MITEL MH88612BV-4K, MH88612BV-K Datasheet

1

Features

• Transformerless 2-wire to 4-wire conversion

• Battery and ringing feed to line.

• Off-hook and dial pulse detection.

• Ring ground over-current protection.

• Constant current feed with constant voltage
foldover.

• Relay driver.

• Power Denial.

• Wide Operating Range.

• Pin for pin compatible 600R and 900R variants.

Applications

Line interface for:

• PABX

• Intercoms

• Key Telephone Systems

• Control Systems

Description

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 with constant voltage foldback,
and dial pulse detection. The device is fabricated
using thick film hybrid technology in a 20-pin single
in-line package.

Figure 1 - Functional Block Diagram

Current and

Voltage

Sensing

Relay

Drive

Circuit

Constant

Current

Feed

Over Current

Protection

2-Wire to

4-Wire

Conversion

Loop

Supervision

Power

Constant

Management

Voltage

Foldback

TF

TIP

RING

RF

RV

VRLY

RC
RD
CD

LPGND VBAT PD VCC VEE AGND

VR

VX

CAP

SHK

DS5066 ISSUE 2 January 1999

Ordering Information

MH88612BV-K 20 Pin SIL Package (600R)
MH88612BV-4K 20 Pin SIL Package (900R)

0°C to 70°C

.

MH88612BV-K

Subscriber Line Interface Circuit (SLIC)

Advance Information

MH88612BV-K Advance Information

2

Figure 2 - Pin Connections

Note 1: C4 in Figure 3 can be added permanently to this pin for certain dial pulse applications.

Pin Description

Pin # Name Description

1TFTip Feed. Connects to external diode for protection
2 LPGND Loop Ground. Is the system ground reference with respect to VBAT. Normally connected

to AGND.
3VRVoice Receive (Input). The 4 wire analog signal to the device.
4ICInternal Connection. No connection should be made to this pin
5 VEE Negative Power Supply Voltage (-5V).
6 AGND Analog Ground. Connects to System Ground (Earth).
7PDPower Denial (Input). A logic high will isolate the battery voltage to Tip/Ring.
8VXVoice Transmit (Output). The 4-wire analog signal from the device.
9 TIP Tip Lead. Connects to the "Tip" lead of the telephone line.

10 RING Ring Lead. Connects to the "Ring" lead of the telephone line.
11 RF Ring Feed (Input). Normally connected to Ring relay for negativ e battery feed voltage and

ringing voltage input.

12 VCC Positive Power Supply Voltage (+5V).
13 RC Relay Control (Input). Active high.
14 RD Relay Drive. Open collector sinks current when RC is at logic high.
15 RV Ring Feed Voltage. Normally connected to pin 11 (RF) through a normally closed relay

and to external diode protection.

16 VRLY Relay voltage Supply (+5V).
17 CD Clamping Diode. Normally connected to AGND.
18 VBAT Negative Battery Feed Supply Voltage (-48V). Battery supply for the Subscriber Line.

Typically -48VDC is applied to this pin.

19 CAP Ring Trip Filter Capacitor. Normally connected via an external capacitor to ground to

prevent false ring trip. Must be open circuit to allow dial pulse detection.

20 SHK Switch Hook Detect (Output). A logic level 1 will indicate that the Subscriber Line is in

the ’Off-Hook’ state.

1
2
3
4

5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

TF

LPGND

VR

IC

VEE

AGND

PD
VX

TIP

RING

RF

VCC

RC
RD

RV

VRLY

CD

VBAT

CAP
SHK

Advance Information MH88612BV-K

3

Functional Description

The SLIC performs a transformerless 2-wire to 4­wire 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 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
1100Ω. 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.

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

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.

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

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.