WM8948ECSSLASHR datasheet(67/189 Pages) WOLFSON

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Production Data

WM8948

PD, May 2011, Rev 4.1

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

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

Ω divider (for normal

operation)

10 = 2 x 250k

Ω divider (for low

power standby)

11 = 2 x 5k

Ω 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

requires the analogue bias current to be enabled throughout the signal path prior to the VMID

reference voltage being applied. The WM8948 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 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 40.

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 37.

WM8948ECS/R Datenblatt(PDF) 67 Page - Wolfson Microelectronics plc

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