WM8721

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WOLFSON MICROELECTRONICS LTD

PP Rev 1.3 November 2000

13

The scheme of the circuit is shown in Figure 10.

VMID

HPOUT

FROM

DAC VIA

LINEOUT

Figure 10 Headphone Amplifier Schematic

LHPOUT and RHPOUT volumes can be independently adjusted under software control using the

LHPVOL[6:0] and RHPVOL[6:0] bits respectively of the headphone output control registers. The

adjustment is logarithmic with an 80dB range in 1dB steps from +6dB to –73dB.

The headphone outputs can be separately muted by writing codes less than 0110000 to

LHPVOL[6:0] or RHPVO[6:0]L bits. Whenever the headphone outputs are muted or the device

placed into standby mode, the DC voltage is maintained at the line outputs to prevent any audible

clicks from being present.

A zero cross detect circuit is provided at the input to the headphones under the control of the LZCEN

and RZCEN bits of the headphone output control register. Using these controls the volume control

values are only updated when the input signal to the gain stage is close to the analogue ground level.

This minimises and audible clicks and zipper noise as the gain values are changed or the device

muted. Note that this circuit has no time out so if only DC levels are being applied to the gain stage

input of more than approximately 20mv, then the gain will not be updated. This zero cross function is

enabled when the LZCEN and RZCEN bit is set high during a volume register write. If there is

concern that a DC level may have blocked a volume change (one made with LZCEN or RZCEN set

high) then a subsequent volume write of the same value, but with the LZCEN or RZCEN bit set low

will force a volume update, regardless of the DC level.

LHPOUT and RHPOUT volume and zero-cross setting can be changed independently. Alternatively,

the user can lock the two channels together, allowing both to be updated simultaneously, halving the

number of serial writes required, provided that the same gain is needed for both channels. This is

achieved through writing to the HPBOTH bit of the control register. Setting LRHPBOTH whilst writing

to LHPVOL and LZCEN will simultaneously update the Right Headphone controls similarly. The

corresponding effect on updating RLHPBOTH is also achieved.