WM8994 Datasheet, PDF (229/359 Pages) Wolfson Microelectronics plc – Multi-Channel Audio Hub CODEC for Smartphones

Production Data
WM8994
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 noted 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.
REGISTER BIT
ADDRESS
LABEL DEFAULT
DESCRIPTION
R1 (0001h)
2:1 VMID_SEL
Power
[1:0]
Management
(1)
00
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
0 BIAS_ENA
0
Enables the Normal bias current generator (for
all analogue functions)
0 = Disabled
1 = Enabled
R57 (0039h) 3 VMID_BUF_
0
VMID Buffer Enable
AntiPOP (2)
ENA
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.
w
PD, April 2012, Rev 4.4
229