MH88612B Просмотр технического описания (PDF) - Zarlink Semiconductor Inc

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MH88612B

Subscriber Line Interface Circuit (SLIC)

Zarlink Semiconductor Inc 

Data Sheet

MH88612B

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, VREFcan be set between 8.75 Vdc and -6.35 Vdc. The

relationship is defined by the equation:

ILOOP = VBAT - VREF mA (+2 mA)

1.89 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 (eg. 90 Vrms and -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 as shown in

Figure 4 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.

SEMICMF.019

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