MC33215
Figure 6. Available Current at VCC
100
90
80
70
60
50
40
30
20
10
0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
I
I
at 98% of
VCC
VCC(max)
I
I
at 50% of
VCC
VCC(max)
I
/l
(%)
VCC line
V
to VLS
Open
CC
V
CC
I
VCC(max) (mA)
0
20
40
60
80
100
0
20
40
60
80
100
I
(mA)
I
(mA)
line
line
A. Maximum Available Current at V
B. Voltage Drop to V
CC
CC
For instance, at a line current of 20 mA a maximum of
15 mA of current is available at VCC. If all this current is
taken, VCC will be 1.7 V below VLN. When not all this current
is drawn from VCC, but for instance only 1.0 mA for biasing of
the loudspeaker amplifier, the voltage at VCC will be 1.2 V
below VLN. Although the measurements for Figure 6 are
done with RREG1 = 365 k, the results are also globally valid
for other dc settings.
by adjusting the sensitivity of the handset microphone by
adjusting the resistors RHM1 and RHM2. It is not advised to
adjust the gain by including series resistors towards the Pins
HM1 and HM2.
A high pass filter is introduced by the coupling capacitors
C
HM1 and CHM2 in combination with the input impedance. A
low pass filter can be created by putting capacitors in parallel
with the resistors RHM1 and RHM2
.
As can be seen from Figure 6, the voltage at VCC is limited
by the voltage at VLN minus 1.0 V. This means that the
voltage at VCC is limited by the external zener at VLN. If it is
necessary to limit the voltage at VCC in order to protect
peripheral circuits, a zener from VCC to Gnd can be added. If
The transmit noise is measured as –72 dBmp with the
handset microphone inputs loaded with a capacitively
coupled 200 Ω. In a real life application, the inputs will be
loaded with a microphone powered by VMC. Although VMC
is a stablized supply voltage, it will contain some noise which
can be coupled to the handset microphone inputs, especially
when a microphone with a poor supply rejection is used. An
additional RC filter on VMC can improve the noise figure, see
also the base microphone section.
the supply of the loudspeaker VLS is also connected to VCC
it is advisable that VCC does not exceed 8.0 V.
,
The high efficiency of the VCC power supply contributes
to a high loudspeaker output power at moderate line
currents. More details on this can be found in the handsfree
operation paragraph.
Handset Earpiece Amplifier
The handset earpiece is to be capacitively connected to
the RXO output. Here, the receive signal is available which is
amplified from the line via the sidetone network and the Rx
and EAR amplifiers. The sidetone network attenuates the
receive signal from the line via the resistor divider composed
of RSLB and Zbal, see also the sidetone section. The
attenuation in the typical application by this network equals
24.6 dB. Then the signal from the sidetone network is
pre–amplified by the amplifier Rx with a typical gain of 6.0 dB.
This amplifier also performs the AGC and MUTE functions,
see the related paragraphs. Finally, the signal is amplified by
the noninverting voltage amplifier EAR. The overall receive
gain ARX from the line to the earpiece output then follows as:
HANDSET OPERATION
During handset operation, the MC33215 performs the
basic telephone functions for the handset microphone and
earpiece. It also enables DTMF transmission.
Handset Microphone Amplifier
The handset microphone is to be capacitively connected
to the circuit via the differential input HM1 and HM2. The
microphone signal is amplified by the HMIC amplifier and
modulates the line current by the injection of the signal into
the line driver. This transfer from the microphone inputs to the
line current is given as 15/(RSLP/11), which makes a total
transmit voltage gain AHM from the handset microphone
inputs to the line of:
V
R
R
RXO
RXO
GRX
A
A
x A
x
1
RX
ST
RXI
V
V
Z
x Z
set
line
line
HM
15
line
A
x
HM
V
Z
Z
R
11
set
line
With: AST = Attenuation of the Sidetone Network
RXI = Gain of the Pre–Amplifier Rx
SLP
With the typical application and Zline = 600 Ω the transmit
gain calculates as 47 dB.
A
For the typical application an overall gain from the line to
the earpiece is close to 0 dB.
In case an electret microphone is used, it can be supplied
from the stabilized microphone supply point VMC of 1.75 V
properly biased with resistors RHM1 and RHM2. This allows
the setmaker to use an electret microphone with poor supply
rejection to reduce total system costs. Since the transmit gain
The receive gain can be adjusted by adjusting the resistor
ratio RRXO over RGRX. However, RRXO also sets the
confidence tone level during dialing which leaves RGRX to be
chosen freely. A high pass filter is introduced by the coupling
capacitor CRXI together with the input impedance of the input
A
HM is fixed by the advised RSLP = 220 Ω and the constraints
of set impedance and line impedance, the transmit gain is set
10
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
