Table 5. beep output gain – Rainbow Electronics MAX9758 User Manual
Page 20
MAX9756/MAX9757/MAX9758
BIAS
The MAX9756/MAX9757/MAX9758 feature an internally
generated, power-supply independent, common-mode
bias voltage of 2.5V referenced to GND. BIAS provides
both click-and-pop suppression and sets the DC bias
level for the amplifiers. Choose the value of the bypass
capacitor as described in the BIAS Capacitor section.
No external load should be applied to BIAS. Any load
lowers the BIAS voltage, affecting the overall perfor-
mance of the device.
BEEP Input
The MAX9756/MAX9757/MAX9758 feature an audible
alert beep input (BEEP) that accepts a mono system
alert signal and mixes it into the stereo audio path.
When the amplitude of V
BEEP
exceeds 300mV
P-P
and
the frequency of the beep signal is greater than 300Hz,
the beep signal is mixed into the active audio path
(speaker or headphone). If the signal at V
BEEP
is either
< 300mV
P-P
or < 300Hz, the BEEP signal is not mixed
into the audio path. The amplitude of the BEEP signal at
the device output is roughly the amplitude V
BEEP
times
the gain of the selected signal path.
The input resistor (R
B
) sets the gain of the BEEP input
amplifier, and thus the amplitude of V
BEEP
. Choose R
B
based on:
The total BEEP gain is given by:
where 47k
Ω is the value of the BEEP amplifier feedback
resistor, V
BEEP
is the BEEP amplifier output, V
IN(BEEP)
is the BEEP input amplitude, and V
OUT(BEEP)
is the total
BEEP output signal. A
V(BEEPOUT
) is given by the values
listed in Table 5. Note that V
BEEP
must be higher than
300mV
P-P
. The BEEP amplifier can be set up as either
an attenuator, if the original alert signal amplitude is too
large, or to gain up the alert signal if it is below
300mV
P-P
. AC-couple the alert signal to BEEP. Choose
the value of the coupling capacitor as described in the
Input Filtering section. Multiple beep inputs can be
summed (Figure 12).
V
V
k
R
A
OUT BEEP
IN BEEP
B
V BEEPOUT
(
)
(
)
(
)
=
+
⎛
⎝
⎜
⎞
⎠
⎟
47
Ω
R
V
k
V
B
IN BEEP
BEEP
(
)
≤
× 47 Ω
2.3W Stereo Speaker Amplifiers and DirectDrive
Headphone Amplifiers with Automatic Level Control
20
______________________________________________________________________________________
MAX9756/
MAX9757/
MAX9758
R
B
47k
Ω
BEEP
0.47
μF
SOURCE 3
R
B
47k
Ω
0.47
μF
SOURCE 2
R
B
47k
Ω
0.47
μF
SOURCE 1
47k
Ω
BIAS
WINDOW
DETECTOR
(0.3V
P-P
THRESHOLD)
FREQUENCY
DETECTOR
(300Hz THRESHOLD)
SPEAKER/HEADPHONE
AMPLIFIER INPUTS
V
BEEP
Figure 12. Beep Input
Table 5. BEEP Output Gain
A
V(BEEPOUT)
HEADPHONE *
(V/V)
SPEAKER*
(V/V)
GAIN3
GAIN2
GAIN1
1.5
8.4
0
0
0
1.5
9.4
0
0
1
1.78
10
0
1
0
1.78
10
0
1
1
1.5
15.8
1
0
0
1.5
18.8
1
0
1
1.78
20
1
1
0
1.78
20
1
1
1
*All output gains are for V
VOL
= GND.