Preliminary 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.
MH88612C
For applications requiring dial pulse detection the
capacitor connection to ground should be controlled
so that the capacitor is only connected during
ringing.
During off-hook conditions (closed loop), the
capacitor should be switched out. This can be
performed using a transistor, relay to system drive
output of a codec. Applying GNDA to Ring Trip Filter
Control pin will switch in the filter, whilst removing
GNDA, (with the switch in a tristate condition), will
switch out the filter.
For applications using DTMF signalling, the
capacitor can be permanently connected to ground.
Once an off-hook condition has been detected a
logic low on the Relay Control (RC) input deactivates
the relay (K1) to disconnect the ringing voltage from
pin 11 (RF)and reconnect to pin 15 (RV). At that time
the SLIC will revert to constant current feed
operation.
Constant Current Feed
The SLIC employs a complex feedback circuit to
supply a constant feed current to the line. This is
done by sensing the sum of the voltages across the
internal feed resistors and comparing it to an input
reference voltage (V
Ref
) that determines the constant
feed current. If V
Ref
(pin 4) is externally grounded,
the constant current is set at 21mA. For a constant
current setting between 18mA and 30mA, V
Ref
can
be set between -8.75 Vdc and -6.35 Vdc. The
relationship is detailed in the formula below:
I
Loop
= [-2.6(V
Ref
) + 21.5] mA +2mA
Current Limit
The Tip or Ring may accidentally short 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.
Switch Hook Detection
When the DC current exceeds an internal threshold
level, the switch hook (SHK) will go high. If the loop
resistance is so high that V
Bat
can no longer supply
the required amount of loop current as determined
by constant current supply circuit, the output of the
switch hook (SHK) will go low to indicate that the
loop resistance is too high and the line is on hook.
Receive Gain
The MH88612C Receive Gain (VR to 2-wire) can be
adjustable by -0.5dB if GS is taken logic low or by
0dB if GS is taken logic high. When the MH88612C
issued with an off hybrid MT896x codec the Receive
Gain can be set to either -3.5dB or -7dB by writing to
control Register A of Codec and programming GS
appropriately.
Ringing And Ring Trip Detection
In Figure 3 a ringing signal (e.g. 90 Vrms
superimposed on -48 VDC) is applied to the line by
disconnecting pin 15 (RV) from pin 11 (FR), 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 1µF capacitor connected from pin 19 (CAP) to
ground will provide adequate attenuation when
ringing is applied.
2-115
MITEL [ MITEL NETWORKS CORPORATION ]
