wm9081 Wolfson Microelectronics plc, wm9081 Datasheet - Page 77

• Manufacturer Part Number: wm9081
• Description: Mono Dac With 2.6w Class Ab/d Speaker Driver, Dynamic Range Controller And Retune Mobile Parametric Equalizer
• Manufacturer: Wolfson Microelectronics plc

Pre-Production
REFERENCE VOLTAGES AND MASTER BIAS
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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. Note that, under the
recommended usage conditions of the WM9081, these features will be configured by running the
default control sequences as described in the “Control Write Sequencer” section. In these cases, the
user does not need to set these register fields directly.
The analogue circuits in the WM9081 require a mid-rail analogue reference voltage, VMID. This
reference is generated from AVDD via a programmable resistor chain. Together with the external
VMID decoupling capacitor, the programmable resistor chain results in a slow, normal, or fast
charging characteristic on VMID. This is controlled by VMID_SEL[1:0], and can be used to optimise
the reference for normal operation, low power standby or for fast start-up as described in Table 46.
The analogue circuits in the WM9081 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.
Table 46 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 WM9081 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. The soft-start feature is selected
by setting VMID_RAMP; the slew rate is controlled by VMID_FAST_ST. 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 slew rate is controlled by VMID_FAST_ST.
The VMID soft-start register controls are defined in Table 47.
R4 (04h)
VMID
Control
R5 (05h)
Bias Control
1
REGISTER
ADDRESS
BIT
2:1
1
VMID_SEL
[1:0]
BIAS_ENA
LABEL
DEFAULT
00
0
VMID Divider Enable and Select
00 = VMID disabled (for OFF mode)
01 = 2 x 40kΩ divider (for normal operation)
10 = 2 x 240kΩ divider (for low power standby)
11 = 2 x 5kΩ divider (for fast start-up)
Enables the Normal bias current generator (for
all analogue functions)
0 = Disabled
1 = Enabled
DESCRIPTION
PP, Rev 3.0, April 2009
WM9081
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