MH88612 Ver la hoja de datos (PDF) - Mitel Networks

Número de pieza

componentes Descripción

Fabricante

MH88612

Subscriber Line Interface Circuit (SLIC)

Mitel Networks 

Preliminary Information

MH88612

Functional Description

The SLIC 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 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 voltage (VRef) that determines the

constant feed current. If VRef (pin 4) is externally

grounded, the constant current is set at 25mA. For a

constant current setting between 18mA and 30mA,

VRef can be set between 8.75 Vdc and

-6.35 Vdc. The relationship is defined by the

equation:

ILOOP = 25 - (VRef) mA (+2 mA)

1.25

Switch Hook Detection

When the DC current exceeds an internal threshold

level, the switch hook (SHK) will go high. If the loop

resistance is so high that VBAT can no longer supply

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.

Ringing And Ring Trip Detection

In Figure 3 a ringing signal (e.g.90 Vrms=+/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 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 AGND to the Ring Trip

Filter Control pin will switch in the filter, whilst

removing AGND, (with the switch in a tri-state

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.

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

ILoop is negligible and power consumption is

minimised. The power denial function is useful for

disabling a loop which may have a ground fault.

2-107

Share Link: