WM9090
Production Data
w
PD, November 2010, Rev 4.1
52
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 5k
Ω
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.
REGISTER
ADDRESS
BIT LABEL
DEFAULT
DESCRIPTION
R1 (01h)
Power
Management
(1)
2:1
VMID_RES
[1:0]
11
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)
0
BIAS_ENA
0
Enables the Normal bias current generator (for
all analogue functions)
0 = Disabled
1 = Enabled
R57 (39h)
AntiPOP2
0
VMID_ENA
0
Enable VMID master bias current source
0 = Disabled
1 = Enabled
Table 31 Reference Voltages and Master Bias Enable
