Preliminary Information
Pin Description (Continued)
Pin #
23
24
25
26
27
28
29
30
Name
RV
PD
Vx
IC/NC
IC/NC
VRLY
RC1
RD1
Description
MH88612K
Ring Feed Voltage
connects to (RF) through a normally closed ring relay.
Power Denial (Input).
A logic high will isolate the battery voltage to tip/ring.
Voice Transmit (Output).
is the 4-wire analog signal from the SLIC.
No connections to be made to this pin, used for internal connection.
No connections to be made to this pin, used for internal connection.
Relay Voltage Supply (+5V).
This pin is used to power the on board relay.
Relay Control 1 (Input).
Active high. Normally used for ringing.
Open collector sinks current when RC1 is at logic high.
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.
use of the relay K1. An internal clamp diode is
provided between VRLY and RD1.
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 (Cap) to ground
will provide adequate attenuation when ringing is
applied. Once an off-hook condition has been
detected a logic high should be applied to (RC)
which will activate the relay (K1) to disconnect the
ringing voltage from (RF) and reconnect to (RV). At
that time SLIC will revert to constant current feed
operation.
For applications requiring Dial Pulse Detection the
Capacitor connection to ground should be controlled
such 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 the 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.
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
is externally grounded, the
constant current is set at 25mA. 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 defined by the equation:
I
Loop
= 25 - V
Ref
mA (±2 mA)
1.25
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.
Tip/Ring Reverse
The MH88612K provides a Tip/Ring reversal relay
driver. To activate the internal relay to reversal Tip
and Ring apply a logic input to RC2.
Current Limit
Ringing And Ring Trip Detection
In Figure 3 a ringing signal (e.g., 90 Vrms +-48 Vdc
is applied to the line by disconnecting (RV) from
(RF), and connecting the ringing voltage at (RF) by
The Tip or Ring may be accidentally shorted to
ground. In such a case, current will only flow through
the feed resistor. This high current will be sensed
2-123
MITEL [ MITEL NETWORKS CORPORATION ]
