MITEL MH88612C Datasheet

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MH88612C

Subscriber Line Interface Circuit

Preliminary Information

Features

• Input impe danc e 200Ω + 560Ω // 0.1µF

• Transformerless 2 -wire to 4- wire con versi on

• Battery and ringing feed to line

• Off-hook and dial pulse detection

• Ring gr oun d over-curren t protection

• Adjustable consta nt curre nt fee d

• Relay drive outp ut

• Power VBat De nial

• Wide Operating Range

• Programmable Receive Gain

Applications

Line interface for:

• PABX

• Intercom s

• Key Telephone Sys tems

• Control Syst ems

ISSUE 4 April 1995

Ordering Information

MH88612C 20 Pin SIL Package

0°C to 70°C

Descript io n

The SLIC provides a complete interface between a
switching system and 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

Relay

Drive

Circuit

VBat

Current & Voltage

Sensing

Over Current

Protection

Constant

Current

Feed

FIgure 1 - Functional Block Diagram

VDD VEE GNDA

GS

2-Wire to

4-Wire

Conversion

Loop

Supervision

SHK

VR

VX

VRef

CAP

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MH88612C Preliminary Information

IC

TF

IC

VR

VRef

VEE

GNDA

GS
VX

TIP

RING

RF

VDD

RC
RD
RV

VRLY

IC

VBat

CAP

SH K

0
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

0ICInternal Connection. No pin fitted in this position.
1TFTip Feed. Connects to external diode to LPGND for protection.
2ICInternal Connection. No pin fitted in this position.
3VRVoice Receive (input) is the 4 wire analog sign al to the SLIC.
4VRefVoltage Reference to set the constant current feed to the subscriber line. Constant current

feed can be adjusted by the input voltage on the VRef. This pin can also be grounded for

normal 21mA loop current.
5VEE Negative Power Supply voltage. (-5V).
6GNDAAnalo g Ground. (0V)
7GS

Gain setting (Input). A logic low at this input adds an additional -0.5dB gain in the receive

direction. This gain is in additio n to the gai n set by the Codec. A high adds 0dB gain.
8VXVoice Transmit (Output) is the 4-wire analog signal from the SLIC.
9 TIP Connects to the “Tip” lead of the telephone line.

10 RING Connects to the “Ring” lead of the telephone line.
11 RF Ring Feed (Input). is connected via the ring relay to either negative battery feed voltage,

(RV), or ringing voltage input.

12 VDD Positive Power Supply Voltage. (+5V)
13 RC Relay Con trol (Input). Active high will appl y ringing to th e line.
14 RD Outpu t Drive Sources current when RC is at a logic high.
15 RV Ring Feed Voltage connects to pin 11 (RF) through a normally closed relay contact.
16 VRLY Relay voltage Supply. (+5V)
17 IC Internal Connection. No pin fitted in this position.
18 VBat Negative Battery Feed Supply Voltage. (-48V)
19 CAP Co n nects externa l capacitor to ground for ring trip.
20 SHK Switch Hoo k Detect (Outp ut). Active high.

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Preliminary Information MH88612C

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 v o lt ag e ( V
feed current. If V
the constant current is set at 21mA. For a constant
current setting between 18mA and 30mA, V
be set between -8.75 Vdc and -6.35 Vdc. The
relationship is detailed in the formula below:

) that determines the constant

Ref

(pin 4) is externally grounded,

Ref

Ref

can

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.

Current Limit

I

Loop

= [-2.6(V

) + 21.5] mA +2mA

Ref

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

can no longer supply

Bat

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.

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.

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.

Receive G ain

The MH88612C Receive Gain (VR to 2-wire) can be
adjustable by -0.5dB if GS
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.

is taken logic low or by

A 1µF capacitor connected from pin 19 (CAP) to
ground will provide adequate attenuation when
ringing is applied.

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