Advance Information
Functional Description
The SLIC performs a transformerless 2-wire to 4-
wire conversion of the analog signal. The 2-wire
circuit is the balanced line going to the subscriber
loop, while the 4-wire circuit is the audio signal going
to and from devices such as the voice codec or
switching circuit. The SLIC also provides a switch
hook (SHK) status output which goes high when the
telephone is set off-hook.
MH88612BV-K
disconnect the ringing voltage from pin 11 and
reconnect to pin 15. At that time the SLIC will revert
to constant current feed operation.
For applications requiring Dial Pulse Detection the
Capacitor connection to ground C1 should be
controlled such that the capacitor is disconnected
during Pulse Dialling.
If the signalling method is unknown, a capacitor of
approximately 56nf should be permanently fitted
between CAP (pin 19) and ground. This should be in
addition to the capacitor mentioned in the above
paragraph.
During off-hook conditions (on-line), the capacitor
(C1) should not have a low impedance path to
ground. This can be achieved using a transistor (Q1)
and resistor (R2) as shown in Figure 3, with R2
providing a high impedance path for C1. When
ringing is applied (RC = 5V) the transistor will short
circuit R2 which will give C1 a low impedance path to
ground and provide the required filtering.
Other methods can be used to switch out C1, such
as a relay or a codec system drive output (with a
similar configuration as above).
For applications using DTMF signalling, C1 can be
permanently connected to ground.
Constant Current Feed
The MH88612BV-K family employs complex
feedback circuits to supply a constant current feed
or, if necessary, constant voltage feed to the line.
With a nominal -48V battery the device will supply a
constant current of typically 25mA to a line up to
1100
Ω.
For longer lines there is insufficient battery
voltage so the device automatically switches to
constant voltage mode where Tip and Ring will stay
at their minimum and maximum voltage levels and
the supplied current will be reduced accordingly. The
maximum line resistance supported is 2000Ω
including the telephone set.
Switch Hook Detection
The SHK pin will go to logic 1 to indicate that a
telephone connected to the line has gone off-hook
(on-line). Off-hook will not be indicated if the loop
resistance (including the telephone set) is too high
for the device to support.
The SHK output will toggle to indicate dial pulses on
the line. If the capacitor on CAP (Pin 19) has not
been switched out of circuit the dial pulses will be
distorted.
Current Limit
The Tip or Ring of the telephone line may
accidentally be shorted to ground. In such a case,
current will only flow through the feed resistor. This
high current will be sensed and reduced by the
current limit circuit to a lower value to protect the
internal circuitry.
Power Denial
Ringing And Ring Trip Detection
In Figure 3 a ringing signal (e.g.90 Vrms and -48Vdc)
is applied to the line by disconnecting pin 15 (RV)
from pin 11(RF), and connecting the ringing voltage
at pin 11 (RF) by use of the relay K1.
The SLIC can detect an off-hook condition during
ringing but there is a large AC component which
must be filtered out to give a true off-hook condition
at SHK.
A 1.0µF capacitor connected from pin 19 (CAP) to
ground will provide adequate attenuation when
ringing is applied. Once an off-hook condition has
been detected a logic low should be applied to pin 13
(RC) which will deactivate the relay (K1) to
3
The power denial function is a feature of the
MH88612 which allows for powering down of the
subscriber loop. A logic high voltage applied to the
power denial input effectively removes the battery
voltage from the loop driver circuitry. The resulting
I
LOOP
is negligible and power consumption is
minimised. The power denial function is useful for
disabling a loop which may have a ground fault.
MITEL [ MITEL NETWORKS CORPORATION ]
