WM8987 datasheet(55/59 Pages) WOLFSON

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Production Data

WM8987L

PD Rev 4.0 August 2008

Figure 47 Driving single-ended headphones using DC-blocking capacitors

It requires the following register settings:

•

Set only the LOUT2 and ROUT2 bits in R26 to ‘1’ to enable LOUT2, ROUT2 and the

associated mixers. The ROUT1, MONO and OUT3 bits should remain at ‘0’.

•

Set LD2LO (R34) and RD2RO (R37) to ‘1’ to enable the correct audio paths (in the case

of audio playback from an analogue source, select the input pin using LMIXSEL and

RMIXSEL, and set LI2LO/RI2RO instead of LD2LO/RD2RO).

•

Leave ROUT2INV = 0 in R24, so that the right-channel signal going to ROUT2 is not

inverted.

•

The left channel volume is controlled by LOUT2VOL.

•

The right channel volume is controlled by ROUT2VOL.

This method for driving a headphone requires two capacitors in the LOUT2 and ROUT2 paths. Their

capacitance C and the load resistance R together determine the lower cut-off frequency, fc.

Increasing R or C lowers fc, improving the bass response. Smaller capacitance values will diminish

the bass response. For example, assuming a 16 Ohm load and C = 220

µF:

fc = 1 / 2

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

CAPLESS DRIVE

To drive single-ended headphones without DC blocking capacitors, the analogue outputs should be

used as follows:

•

LOUT2 is the left output

•

ROUT2 is the right output

•

OUT3 is a pseudo-ground output for the headphone

•

ROUT1 is unused and should be disabled

This setup is illustrated in Figure 47.

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