2-253

Features

Loop Start Trunk Interface

600 ohm input impedance, -2 variant

Line state detection outputs: forward loop,
reverse loop, ringing voltage, switch hook
On-hook reception

Transformerless 2W to 4W conversion

One loop start relay driver

+/- 5V operation

Small footprint area (<4.75cm

)

Applications

Interface to Central Office for:

PABX

Key Telephone Systems

Channel Bank

Voice Mail

Terminal Equipment

Digital Loop Carrier

Optical Multiplexer

Description

The Mitel MH88634 Central Office Trunk Interface
circuit provides a complete audio and signalling link
between audio switching equipment and a central
office. The functions provided by the MH88634
include 2-4 Wire conversion, 600 ohm input
impedance and network balance. The device is
fabricated as a thick film hybrid incorporating various
technologies for optimum circuit design and very
high reliability.

Figure 1 - Functional Block Diagram

Status

Detection

Transmit

Gain

2-4 Wire Hybrid

Dummy

Ringer

Loop

Termination

Loop Relay

Driver

Impedance

Matching

Network
Balance

XLA
XLB

XLC
XLD

LRC

LRD

VRLY RGND

RING

TIP

RV FL RL SHK

VDD VEE AGND

Receive

Gain

ISSUE 2

April 1995

Ordering Information

MH88634-2 21 Pin SIL Package

C to 70

MH88634

Central Office Interface Circuit

Preliminary Information

2-254

MH88634

Preliminary Information

Figure 2 - Pin Connections

Pin Description

Pin #

Name

Description

TIP

Tip Lead: Connects to the "Tip" lead of the central office

RING

Ring Lead: Connects to the "Ring" lead of the central office

XLA

Loop Relay Contact A:. Connects to XLB through the Loop relay (K1) contacts when the
relay is activated. This operates the internal active termination circuitry.

XLD

Loop Relay Contact D: See XLC for description

XLB

Loop Relay Contact B: See XLA for description

XLC

Loop Relay Contact C: Connects to XLD through the loop relay (K1) contacts when the
relay is activated. This operates the internal active termination circuitry.

Internal Connection: This pin is internally connected and must be left open.

Internal Connection.: This pin is internally connected and must be left open.

Internal Connection: This pin is internally connected and must be left open.

SHK

Switch Hook (Output). A logic low indicates either forward or reverse loop current.

Receive (Input): 4-Wire ground (AGND) referenced audio input.

VEE

Negative Supply Voltage. -5V

Transmit (Output). 4-Wire ground (AGND) referenced audio output.

Ring Voltage Detect (Output). A logic low indicates that ringing voltage is across the Tip
and Ring leads. Note that this output toggles at the ringing cadence and not at the ringing
frequency.

Forward Loop Detect (Output): In the on-hook state, a logic low output indicates that
forward loop battery is present. In the off-hook state, a logic low indicates that forward loop
current is present.

Reverse Loop Detect (Output). In the on-hook state, a logic low output indicates that
reverse loop battery is present. In the off-hook state, a logic low output indicates that
reverse loop current is present. Reverse loop refers to the Tip lead negative respect to the
Ring Lead.

VDD

Positive Supply Voltage: +5V

TIP

RING

XLA

XLB

XLC

IC
IC
IC

SHK

VEE

VDD

AGND

LRC

VRLY

LRD

XLD

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21

2-255

Preliminary Information

MH88634

AGND

Analog Ground. 4-wire ground. Connected to system ground

LRC

Loop Relay Control (Input): A logic high activates the Loop Relay Driver output (LRD).
The Loop Relay activates internal circuitry which provides a DC termination across Tip and
Ring. Used for line seizure and dial pulsing.

VRLY

Relay Positive Supply Voltage. Typically +5V. Connects to the relay coil and the relay
supply voltage

LRD

Loop Relay Drive (Output). Connects to the Loop Relay coil. A logic low activates the
relay. An internal clamp diode from VRLY to LRD is provided.

Pin Description (Continued)

Pin #

Name

Description

Functional Description

The MH88634 is a COIC (Central Office Interface
Circuit) used to interface the Central Office 2-Wire
analog trunks. The COIC provides a Loop Start
interface function.

Incoming analog (voiceband) signals from the 2-Wire
circuit are applied differentially across Tip and Ring
the output of which is applied to the 2 to 4 wire
converter with a fixed gain to the Tx output.

The outgoing analog signals are applied to Rx. The
audio signals undergo 4 to 2 wire conversion with a
fixed gain, and are differentially applied to Tip and
Ring.

DC Loop Termination

The line is provided with an active DC load
termination when a logic high is applied to the LRC
(Loop Start Relay) input. The termination is similar to
a DC resistance of less than 275 ohms. An external
relay is used to activate internal circuitry which
switches the termination in and out of the loop. This
is used for both seizing the line as well as generating
dial pulses.

Supervision Features

The supervision circuitry provides, the signally status
outputs. The ringing voltage, forward current,
reverse current and switch hook. Ringing Voltage
detect - the RV output provides a logic low when
ringing voltage is detected. This sector includes a
ringing filter which ensures that the output toggles at
the ringing cadence and not at the ringing frequency.
Typically, this output goes low 50ms after the ringing

voltage is applied and remains low for 50ms after
ringing voltage is removed.

Forward loop and reverse loop detect - the FL output
provides a logic low when either forward loop battery
or forward loop current is detected (ring lead voltage
negative with respect to tip lead). The RL output
provides a logic low when either reverse loop battery
or reverse loop current is detected (tip lead voltage
negative with respect to ring lead)

SHK output is active if either forward loop or reverse
loop current is detected.

