WM8956 View Datasheet(PDF) - Cirrus Logic
Part Name
Description
MFG CO.
'WM8956' PDF : 80 Pages View PDF
Production Data
WM8956
The speaker output enable bits SPK_OP_EN[1:0] should not be enabled until there is a valid
switching clock to drive the class D outputs. This means that SYSCLK must be active, and DCLKDIV
set to an appropriate value to produce a class D clock of between 700kHz and 800kHz for best
performance (See "Class D Speaker Outputs" and "Clocking and Sample Rates" sections for more
information).
Whenever an analogue output is disabled, it remains connected to VREF through a resistor. This
helps to prevent pop noise when the output is re-enabled. The resistance between VREF and each
output can be controlled using the VROI bit in register 27. If a high impedance is desired for disabled
outputs, VROI can then be set to 1, increasing the resistance to about 20k.
REGISTER
ADDRESS
R27 (1Bh)
Additional (1)
BIT
LABEL
6
VROI
DEFAULT
0
Table 21 Disabled Outputs to VREF Resistance
DESCRIPTION
VREF to Analogue Output Resistance
(Disabled Outputs)
0 = 500 VMID to output
1 = 20k VMID to output
HEADPHONE OUTPUT
Analogue outputs HP_L/HP_R, and OUT3, can drive a 16 or 32 headphone load, either through
DC blocking capacitors, or DC coupled without any capacitor.
Headphone Output using DC blocking capacitors
DC Coupled Headphone Output
(L2MO=0; R2MO=0)
Figure 15 Recommended Headphone Output Configurations
When DC blocking capacitors are used, then their capacitance and the load resistance together
determine the lower cut-off frequency, fc. Increasing the capacitance lowers fc, improving the bass
response. Smaller capacitance values will diminish the bass response. Assuming a 32 load and C1,
C2 = 100F:
fc = 1 / 2 RLC1 = 1 / (2 x 32 x 100F) = 50 Hz
In the DC coupled configuration, the headphone “ground” is connected to the OUT3 pin, which must
be enabled by setting OUT3 = 1 and muted by setting L2MO=0 and R2MO=0. As the OUT3 pin
produces a DC voltage of AVDD/2 (=VREF), there is no DC offset between HP_L/HP_R and OUT3,
and therefore no DC blocking capacitors are required. This saves space and material cost in portable
applications.
It is recommended to connect the DC coupled headphone outputs only to headphones, and not to the
line input of another device. Although the built-in short circuit protection will prevent any damage to
the headphone outputs, such a connection may be noisy, and may not function properly if the other
device is grounded.
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PD, November 2011, Rev 4.1
35
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