MITEL MH88612K Datasheet

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MH88612K

Subscriber Line Interface Circuit

Preliminary Information

Features

• Input impe danc e 600 Ω

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

• Battery and ri ngin g feed to line

• Off-hook and dial pulse detection

• Ring gr oun d over-current protec tion

• Adjustable consta nt curre nt fee d

• Relay drive output with diode protection

• Power Denial

•Wide V

Operatin g Range

BAT

•Tip/Ring Reversal

Applications

Line interface for:

• PABX

• Intercoms

• Key Telephone Syst ems

• Control S ystem s

ISSUE 3 April 1995

Ordering Information

MH88612K 30 Pi n SIL Package

0°C to 70°C

Description

The SLIC provides a complete interface between a switching system and subscriber loop. Functions provided include battery feed, ringing feed and tipring reversal to the subscriber line, 2-Wire to 4-Wire hybrid interfacing, constant current feed and dial pulse detection. The device is fabricated as a thick film hybrid using various technologies.

TIP

RING

RC2

VRLY

RD1

RC1

T/R

Reverse

T/R Reverse

Relay Driver

Relay

Drive

Circuit

PDLPGND

VBat VDD

Power Denial

Battery

Feed

Over Current

Protection

Constant

Current

Feed

Figure 1 - Functional Blo ck Diag ram

VEE

2-Wire to

4-Wire

Conversion

Loop

Supervision

SHK

GNDA

VRef

CAP

2-121

MH88612K Preliminary Information

TIP

RING

RC2

LPGND

VRef

CAP SHK

GNDA

VEE

VDD

2 IC IC IC

IC IC

VBat

RV PD VX

IC IC

VRLY

RC1 RD1

21 22 23 24 25 26 27 28 29 30

Figure 2 - Pin Connections

Pin Description

Pin # Name Description

1TFTip Feed. Connects to external diode for protection. Internally co nnecte d to pin 21. 2RFR ing Feed (Inp ut). is normally co nnecte d to ring relay for negative batt ery feed voltage

and ringing voltage inpu t. 3 IC/NC No Connections to be made to this pin, used for internal connect ion. 4 IC/NC No Connections to be made to this pin, used for internal connect ion. 5 IC/NC No Connections to be made to this pin, used for internal connect ion. 6 IC/NC No Connections to be made to this pin, used for internal connect ion. 7 IC/NC No Connections to be made to this pin, used for internal connect ion. 8 TIP Connects to the “Tip” lead of the telephone line. 9 IC/NC No Connections to be made to this pin, used for internal connect ion.

10 RING Connects to the “Ring” lead of the telephone line. 11 RC2 Relay Control 2 (Input). Active high to reverse Tip/Ring. 12 LPGND Loop Ground is the system ground reference wit h respect to V

GNDA.

13 IC/NC No Connections to be made to this pin, used for internal connect ion. 14 VR Voice Receive (Input) is the 4-wire analog signal to the SLIC. 15 VRef Voltage Reference (In pu t). to set the constant current feed to the subscriber line.

Constant current feed can be adjusted by the input voltage on the V

grounded for normal 25mA loop current.

16 CA P Connects external ca pacit or to ground for ring tri p. 17 SHK Switch Hook Detect (Outp ut). Active high 18 GNDA Analog Ground (0V) 19 VEE Negative Po wer Supply Voltage (-5V). 20 VDD Positive Power Supply Voltage (+5V). 21 TF Ti p Fe ed. Connect an external diode for protection. Internally connected to pin 1. 22 VBat N egati ve Battery Feed Supp ly Voltage (-48V).

. Normally tied with

Bat

. This pin can also be

Ref

2-122

Preliminary Information MH88612K

Pin Description (Continued)

Pin # Name D es crip tio n

23 RV R in g Feed Voltage connects to (RF) through a normally closed ring relay. 24 PD Power Denial (Input). A logic high will isolate the bat tery volt age to tip/ri ng. 25 Vx Voice Transmit (Output). is the 4-wire analo g signal from the SLI C. 26 IC/NC No connections to be made to this pin, used for internal connection. 27 IC/NC No connections to be made to this pin, used for internal connection. 28 VRLY Relay Voltage Supp ly (+5V). This pin is used to power the on board relay. 29 RC1 Relay Contro l 1 (Input). Active high. Normally used for ringi n g. 30 RD1 Open collector sinks current when RC1 is at logic high.

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 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 constant current is set at 25mA. For a constant current setting between 18mA and 30mA. V be set between 8.75 Vdc and -6.35 Vdc. The relationship is defined by the equation:

Loop

) that determines the constant

Ref

is externally grounded, the

REF

= 2 5 - V

mA (±2 mA)

Ref

1.25

Ref

can

Switch Ho ok Det ecti on

use of the relay K1. An internal clamp diode is provided between VRLY and RD1.

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 (Cap) to ground will provide adequate attenuation when ringing is applied. Once an off-hook condition has been detected a logic high should be applied to (RC) which will activate the relay (K1) to disconnect the ringing voltage from (RF) and reconnect to (RV). At that time 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 to 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.

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 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.

can no longer supply

BAT

Ringing And Ring Trip Detection

In Figure 3 a ringing signal (e.g., 90 Vrms +-48 Vdc is applied to the line by disconnecting (RV) from (RF), and connecting the ringing voltage at (RF) by

Tip/Ring Reverse

The MH88612K provides a Tip/Ring reversal relay driver. To activate the internal relay to reversal Tip and Ring apply a logic input to RC2.

Current Limit

The Tip or Ring may be accidentally shorted to ground. In such a case, current will only flow through the feed resistor. This high current will be sensed

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