WM8978CGEFLSLASHV datasheet(59/118 Pages) WOLFSON

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Production Data

WM8978

PD, Rev 4.5, October 2011

Headphone Output using DC Blocking Capacitors:

DC Coupled Headphone Output:

Figure 28 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

 load and C1,

C2 = 220

F:

fc = 1 / 2 RLC1 = 1 / (2 x 16 x 220F) = 45 Hz

In the DC coupled configuration, the headphone “ground” is connected to the VMID pin. The OUT3/4

pins can be configured as a DC output driver by setting the OUT3MUTE and OUT4MUTE register bit.

The DC voltage on VMID in this configuration is equal to the DC offset on the LOUT1 and ROUT1

pins therefore no DC blocking capacitors are required. This saves space and material cost in portable

applications.

Note that OUT3 and OUT4 have an optional output boost of 1.5x. When these are configured in this

output boost mode (OUT3BOOST/OUT4BOOST=1) then the VMID value of these outputs will be

equal to 1.5xAVDD/2 and will not match the VMID of the headphone drivers. Do not use the DC

coupled output mode in this configuration.

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.

SPEAKER OUTPUTS (LOUT2 AND ROUT2)

The outputs LOUT2 and ROUT2 are designed to drive an 8Ω BTL speaker but can optionally drive

two headphone loads of 16

/32 or a line output (see Headphone Output and Line Output sections,

respectively). Each output has an individual volume control PGA, an output boost/level shift bit, a

mute and an enable as shown in Figure 29. LOUT2 and ROUT2 output the left and right channel

mixer outputs respectively.

The ROUT2 signal path also has an optional invert. The amplifier used for this invert can be used to

mix in the AUXR signal with an adjustable gain range of -15dB -> +6dB. This allows a ‘beep’ signal

to be applied only to the speaker output without affecting the HP or line outputs.

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