MITEL MH88610 Datasheet

MH88610



Subscriber Line Interface Circuit (SLIC)

Preliminary Info rm atio n

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

• Loop leng th det ectio n

• Constan t c u rrent feed

Applications

Line interface for:

• PABX

• Intercoms

• Key Telephone Syst ems

• Control S ystem s

ISSUE 6 April 1995

Ordering Information

MH88610 20 Pin SIL Pac kage

0°C to 70°C

Descript io n

The Mitel MH88610 Subscriber Line Interface Circuit 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, loop length and dial pulse detection. The device is fabricated using thick film hybrid technology in a 20-pin single in-line package.

TIP

RING

LPGND

Current &Voltage

Sensing

Constant

Current

Feed

Figure 1 - Functional Block Diagram

VBat VDD VEE

Over Current

Protection

2-Wire to

4-Wire

Conversion

Loop

Supervision

GNDA

VR VX

VRef

LOOP SHK

CAP

2-97

MH88610 Preliminary Information

LPGND

VRef

VEE

GNDA

LOOP

TIP

RING

VDD

IC IC

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

1TFTF Tip Feed. Internal connection. Normally connect s a pair of external diodes for

protection. 2 LPGND Loop Ground. is the system ground reference wit h respect to VBat. 3VRVoice Receive (inpu t ) is the 4 wire analog signal to the SLIC. 4V 5V

Voltage Reference (Input) to set the line current feed to the subscriber line.

Ref

Negative Pow er Su pp ly Voltage (-5V).

6GNDAAnalog G round (0V). 7LOOPLoop Monitor V o ltage (Output). is proportional to the loop length. 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 (In pu t) is normally connected to Ring relay for negative bat tery feed voltage

and ringing voltage input.

12 V

Positive Power Supply Voltage.

13 IC Internal Connection. Pin cut short. 14 IC Internal Connection. Pin cut short. 15 RV Ring Feed Voltage connects to pin 11 (RF) through a normally closed ring relay. 16 IC Internal Connection. Pin cut short. 17 NC No Connection. 18 V

Negative Battery Feed Sup ply Voltage.

Bat

19 CAP Connects external capacitor and resistor to ground for ring trip filter control. 20 SHK

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Switch Hook Detect (Output). Digital output of an open-collector com parator. This output

will go low (V

) when the subscriber line resistance falls below a set threshold value

indicating that the telephon e set has gone off-hook. This output can be monitored for dial

pulse collection.

Preliminary Information MH88610

Functional Description

The MH88610 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 an from devices such as the voice codec or switching circuit. The SLIC also provides two status signals. switch hook (SHK

) and loop length (Loop). The Loop signal is an analog voltage which is proportional to the loop length and the SHK

goes low

when the telephone set is off-hook.

Constant Current Feed

The MH88610 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. This gives the loop current as:

= 25 - (VR e f) mA (+2 mA)

Loop

1.25

Switch Hook De tect io n

been detected an external circuit 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 GNDA to the 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.

Current Limit

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.

When the DC current exceeds an internal threshold level, the switch hook (SHK resistance is so high that V

) will go low. If the loop

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 high impedance (open collector output) 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., 90Vrms=+/-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 S H K

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

Components List

R1 =Resistor 1/4w, + 5%, 30.0kΩ R2 =Resistor 1/4w, + 5%, 750kΩ R3 =Resistor 1/4w, + 5%, 3.9kΩ R4 =Resistor 1/4w, + 5%, 4.7kΩ R5 =Resistor 1/4w, + 5%, 43kΩ R6 =Resistor 1/4w, + 5%, 470kΩ RV1*, RV2* =Var. 160 VRMS, 250V,10J D1*, D3*, D4* =Diode Rect., 200V, 1A IN4003 D2 =Diode Rect., 200V, 1A IN4003 D5 =Diode Sw., 75V, 02A, 0.5W IN4148 Q =NPN, 2N2222 or equivalent K1 =Relay E/M, 12V, 1 form C C1*,C2*,C3*,C4*= Cap. C er., 1.0µF, 50V, 20% C5 =Cap. Elec., 1.0µF, 16V 10%

Note: Components marked * are optional for extra protection of the hybrid.

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