MITEL MH88612 Datasheet
MH88612
Subscriber Line Interface Circuit (SLIC)
Preliminary Information
Features
• Transformerless 2 -wire to 4- wire con versi on
• Battery and ri ngin g feed to line
• Off-hook and dial pulse detection
• Ring g roun d over-current protec tion
• Adjustable consta nt curre nt f eed
• Relay drive outp ut
•Power V
Denial
Bat
• Wide operating ra nge
Applications
Line interface for:
• PABX
• Intercoms
• Key Telephone Syst ems
• Control S ystem s
ISSUE 4 April 1995
Ordering Information
MH88612 20 Pin SIL Package
0°C to 70°C
Descript io n
The SLIC provides a complete interface between a
switching system and a subscriber loop. Functions
provided include battery feed and ringing feed to the
subscriber line, 2-Wire to 4-Wire hybrid interfacing,
constant current feed and dial pulse detection. The
device is fabricated using thick film hybrid
technology in a 20-pin single in-line package.
TF
TIP
RING
RF
RV
VRLY
RC
RD
CD
Relay
Drive
Circuit
LPGND
Current & Voltage
Sensing
Constant
Current
Feed
FIgure 1 - Functional Block Diagram
VBat
Power
Denial
Over Current
Protection
PD
VDD VEE
2-Wire to
4-Wire
Conversion
Loop
Supervision
SHK
AGND
VR
VX
VRef
CAP
2-105
MH88612 Preliminary Information
TF
LPGND
VR
VRef
VEE
AGND
PD
VX
TIP
RING
RF
VDD
RC
RD
RV
VRLY
CD
VBat
CAP
SHK
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Figure 2 - Pin Connections
Pin Description
Pin # Name Description
1TFTip Feed. Connects to external diode for protection.
2LPGNDLoop Ground. is the system ground reference with respect to VBat. Normally tied with
AGND.
3VRVoice Receive (input) is the 4 wire analog signal to the SLIC.
4V
Ref Voltage Reference (Input) to set the constant current feed to the subscriber line. Constant
current feed can be adjusted by the input voltage on the V
. This pin can also be
Ref
grounded for normal 25 mA loop current.
5 VEE Negative Power Suppl y Voltage (-5V).
6AGNDAnalog Ground (0V).
7PDP ower D en ial (Input) . A logic high will isolate the battery volt age to Tip/Ring.
8VXVoice Transmit (Outpu t) is the 4-wire ana log signal from the SLI C.
9 TIP Connects to the “Tip” lead of the telephone line .
10 RING Connects to the “Ring” lead of the telephone line
11 RF R in g Feed (Inp ut) is normall y connected to Ring relay for ne gati ve battery f eed volt age
and ringing voltage inpu t.
12 VDD Po siti ve Power Supply Voltage (+5V).
13 RC Relay Control (Input). Active high.
14 RD O pen collect or sink s current when RC is at logic high.
15 RV Ring Feed Voltage connects to pin 11 (RF) through a normally closed relay.
16 VRLY Relay voltage Supply (+5V).
17 CD C lamp in g Diode. Connect s to GNDA .
18 VBat Negative Battery Feed Supp ly Voltage (-48V).
19 CAP Connects external ca pacit or to ground for ring tri p.
20 SHK Switch Hook Detect (Outp ut). Active high.
2-106
Preliminary Information MH88612
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.
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 (VRef) that determines the
constant feed current. If VRef (pin 4) is externally
grounded, the constant current is set at 25mA. For a
constant current setting between 18mA and 30mA,
VRef can be set between 8.75 Vdc and
-6.35 Vdc. The relationship is defined by the
equation:
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
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 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 or system drive
output of a codec. Applying AGND to the Ring Trip
Filter Control pin will switch in the filter, whilst
removing AGND, (with the switch in a tri-state
condition), will switch out the filter.
For applications using DTMF signalling, the
capacitor can be permanently connected to ground.
Current Limit
= 25 - (V Re f) mA (+2 mA)
I
LOOP
1 .2 5
Switch Ho ok Det ecti on
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
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.
can no longer supply
BAT
Ringing And Ring Trip Detection
In Figure 3 a ringing signal (e.g.90 Vrms=+/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.
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.
Powe r D e n i a l
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
is negligible and power consumption is
Loop
minimised. The power denial function is useful for
disabling a loop which may have a ground fault.
A 1.0µF capacitor connected from pin 19 (CAP) to
ground will provide adequate attenuation when
2-107
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