Datasheet

ADAU1761
Rev. C | Page 36 of 92
HEADPHONE OUTPUT
The LHP and RHP pins can be driven by either a line output
driver or a headphone driver by setting the HPMODE bit in
Register R30 (playback headphone right volume control register,
Address 0x4024). The headphone outputs can drive a load of at
least 16 .
Separate volume controls for the left and right channels range
from −57 dB to +6 dB. Slew can be applied to all the playback
volume controls using the ASLEW[1:0] bits in Register R34
(playback pop/click suppression register, Address 0x4028).
Capless Headphone Configuration
The headphone outputs can be configured in a capless output
configuration with the MONOOUT pin used as a dc virtual
ground reference. Figure 45 depicts a typical playback path in
a capless headphone configuration. Tabl e 19 lists the register
settings for this configuration. As shown in this table, the
MONOOUT pin outputs common mode (AVDD/2), which
is used as the virtual headphone reference.
LHP
RHP
MONOOUT
MIXER 3
LEFT
DAC
MX3LM
LHPVOL[5:0]
MX3EN
MIXER 4
RIGHT
DAC
MX4RM
RHPVOL[5:0]
MX4EN
MIXER 7
MX7[1:0]
MONOM
MOMODE
MX7EN
0
7680-062
Figure 45. Capless Headphone Configuration Diagram
Table 19. Capless Headphone Register Settings
Register Bit Name Setting
R36 DACEN[1:0] 11 = both DACs on
MX3EN 1 = enable Mixer 3 R22
MX3LM 1 = unmute left DAC input
MX4EN 1 = enable Mixer 4 R24
MX4RM 1 = unmute right DAC input
MX7EN 1 = enable Mixer 7 R28
MX7[1:0] 00 = common-mode output
MONOM 1 = unmute mono output R33
MOMODE 1 = headphone output
LHPVOL[5:0] Desired volume for LHP output R29
LHPM 1 = unmute left headphone output
HPMODE 1 = headphone output
RHPVOL[5:0] Desired volume for RHP output
R30
RHPM 1 = unmute right headphone output
Headphone Output Power-Up/Power-Down Sequencing
To prevent pops when turning on the headphone outputs, the
user must wait at least 4 ms to unmute these outputs after
enabling the headphone output with the HPMODE bit. This is
because of an internal capacitor that must charge before these
outputs can be used. Figure 46 and Figure 47 illustrate the
headphone power-up/power-down sequencing.
For capless headphones, configure the MONOOUT pin before
unmuting the headphone outputs.
4ms
USER
DEFINED
HPMODE
1 = HEADPHONE
INTERNAL
PRECHARGE
RHPM AND LHPM
1 = UNMUTE
07680-046
Figure 46. Headphone Output Power-Up Timing
HPMODE
0 = LINE OUTPUT
RHPM AND LHPM
0 = MUTE
USER DEFINED
07680-047
Figure 47. Headphone Output Power-Down Timing
Ground-Centered Headphone Configuration
The headphone outputs can also be configured as ground-
centered outputs by placing coupling capacitors on the LHP
and RHP pins. Ground-centered headphones should use the
AGND pin as the ground reference.
When the headphone outputs are configured in this manner,
the capacitors create a high-pass filter on the outputs. The
corner frequency of this filter, at which point its attenuation
is 3 dB, is calculated by the following formula:
f
3dB
= 1/(2π × R × C)
where:
C is the capacitor value.
R is the impedance of the headphones.
For a typical headphone impedance of 16  and a 47 F
capacitor, the corner frequency is 211 Hz.