MITEL MH88520-1 Datasheet

Features



MH88520-1

German Subscriber Line Interface Circuit (SLIC)

Preliminary Information

ISSUE 4 April 1995

• Differential to s ingle en ded conv ersion

• Transformerless 2-2 wire conversion

• Minim um in sta lla tio n s pa ce

• Off-Hook detection and LED

indic ato r d riv e

• Relay drive outp ut

• Battery and ri ngin g feed to line

• Logic interface: MUTE

, SHK, RRC

• Mute of inco ming aud io

• Dial pulse detection

• Voltage surge protecti on

• German compl ex inpu t impe da nce

Applications

Line Interfac e for:

• Intercoms

• Key System s

• PABX

Ordering Information

MH88520-1 20 Pin SIL Pa ckag e

0°C to 70°C

Descript io n

The Mitel MH88520-1 German Subscriber Line
Interface Circuit provides a complete interface
between the telephone line and a speech switch
requiring only single bidirectional switch per
crosspoint. The functions provided by the
MH88520-1 include bidirectional differential to single
ended conversion in the speech path, line battery
feed, ringing feed and loop and dial pulse detection.
The device is fabricated as a thick film hybrid in a
20-pin ‘single-in-line’ (SIL) package allowing
optimum circuit board packing density and very high
reliability.

TIP (A)

RING (B)

RING FEED 1

RING FEED 2

VRLY

RRD
RRC

RGND

Loop Feed/

Ring Feed

Relay Drive

Circuit

VBAT

AGND VDD VEE

Speech Circuit

Figure 1 - Functional Block Diagram

Loop Detectio n

JUNC

MUTE

SHK

LED

THRESH ADJ

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MH88520-1 Preliminary Information

TIP

IC

RING

RF1

IC

RF2

IC

VEE
LED

THRESH ADJ

SHK

VDD

AGND

VBAT

MUTE

JUNC

RRD

RGND

RRC

VRLY

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

1TIPTip Lead. Connects to the “Tip” lead (A-wire) of the telephone line.
2ICInternal Connec tion . Leave open circuit.
3 RING Ring Lead. Connects to the “Ring” lead (B-wi re) of the telephon e line.
4RF1Ring Fe ed 1. Connect to the Ring Relay contact. See Figure 5.
5ICInternal Connec tion . Leave open circuit.
6RF2Ring Fe ed 2. Connect to normall y closed contact of Ring Relay. See Figure 5.
7ICInternal Connec tion . Leave open circuit.
8V

EE

9LED

10 SHK

11 THRESH ADJ Allows adjustment of SHK
12 V

DD

Negative Pow er Su pp ly Voltage. Normally -5V.
LED Drive (Output). Drives an LED directly. A logic low indicates an off-hook condition.
Switch Hook Detect (Output). A logic low indicate s an off-hook and dial pulsing

condition. Open collector out put wit h 10k Ω internal pull up to V

DD

.

detection threshold as shown in Figure 6.

Positive Power Supply Voltage. Normally +5V. This provides current for both internal

circuitry as well as the loop.
13 AGND Analog Grou nd . Supply and Bat tery Ground.
14 V

BAT

15 MUTE

Battery Supply Voltage. Normally -24V. This provides current to the loop.

MUTE (Input). A logic low will mute signals coming from Tip-Ring to the JUNC pin.

16 JUNC JUNCTOR. Ground (AGND) referenced transmit and receive speech path.
17 RRD

Ring Relay Drive (Outpu t). Connects to the ring relay coil. A logic low activates the

relay.
18 RGND Relay Ground. Return path for relay supply voltage. Normally connected to AGND.
19 RRC Ring Relay Control (Input). A logic high activates the Ring Relay Drive (RRD

) outputs.

20 VRLY Relay Positive Supply voltage. Normally +5V. Connects to the relay coil and the relay

supply voltage. An internal clamp diode from VR LY to GRND is provided.

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Preliminary Information MH88520-1

Functional Description

The BORSH Functions

The MH88520-1 performs all of the Borsh functions
of Battery Feed Overvoltage Protection, Ringing,
Supervision and Hybrid (2-2 Wire).

Return Loss at Tip-Ring

To maximise return loss, the impedance at Tip-Ring
should match the SLIC’s impedance (220R + 820R //
115nF). However, the SLIC’s input impedance is
dependent on the JUNCTOR termination resist ance.
Therefore the JUNCTOR must be terminated with
754Ω.

Figure 3 illustrates a typical connection between two
SLICs through two crosspoint switches. Optimum
return loss occurs when the JUNCTOR is terminated
with 754Ω. Since the JUNCTOR input/output
impedances is 604Ω and the crosspoint switch
resistances are 75Ω + 75Ω, this configuration gives
optimum return loss as shown in Figure 4.

Hybrid

The 2-2 wire hybrid circuit converts the incoming
balanced signal at Tip and Ring of the telephone line
into a ground referenced output signal at JUNC of

the SLIC, and converts the ground referenced input
signal at JUNC of the SLIC into the non-balanced
output signal at Tip-Ring of the telephone line.

Mute

A logic low at the MUTE input results in muted
signals coming from Tip and Ring to the JUNC
terminal while allowing signals from the JUNC
terminal to Tip and Ring to be transmitted.

Overvoltage Protection

The MH88520-1 is protected from short term (1ms)
(+250V) between Tip and Ring, Tip and Ground, and
Ring and Ground. However, additional protection
circuitry may be needed depending upon the
requirements which must be met for the final
equipment.

Loop Detection

The loop detection circuit determines whether a low
enough path is across Tip and Ring to be recognised
as an off-hook condition. (Threshold impedance =

5.4kΩ with no adjustment). This threshold level can
be adjusted by the use of external resistors as
shown in Figure 6. When an off-hook condition
occurs the SHK
low level. These outputs also toggle during incoming
dial pulses.

and LED outputs toggle to a logic

TIP

RING

TIP

RING

1

TIP

3

RING

MH88520-1

16

JUNC

75Ω

CROSSPOINT

SWITCH

SLIC 1

MH88520-1

1

TIP

8

RING

JUNC

16

SLIC 2

75Ω

CROSSPOINT

SWITCH

Figure 3 - SLIC Crosspoi nt S witc h Connecti on

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