max9791 Maxim Integrated Products, Inc., max9791 Datasheet - Page 20

• Manufacturer Part Number: max9791
• Description: Windows Vista-compliant Class D Speaker Amplifiers With Directdrive Headphone Amplifiers
• Manufacturer: Maxim Integrated Products, Inc.

Windows Vista-Compliant Class D Speaker
Amplifiers with DirectDrive Headphone Amplifiers
Windows Vista-compliant platforms are restricted to only
115mΩ of ground return impedance. If the headphone
jack ground is connected close to the audio device
ground using a solid ground plane, the return path resis-
tance can be quite low. However, it is often necessary to
locate some jacks far from the audio device. The
MAX9791/MAX9792 COM and SENSE inputs allow the
headphone jack to be placed further away from the
device without degrading crosstalk performance.
The MAX9791/MAX9792 SENSE and COM inputs sense
and correct for the difference between the headphone
return and device ground. When using common-mode
sense, connect COM through a resistor to GND of the
device (Figure 4). For optimum common-mode rejec-
tion, use the same value resistors for R
To improve AC CMRR, add a capacitor equal to C
between GND and R
Configuring SENSE and COM in this way improves sys-
tem crosstalk performance by reducing the negative
effects of the headphone jack ground return resistance.
Figure 4. Connecting COM for Ground Sense
20
______________________________________________________________________________________
C
C
C
COM
IN2
IN2
Crosstalk in dB
R
COM
R
R
IN2
IN2
HP_INL
HP_INR
COM
COM
=
.
20 log
R
R
R
FHP
FHP
FHP
Common-Mode Sense
⎛
⎜
⎝
R
L
R
+
G
R
S
IN2
⎞
⎟
⎠
HPL
HPR
SENSE
and R
COM
IN2
.
The headphone amplifier output impedance, trace
resistance, and contact resistance of the jack are
grouped together to represent the source resistance,
R
the sleeve contact resistance, and the system ground
return resistance are grouped together to represent the
ground resistance, R
Assuming a typical source resistance of 5Ω, the ground
return impedance would need to be limited to 115mΩ
to meet Windows Vista’s crosstalk specification of 50dB
(Figure 5). This is further complicated by the fact that
the impedance of the sleeve connection in the 3.5mm
stereo jack can make up 30mΩ–90mΩ alone.
The MAX9791/MAX9792 COM and SENSE inputs
reduce crosstalk performance by eliminating effects of
28.5mΩ of ground return path resistance. If ground
sensing is not required, connect COM directly to GND
and leave SENSE unconnected (Figure 6).
The MAX9791/MAX9792 beep-detection circuit wakes
up the device (speaker and headphone amplifiers)
once a qualified beep signal is detected at BEEP and
the LDO is enabled. The amplifier wake command from
the beep-detection circuit overrides the logic signal
applied at HP_EN and SPKR_EN.
Figure 5. Crosstalk vs. Ground Resistance
S
. The resistance between the load and the sleeve,
-40
-45
-50
-55
-60
-65
-70
-75
-80
0
R
R
S
L
= 32Ω
vs. GROUND RESISTANCE (RG)
= 5Ω
0.025
G
.
0.050
CROSSTALK
RG (Ω)
0.075 0.100 0.125 0.150
Wake-on-Beep