MC33219A Datasheet, PDF (18/28 Pages) Motorola, Inc – Voice Switched Speakerphone

Freescale SMeCm33i2c1o9Anductor, Inc.
Signal ^{C R}
Input
XDI
(RLI)
VB
External Component Values are
Application Dependent.
Figure 27. Background Noise Monitor
500 Ω
2.0 µA
CP2
(RLO)
Background
Noise Monitor
1.0 µF
31.7 k
18.6 k
VB
36 mV
CPT
(CPR) 100 k
VCC
47 µF
To Attenuator
Control Circuit
Volume Control
Dial Tone Detector
The volume control input at VLC (Pin 19) is sensed as a
voltage with respect to VB. The volume control affects the
attenuators in the receive mode only. It has no effect in the
idle or transmit modes.
By varying the voltage at the VLC pin (Pin 19), the volume
control varies the gain of the attenuators. Maximum receive
attenuator gain (6.7 dB) occurs when VLC = VB. As VLC is
reduced below VB, the gain of the receive attenuator is
reduced, and the transmit attenuator gain increases in a
complementary manner. The usable range of the VLC pin is
≈ 1.1 V for VCC ≥ 3.5 V, providing a range of ≈ 40 dB (see
Figure 4). At VCC < 3.5 V, the range is reduced due to the
lower VB voltage, and the AGC function.
The configuration of the external volume control
potentiometer circuit depends on whether the VCC supply
voltage is regulated or if it varies, such as in a phone line
powered circuit (see Figure 28). If the supply voltage is
regulated, the circuit on the left can be used. The value of the
lower resistor (R1) depends on the value of VCC, so that
Pin 19 can be varied from VB to ≈ 1.1 V below VB.
In a phone line powered circuit, the value of VCC, and
consequently VB, will vary with line length and with the
amount of sound at the speaker. In this case, the circuit on
the right side of Figure 28 must be used to provide a fixed
reference voltage for the potentiometer. With this circuit, the
volume setting will not vary when VCC is ≥ 3.5 V. As VCC falls
below 3.5 V, the zener diode will drop out of regulation, but
the AGC circuit will ensure that instabilities do not occur.
The bias current at VLC flows out of the pin and depends
on the voltage at the pin (see Figure 16). The capacitor from
VLC to VB helps reduce any effects of ripple or noise on VB.
Figure 28. Volume Control
Regulated Supply
Unregulated Supply
VB
VB
0.1
Volume
Control _{50 k}
VCC R1
6.5 V 86 k
R1
To VLC
(Pin 19)
0.1
50 k
To VLC
Volume ^{(Pin 19)}
Control
6.0 V 72 k
3160
5.0 V 50 k
4.0 V 25 k
When the speakerphone is initially taken off–hook, the dial
tone signal will switch the circuit to the receive mode.
However, since the dial tone is a continuous signal, the
MC33219A would consider it as background noise rather
than speech, and would therefore switch from receive to idle,
causing the dial tone sound level to fade. The dial tone
detector prevents the fading by disabling the background
noise monitor.
The dial tone detector is a comparator with one side
connected to the receive attenuator input (RAI), and the other
input connected to VB with a – 20 mV offset (see Figure 29).
If the circuit is in the receive mode and the incoming signal
has peaks greater than 20 mV (14 mV rms), the comparator’s
output will change, disabling the receive idle mode. The
receive attenuator will then be at a setting determined solely
by the volume control. NOTE: The dial tone detector is not a
frequency discriminating circuit.
Figure 29. Dial Tone Detector
To Rx
Attenuator
RAI
To Attenuator
Control Circuit
20 mV
VB
Microphone Amplifier, Mute
The microphone amplifier (Pins 20, 21) has the
non–inverting input internally connected to VB, while the
inverting input and the output are pinned out. Unlike most op
amps, the amplifier has an all NPN output stage, which
maximizes phase margin and gain–bandwidth. This feature
ensures stability at gains less than unity, as well as with a
wide range of reactive loads. The open loop gain is typically
70 dB (f < 100 Hz), and the gain–bandwidth is typically
1.5 MHz. The maximum p–p output swing, for 1.0% or less
distortion, is shown in Figure 14. The output impedance is
<10 Ω until current limiting is reached (typically 2.0 mA peak).
The input bias current at MCI is typically 30 nA out of the pin.
The mute function (Pin 18), when activated, will reduce the
gain of the amplifier by shorting the external feedback
resistor (RMF in Figure 30). The amplifier is not disabled in
this mode; MCO remains a low impedance output, and MCI
remains a virtual ground at VB. The amount of muting (the
18
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