Line Impedance

The input impedance of the MH88634-2 is 600 ohms.
The network balance is also set at 600 ohms to
maximize the Transhybrid Balance.

Transmit and Receive Gain

Transmit Gain (Tip-Ring) is set at 0dB and Receive
Gain (Rx to Tip-Ring) is set at -2dB.

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MH88634

Preliminary Information

Absolute Maximum Ratings*

Voltages are with respect to AGND

* Exceeding these values may cause permanent damage. Functional operation under these conditions is not implied.

Recommended Operating Conditions

* Exceeding these values may cause permanent damage. Functional operation under these conditions is not implied.

DC Electrical Characteristics

DC Electrical Characteristics are over recommended operating conditions unless otherwise stated.
* Typical figures are at 25°C with nominally +5V supplies and are for design use only.

Parameters

Sym

Min

Max

Units

DC Supply Voltages

BAT

-0.3

0.3

-60

-7

V
V
V

DC Ring Relay Voltage

RLY

-0.3

Storage Temperature

-55

+125

°C

Parameters

Symbol

Min

Typ*

Max

Units

Comments

DC Supply Voltages

4.75

-4.75

5.0

-5.0

5.25

-5.25

V
V

DC Ring Relay Voltage

RLY

5.0

Operating Temperature

°C

Characteristics

Sym

Min

Typ*

Max

Units

Test Conditions

Supply Current

13
13

mA
mA

Power Consumption

137

BAT

not connected

SHK

Low Level Output Voltage

High Level Output Voltage

-.3

3.7

0.5

5.25

= 4mA

= 100

LRD

Sink Current, Relay to V

Clamp Diode Current

100
150

mA
mA

= 0.35V not

continuous

LRC

Low Level Input Voltage
High Level Input Voltage

2.0

0.8

V
V

High Level Input Current
Low Level Input Current

40
40

= 5.0V

2-257

Preliminary Information

MH88634

Loop Electrical Characteristics*

* DC Electrical Characteristics are over recommended operating conditions unless otherwise stated.
Typical figures are at 25°C and are for design aid only.

AC Electrical Characteristics *

* AC Electrical Characteristics are over recommended operating conditions unless otherwise stated.
Typical figures are at 25°C and are for design aid only.

Characteristics

Symbol

Min

Typ

Max

Units

Test Conditions

Ringing Voltage

110

rms

Ringing Frequency

Ringer Equivalence Number

REN

0.5

Dummy ringer is 17K in
series with 330nF.
Type A ringing

Operating Loop Current

Off-Hook DC Resistance

270

280

@ 20mA

Leakage Current (Tip-Ring to
AGND)

@ 1000Vac

FL Threshold
Tip-Ring (On-hook)
Tip-ring Current (Off-Hook)

+30

+10

+40

Vdc

LRC = 0v
LRC = 5V

RL Threshold
Tip-Ring (On-Hook)
Tip-Ring (Off-Hook)

-30

-8

-40
-15

Vdc

LRC = 0v
LRC = 5V

Characteristics

Symbol

Min

Typ

Max

Units

Test

Conditions

2-wire Input Impedance (-2
variant)

600

Return Loss at 2-wire
(Zin = Ref = 600)

200-3400 Hz

Longitudinal to Metallic
Balance

58
58
53

200Hz
1000Hz
3.4kHz

Transhybrid Loss

THL

200-3400Hz

Gain (voltage) 2 wire to Tx

1024Hz

Gain (Voltage)
Rx to 2 wire

-2

Spec Freq

Input impedance at Rx

Output impedance at Rx

Signal Overload Level
at 2-wire output
at Tx

4.0

0.95

dBm

Vrms

% THD < 5%
Ref 600

@ 20mA

Total Harmonic Distortion
at 2-wire output
at Tx

THD

1.0
1.0

%
%

Input 0.5V, 1kHz @ Rx
Input 0.5V, 1kHz @
Tip-Ring

Idle Channel Noise
at 2-Wire
at Tx

15
15

dBrnC

Power Supply Rejection Ratio
at 2-wire and Tx

PSRR

25 dB
25 dB

dB
dB

Ripple 0.1V, 1kHz

2-258

MH88634

Preliminary Information

Figure 3 - Mechanical Data

Figure 4 - Typical LS-GS Application Circuit

2.10 + 0.040
(53.3 + 1.0)

0.56 Max

(14.2 Max)

0.27 Max
(6.9 Max)

0.010 + 0.002

(0.25 + 0.05)

0.080 Max
(2.0 Max)

Side View

0.05 + 0.01

(1.3 + 0.5)

0.05 + 0.02

(1.225 + 0.05)

0.020 + 0.005

(0.51 + 0.13)

0.100 + 0.010

(2.54 + 0.26)

0.18 + 0.02
(4.6 + 0.5)

Notes:
1) Not to scale
2) Dimensions in inches).
3) (Dimensions in millimetres).

*Dimensions to centre of pin &

tolerance non accumulative.

MH88634

Audio Out

Audio In

Ring Detect

Forward Loop

Reverse Loop

Switch Hook

-5V

+5V

Protection

Circuit

Tip

Ring

Loop Relay Control

Tip

Ring

LRD

VRLY

LRC

XLA

XLB

XLC

XLD

VDD

AGND

VEE

NOTES

1) See Figure 5 for
Protection Circuit.
2) K1 Electro Mechanical
2 form A.

SHK

+5V

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