Rainbow Electronics MAX9792 - Manual (Page 20)

MAX9791/MAX9792
Windows Vista-Compliant Class D Speaker
Amplifiers with DirectDrive Headphone Amplifiers
20 ______________________________________________________________________________________
Common-Mode Sense
Windows Vista-compliant platforms are restricted to only
115m
Ω 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
IN2 and R COM .
To improve AC CMRR, add a capacitor equal to C
IN2
between GND and R COM .
Configuring SENSE and COM in this way improves sys-
tem crosstalk performance by reducing the negative
effects of the headphone jack ground return resistance.
The headphone amplifier output impedance, trace
resistance, and contact resistance of the jack are
grouped together to represent the source resistance,
R
S . The resistance between the load and the sleeve,
the sleeve contact resistance, and the system ground
return resistance are grouped together to represent the
ground resistance, R
G
.
Assuming a typical source resistance of 5
Ω, the ground
return impedance would need to be limited to 115m
Ω
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 30m
Ω–90mΩ alone.
The MAX9791/MAX9792 COM and SENSE inputs
reduce crosstalk performance by eliminating effects of
28.5m
Ω of ground return path resistance. If ground
sensing is not required, connect COM directly to GND
and leave SENSE unconnected (Figure 6).
Wake-on-Beep
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.
Crosstalk in dB
R
R R
G
L S
=
+
⎛
⎝⎜ ⎞
⎠⎟
20 log
R IN2
R FHP
R FHP
R FHP
COM
HP_INL
HPL
R COM
SENSE
R IN2
HP_INR
HPR
C IN2
C COM
C IN2
Figure 4. Connecting COM for Ground Sense
-80
-75
-70
-65
-60
-55
-50
-45
-40
0 0.050 0.025 0.075 0.100 0.125 0.150
CROSSTALK
vs. GROUND RESISTANCE (RG)
RG ( Ω)
CROSSTALK (dB)
R S = 5 Ω
R
L
= 32 Ω
Figure 5. Crosstalk vs. Ground Resistance