MC33215
Figure 11. Loudspeaker Amplifier Output
Power when Line Powered
loudspeaker amplifier is muted which is needed for correct
handset operation.
100
90
The volume of the loudspeaker signal can be varied via a
potentiometer at VOL. A fixed current of 10 µA is running
through the potentiometer and the resulting voltage at VOL
is a measure for the gain reduction. The relation between
the voltage at VOL and the obtained gain reduction is given
in Figure 13.
R
R
R
= 365 k
= 220 k
REG1
REG2
LSP
= 25
Ω
80
R
R
R
= 365 k
= 220 k
REG1
REG2
LSP
70
R
R
R
= 365 k
= Infinite
= 50
Ω
REG1
REG2
LSP
60
50
40
30
20
10
0
= 25
Ω
R
R
R
= 365 k
= Infinite
REG1
REG2
LSP
Figure 13. Volume Reduction
0
= 50
Ω
–5.0
–10
0
20
40
60
80
100
–15
–20
I
(mA)
line
The quality of the audio output of the loudspeaker amplifier
is mainly determined by the distortion level. To keep high
quality under difficult supply conditions, the MC33215
incorporates a peak–to–peak limiter. The peak–to–peak
limiter will detect when the output stage gets close to its
maximum output swing and will then reduce the gain from LSI
to LSF. The attack and release of the limiter is regulated by
the CPPL capacitor. Figure 12 depicts the limiter’s attack
behavior with CPPL = 100 nF. The release time is given as
3 x CPPL x RPPL. In the typical application this leads to a
release time of 300 ms.
–25
–30
–35
–40
0
100
200
(mV), dA
300
(dB)
400
500
V
VOL
LSP
It can be seen from Figure 13 that a linear variation of
RVOL will give a logarithmic gain reduction which adapts
better to the human ear than a linear gain reduction.
During DTMF dialing, see Table 2, a confidence tone is
audible at the loudspeaker of which the level is proportional
to the feedback resistor RLSF only. At RLSF = 180 kΩ the gain
from MFI to LSO equals 28.5 dB.
Figure 12. Peak–to–Peak Limiter Dynamic Behavior
Half Duplex Controller
V
LSO
To avoid howling during speakerphone operation, a half
duplex controller is incorporated. By monitoring the signals in
both the transmit and receive channel the duplex controller
will reduce the gain in the channel containing the smallest
signal. A typical gain reduction will be between 40 dB and
52 dB depending on the setting, see below. In case of equal
signal levels or by detection of noise only, the circuit goes into
idle mode. In this mode the gain reduction in both channels is
halfway, leading to 20 dB to 26 dB of reduction.
V
PPL
V
in
0
2.0
5.0
In a speakerphone built around the MC33215, following
the signal path from base microphone to the line and via
sidetone, loudspeaker and acoustic coupling back to the
microphone, the loop gain can be expressed as a sum of the
gains of the different stages. However, since the transmit and
receive gains are dependent on AGC and the sidetone
suppression is dependent on matching with the different lines
we are mostly interested by the maximum possible loop gain
1.0
3.0
4.0
6.0
t, TIME (ms)
Figure 12 clearly shows that due to the action of the
peak–to–peak limiter, the output swing and thus the output
power is reduced with respect to the maximum possible as
already indicated in Figure 10. The peak–to–peak limiter can
be disabled by connecting the PPL pin to ground.
On top of the peak–to–peak limiter, the MC33215
incorporates a supply limiter, which reduces the gain rapidly
when the supply voltage VLS drops too much. This will
avoid malfunctioning of the amplifier and unwanted
oscillations. The voltage drop is detected via the BVO input
and for that reason the CBVO has to be connected to VLS
and not to Gnd.
The amplifier can be activated by making Pin LSM high. In
the typical application this pin is connected to SPS, which
activates the loudspeaker amplifier automatically when the
speakerphone mode is entered. When LSM is made low, the
A
LOOP(max). It follows:
A
LOOP(max) = ABMRX(max) + ARXBM(max) – ASWD (dB)
With: ABMRX(max) = Maximum gain from BM1 and BM2 to
RXI as a function of line length AGC and line
impedance matching
ARXBM(max) = Maximum gain from RXI to BM1 and
BM2 as a function of line length AGC and acoustic
coupling
14
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
