WM8721_04 Datasheet, PDF (14/37 Pages) Wolfson Microelectronics plc – Internet Audio DAC with Integrated Headphone Driver

WM8721 / WM8721L
Production Data
HEADPHONE AMPLIFIER
The WM8721 has a stereo headphone output available on LHPOUT and RHPOUT. The output is
designed specifically for driving 16 or 32Ω headphones with maximum efficiency and low power
consumption. The headphone output includes a high quality volume level adjustment and mute
function.
The scheme of the circuit is shown in Figure 10.
FROM
DAC VIA
LINEOUT
VMID
HPOUT
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 any 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.
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PD Rev 4.0 November 2004
14