233 Page - Cirrus Logic

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