WM8974 datasheet(39/63 Pages) WOLFSON

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WM8974

PP Rev 1.1 September 2004

HEADPHONE OUTPUT

The speaker outputs 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:

Figure 18 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 16

Ω load and

C1, C2 = 220

µF:

fc = 1 / 2

π RLC1 = 1 / (2π x 16Ω x 220µF) = 45 Hz

In the DC coupled configuration, the headphone “ground” is connected to the MONOOUT pin. The

MONOOUT pin can be configured as a DC output driver by setting the MONOMUTE register bit.

The DC voltage on MONOOUT in this configuration is equal to the DC offset on the SPROUTP and

SPKOUTN pins therefore no DC blocking capacitors are required. This saves space and material

cost in portable applications.

It is recommended to connect the DC coupled 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.

MONO OUTPUT

The mono output, can be used as a line output, a headphone output or as a puedo ground for cap-

less driving of loads by SPKROUT. Recommended external components are shown below.

Figure 19 Recommended Circuit for Line Output

The DC blocking capacitors and the load resistance together determine the lower cut-off frequency,

fc. Assuming a 10 k

Ω load and C1 = 1µF:

fc = 1 / 2

π (RL+R1) C1 = 1 / (2π x 10.1kΩ x 1µF) = 16 Hz

Increasing the capacitance lowers fc, improving the bass response. Smaller values of C1 will

diminish the bass response. The function of R1 is to protect the line outputs from damage when

used improperly.

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