TCA3388 Datasheet, PDF (11/16 Pages) Motorola, Inc – TELEPHONE LINE INTERFACE

TCA3388
factor m. The relation between the line current and the factor
m is depicted in Figure 11.
Figure 11. Modulation Factor m as a Function
of Line Current
m
1.0
0.5
ILine
0
ILstart
ILm
ILstop
For low line currents below ILstart, thus long lines, the
factor m equals 1. This means the hybrid network ZHL is fully
used. For high line currents above ILstop, thus short lines, the
factor m equals 0. This means the hybrid network ZHS is fully
used. Both networks are used 50% for the intermediate line
current Ilm.
The switch over between the 2 networks takes place in
region 3 for the French and U.K. mask and in region 2 for the
Low Voltage mask.
LINE LENGTH AGC
The TCA3388 offers the possibility to vary the transmit and
receive gain over line length in order to compensate for the
loss in gain at longer line lengths. In the block diagrams of the
transmit and receive channels (Figures 9, 10) the line AGC is
drawn. The line AGC can be switched off by connecting a
150 kΩ resistor between HSO and Gnd. In this case, the
transmit and receive gain are lowered by 2.0 dB with respect
to the value calculated in the formulas above.
The line AGC characteristics for both transmit and receive
channel have the general shape depicted in Figure 12.
Figure 12. General Line AGC Characteristics
Gain
Nominal
Gain
+ Gain ^{Nominal Gain}
) 1
IL – ILstart
ILrange
Reduced
Gain
ILine
ILstart ILrange
For low line currents, and thus long lines, the gains are
nominal. When the line current has increased above ILstart
with a current ILrange, the gain is reduced by 6.0 dB. Due to
the general characteristics of the line AGC curve, the gain will
be decreased further for higher currents.
For France and U.K., the line AGC will be active in region
3 of the dc characteristics. The ILstart is approximately equal
to the ILK. The range is calculated from:
+ ILrange
Z1 x (I2R – I2CD)
RE3
For Low Voltage mask, the line AGC is active in region 2.
DIALING
Pulse–dialing is performed by making pin PI high. As a
result the output LAO goes low and the loop will be
disconnected. Internally the current consumption of the
circuit is reduced and the current through the microphone is
switched off.
DTMF–dialing is performed by supplying a DTMF signal
current to Pin LAI. This is the same node where the
microphone signal currents are internally applied. Therefore,
for the DTMF gain the same formulas apply. Because the
microphone preamplifier is bypassed, there is no influence
on DTMF signals by the line length AGC.
A DTMF confidence tone can be generated on the
earpiece by injecting a signal current at the RXI pin. Because
only the earpiece amplifier itself is used, there are no effects
from AGC or hybrid switchover.
For correct DTMF–dialing the pin MUT has to be made
high. This mutes both the microphone and earphone
preamplifier. In this way signals from the microphone will not
be amplified to the line and signals from the line are not
amplified to the earpiece.
The complete interfacing of the DTMF generator with the
TCA3388 is shown in the typical application.
SUPPORT MATERIAL
Device Specification: Brief description of the TCA3388,
block diagram, device data, test
diagram, typical application
User manual TCA3388: Extended description of the circuit
and its concept, adjustment
procedure, application hints and
proposals
Demonstration board: Shows performance of the TCA3388
in its basic application
TYPICAL APPLICATION
The typical application below is based on the demoboard
of the TCA3388. It contains the speech transmission part,
diode bridge, hook switch and microcontroller interfacing.
The dc mask setting on the bottom left is given for France,
U.K. and Low Voltage applications. The component values
are given in the table of Figure 14. The line driver is extended
with T1, D5 and R3 which increases the signal swing under
low line voltage conditions.
MOTOROLA ANALOG IC DEVICE DATA
11