wm9090 Wolfson Microelectronics plc, wm9090 Datasheet - Page 49

• Manufacturer Part Number: wm9090
• Description: Ultra Low Power Audio Subsystem
• Manufacturer: Wolfson Microelectronics plc

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Pre-Production
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 WM9090 require a mid-rail analogue reference voltage, VMID. This
reference is generated from AVDD via a programmable resistor chain. Together with the external
decoupling capacitor (connected to the VMIDC pin), the programmable resistor chain results in a
slow, normal or fast charging characteristic on VMID.
VMID is enabled by setting the VMID_ENA register bit. The programmable resistor chain is
configured by VMID_RES [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 31.
When VMID is enabled using VMID_ENA, the WM9090 automatically controls VMID using a pop-
suppression circuit to avoid a step change in VMID; this suppresses pop/click noise which could
otherwise occur.
By default, the 2 x 5kΩ VMID divider is selected in order to allow fast start-up. For normal operation
and lower power consumption, the VMID_RES register should be updated after start-up to select
another resistor value.
The analogue circuits in the WM9090 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 31 Reference Voltages and Master Bias Enable
R1 (01h)
Power
Management
(1)
R57 (39h)
AntiPOP2
REGISTER
ADDRESS
2:1
BIT
0
0
VMID_RES
[1:0]
BIAS_ENA
VMID_ENA
LABEL
DEFAULT
11
0
0
VMID Divider Enable and Select
00 = VMID disabled (for OFF mode)
01 = 2 x 50kΩ divider (for normal operation)
10 = 2 x 250kΩ 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
Enable VMID master bias current source
0 = Disabled
1 = Enabled
DESCRIPTION
PP, January 2010, Rev 3.0
WM9090
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