WM8956 datasheet(36/80 Pages) WOLFSON

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WM8956

Preliminary Technical Data

PTD, July 2007, Rev 2.1

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

Ω.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R27 (1Bh)

Additional (1)

VROI

VREF to Analogue Output Resistance

(Disabled Outputs)

0 = 500

Ω VMID to output

1 = 20k

Ω VMID to output

Table 21 Disabled Outputs to VREF Resistance

HEADPHONE OUTPUT

Analogue outputs HP_L/HP_R, and OUT3, can drive a 16

Ω or 32Ω 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

Ω load and

C1, C2 = 100

µF:

fc = 1 / 2

π RLC1 = 1 / (2π x 32Ω x 100µF) = 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.

WM8956 Datasheet(PDF) 36 Page - Wolfson Microelectronics plc

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