WM8944BECSSLASHR datasheet(63/165 Pages) CIRRUS

63 / 165 page

WM8944B

Rev 4.3

has selected the LDOVDD supply voltage

, then the alternate ratio “5/11” may be preferred, as

described below.

Note that the “5/11” ratio is designed for the case where LDOVDD = 3.3V and LDOVOUT = 3.0V. This

results in a VMID = 3.3V x (5/11) = 1.5V which is half of the LDOVOUT voltage.

If these conditions are not being used or the LDO has been bypassed then VMID_REF should be set

to select LDOVOUT as the VMID source and VMID_CTRL should be set to select the ratio “1/2”.

The speaker output drivers require a mid-rail reference voltage. The default operating conditions

assume SPKVDD >= 3.0V, and use VMID as the reference. The reference can also be adjusted for

SPKVDD = 1.8V.

If SPKVDD = 1.8V, then the SPK_LOWVMID_ENA register should be set to 1. This selects a 0.9V

mid-rail reference, enabling optimal power and THD performance at the lower SPKVDD voltage.

Note that the speaker output reference is fixed with respect to LDOVOUT or LDOVDD; it does not

scale with SPKVDD. Therefore, the speaker reference cannot be otimised for all SPKVDD conditions.

If SPKVDD > 1.8V and < 3.0V, then SPK_LOWVMID_ENA should be set to 1, but the THD

performance may be degraded.

The analogue circuits in the WM8944B require a bias current. The normal (Master) bias current is

enabled by setting BIAS_ENA. Note that the Master bias current source requires VMID to be enabled

also.

The Master Reference and Bias Control bits are defined in Table 40.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R7 (07h)

Additional

control

VMID_REF_SEL

VMID Source Select

0 = LDO supply (LDOVDD)

1 = LDO output (LDOVOUT)

VMID_CTRL

VMID Ratio control

Sets the ratio of VMID to the source

selected by VMID_REF_SEL

0 = 5/11

1 = 1/2

VMID_ENA

VMID Enable

0 = Disabled

1 = Enabled

R2 (02h)

Power

Management

SPK_LOWVMID_

ENA

Selects 0.9V midrail voltage for

speaker output drivers

0 = Disabled

1 = Enabled

This bit should be enabled if

SPKVDD = 1.8V

BIAS_ENA

Master Bias Enable

0 = Disabled

1 = Enabled

1:0

VMID_SEL [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 40 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

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