WM8945ECSSLASHR datasheet(63/169 Pages) WOLFSON

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Production Data

WM8945

PD, May 2011, Rev 4.1

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 internal LDO (described in the previous section) requires a voltage reference. Under normal

operating conditions, this is provided from VMID, via the register controls described in Table 39. Note,

however, that VMID is normally generated from the LDO output. Therefore, an alternative voltage

reference is required for start-up, which is not dependent on the LDO output. The

VMID_FAST_START bit enables a ‘Fast-Start’ reference powered from LDOVDD. This alternate

VMID can be selected as the LDO reference using the LDO_REF_SEL_FAST bit as described in

Table 36.

The VMID soft-start and fast start register controls are defined in Table 40.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R7 (07h)

Additional

control

VMID_FAST_

START

VMID (fast-start) Enable

0 = Disabled

1 = Enabled

STARTUP_BIAS_

ENA

Start-Up Bias Enable

0 = Disabled

1 = Enabled

BIAS_SRC

Bias Source select

0 = Normal bias

1 = Start-Up bias

6:5

VMID_RAMP [1:0]

VMID soft start enable / slew rate

control

00 = Disabled

01 = Fast soft start

10 = Normal soft start

11 = Slow soft start

Table 40 Soft Start Control

POP SUPPRESSION CONTROL

The WM8945 incorporates a number of features which are designed to suppress pops normally

associated with Start-Up, Shut-Down or signal path control. These include the option to maintain an

analogue output to VMID even when the output driver is disabled. In addition, there is the ability to

actively discharge an output to GND.

Note that, to achieve maximum benefit from these features, careful attention may be required to the

sequence and timing of these controls.

DISABLED OUTPUT CONTROL

The line outputs and speaker outputs are biased to VMID in normal operation. In order to avoid

audible pops caused by a disabled signal path dropping to GND, the WM8945 can maintain these

connections at VMID when the relevant output stage is disabled. This is achieved by connecting a

buffered VMID reference to the output.

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