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PBL386112

Subscriber Line Interface Circuit

Ericsson  

PBL 386 11/2

Ground Key Detector

The ground key detector indicates when

the ground key is pressed (active) by put-

ting the output pin DET to a logic high level

when selected. The ground key detector

circuit senses the difference between TIPX

and RINGX currents. The detector is trig-

gered when the difference exceeds the

current threshold.

Ring Trip Detector

Ring trip detection is accomplished by con-

necting an external network to a compara-

tor in the SLIC with inputs DT and DR. The

ringing source can be balanced or unbal-

anced e g superimposed on the battery

voltage or ground. The unbalanced ringing

source may be applied to either the ring

lead or the tip lead with return via the other

wire. A ring relay driven by the SLIC ring

relay driver connects the ringing source to

tip and ring.

The ring trip function is based on a polar-

ity change at the comparator input when

the line goes off-hook. In the on-hook state

no dc current flows through the loop and

the voltage at comparator input DT is more

positive than the voltage at input DR. When

the line goes off-hook, while the ring relay

is energized, dc current flows and the com-

parator input voltage reverses polarity.

Figure 13 gives an example of a ring trip

detection network. This network is applica-

ble, when the ring voltage superimposed

on the battery voltage is injected on the ring

lead of the two-wire port. The dc voltage

across sense resistor RRT is monitored by

the ring trip comparator input DT and DR

via the filter network R1, R2, R3, R4, C1 and

C2. DT is more positive than DR, with the

line on-hook (no dc current). The DET

output will report logic level high, i.e. the

detector is not tripped. When the line goes

off-hook, while ringing, a dc current will flow

through the loop including sense resistor

RRT and will cause the input DT to become

more negative than input DR. This chang-

es the output on the DET pin to logic level

low, i.e. tripped detector condition. The

system controller (or line card processor)

responds by de-energizing the ring relay

via the SLIC, i.e. ring trip.

Complete filtering of the 20 Hz ac compo-

nent at terminals DT and DR is not neces-

sary. A toggling DET output can be exam-

ined by a software routine to determine the

duty cycle. Off-hook condition is indicated

when the DET output is at logic level low for

more than half the time.

Detector Output (DET)

The PBL 386 11/2 SLIC incorporates a

detector output driver designed as open

collector (npn) with a current sinking capa-

bility of min 3 mA, and a 10 kΩ pull-up

resistor. The emitter of the drive transistor

is connected to AGND. A LED can be

connected in series with a resistor (≈1 kΩ)

at the DET output to visualize, for example

loop status.

Relay driver

The PBL 386 11/2 SLIC incorporates a ring

relay driver designed as open collector

(npn) with a current sinking capability of 50

mA.The emitter of the drive transistor is

connected to BGND. The relay driver has

an internal zener diode clamp to protect the

SLIC from inductive kick-back voltages. No

external clamp is needed.

Control Inputs

The PBL 386 11/2 SLIC has three digital

control inputs, C1, C2 and C3.

A decoder in the SLIC interprets the con-

trol input condition and sets up the com-

manded operating state.

C1 to C3 are internal pull-up inputs.

Open Circuit State

In the Open Circuit State the TIPX and

RINGX line drive amplifiers as well as other

circuit blocks are powered down. This caus-

es the SLIC to present a high impedance to

the line. Power dissipation is at a minimum

and no detectors are active.

Ringing State

In the ringing state the SLIC will behave

as in the active state with the exception

that the ring relay driver and the ring trip

detector are activated. The ring trip detec-

tor will indicate off hook with a logic low

level at the detector output.

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