Production Data
WM8945
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 39. 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 36.
The VMID soft-start and fast start register controls are defined in Table 40.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R7 (07h)
11
VMID_FAST_
START
0
VMID (fast-start) Enable
0 = Disabled
Additional
control
1 = Enabled
STARTUP_BIAS_
ENA
Start-Up Bias Enable
0 = Disabled
8
7
0
0
1 = Enabled
BIAS_SRC
Bias Source select
0 = Normal bias
1 = Start-Up bias
VMID_RAMP [1:0]
VMID soft start enable / slew rate
control
6:5
00
00 = Disabled
01 = Fast soft start
10 = Normal soft start
11 = Slow soft start
Table 40 Soft Start Control
POP SUPPRESSION CONTROL
The WM8945 incorporates a number of features which are designed to suppress pops normally
associated with Start-Up, Shut-Down or signal path control. These include the option to maintain an
analogue output to VMID even when the output driver is disabled. In addition, there is the ability to
actively discharge an output to GND.
Note that, to achieve maximum benefit from these features, careful attention may be required to the
sequence and timing of these controls.
DISABLED OUTPUT CONTROL
The line outputs and speaker outputs are biased to VMID in normal operation. In order to avoid
audible pops caused by a disabled signal path dropping to GND, the WM8945 can maintain these
connections at VMID when the relevant output stage is disabled. This is achieved by connecting a
buffered VMID reference to the output.
PD, May 2011, Rev 4.1
63
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
