MT91600
The value of k can be set by the designer to be any
value between 20 and 250. Three rules to ensure the
correct operation of the circuit:
(A) R18 + R19 > 50kΩ
(B) R1 = R2.
(C) R11 > =50kΩ
It is advisable to place these components as close
as possible to the SLIC.
Preliminary Information
The MT91600’s programmable current range is
between 18mA to 32mA.
Line Drivers & Overcurrent Protection
The Line Drivers control the external Battery Feed
circuit which provide power to the line and allows bi-
directional audio transmission.
The loop supervision circuitry provides bias to the
line drivers to feed a constant current while the over-
current protection circuitry prevents the ring driver
from causing the ring transistor to overload.
The line impedance presented by the Line Driver
circuitry is determined by the external network,
which may be purely resistive or complex, allowing
the circuit to be configured for use in any application.
The impedance can also be fixed to one value and
modified to look like a different value by reflecting an
impedance through the SLIC from an intelligent
CODEC or DSP module.
There is long term protection on the RING output
against accidental short circuits that may be applied
either across TIP/RING to GND or RING to GND.
This high current will be sensed and limited to a
value that will protect the circuit.
In situations where an accidental short circuit occurs
either across TIP/RING to GND or RING to GND, an
excessive amount of current will flow through the ring
drive transistor, Q3. Although the MT91600 will
sense this high current and limit it, if the power rating
of Q3 is not high enough, it may suffer permanent
damage. In this case, a power sharing resistor, R23,
can be inserted (see Figure 5) to dissipate some of
the power. Capacitor C13 is inserted to provide an
a.c. ground point. The criteria for selecting a value
for the power sharing resistor R23 can be found in
the application section of this datasheet.
Network Balance Impedance
The network balance impedance, (Z
NB
), will set the
transhybrid loss performance for the circuit. The
balance of the circuit is independent of the 4 - 2 Wire
gain but is a function of the 2 - 4 Wire gain.
The method of setting the values for R16 and R17 is
given by the formula:
R17 = [1.782 * Z
o
/ ( Z
o
+Z
NB
) * ( R13 / R12 )]
R17 + R16
[1 + R13 / R12]
where Z
NB
is the network balance impedance of the
SLIC and Z
o
is the line impedance.
(R16 + R17) >= 50kΩ
It is advisable to place these components as close
as possible to the SLIC.
Loop Supervision & Dial Pulse
Detection
The Loop Supervision circuit monitors the state of
the phone line and when the phone goes "Off Hook"
the SHK pin goes high to indicate this state. This pin
reverts to a low state when the phone goes back "On
Hook" or if the loop resistance is too high for the
circuit to continue to support a constant current.
The SHK output can also be monitored for dialing
information when used in a dial pulse system.
Ringing and Ring Trip Detection
Ringing is applied to the line by disconnecting pin 8,
RF, from pin 9, RV, and connecting it to a ringing
source which is battery backed. This may be done by
use of an electro-mechanical relay. The SLIC is
capable of detecing an Off Hook condition during
ringing by filtering out the large A.C. component by
use of the external components connected to pin 23.
This filter allows an Off Hook condition to be
monitored at SHK, pin 24.
Constant Current Control
The SLIC employs a feedback circuit to supply a
constant feed current to the line. This is done by
sensing the sum of the voltages across the feed
resistors, R1 and R2, and comparing it to the input
reference voltage, Vref, that determines the constant
current feed current.
4
MITEL [ MITEL NETWORKS CORPORATION ]
