AN382 - STMicroelectronics

AN382/1093

APPLICATION NOTE

PABXBIG COST REDUCTIONAND PERFORMANCE

IMPROVEMENT AREOBTAINEDWITH A NEW SLIC CHIP SET

by W. Rossi

The new L3000N/L3092SLIC need very few externalcomponents,very low power in ON-HOOK

and provide innovative functions like Ringing injection, message waiting, line length measure-

mentfor autoadaptivesystems.

Abstract

The new SGS-THOMSON SLIC (Subscriber Line

Interface Circuit) kit L3000N/L3092 suitable for

PABX (Private Automatic Branch Exchange), low

end C.O. (Central Office) and ISDN terminal

adaptor applications is described. The two chip

approach allows the integration of innovative

functions as ringing generation and injection, line

length evaluation, message waiting, loop exten-

sion all software programmable. High level

DC/AC performances are provided and very low

power is requested in on hook condition (less

than 10mW from battery and typ. 50mW from ±5V

supplies).

1. INTRODUCTION

The ’90 years represent a transition period from

fully analog solution to fully digital (ISDN) one in

PABX systems design. In particular in this new

systems generation analog and digital line cards

will coexist. The SLICs for this new systems

shouldsatisfy the followingrequirements:

- do not degrade the transmission performances

of the digital part (ex: noise due to relay bounces

during ringinginjection).

- Software programmability of the DC/AC charac-

teristicsin order to have an easy adaptationto dif-

ferent countriesrequirements.

- Signaling self management in order to simplify

the card controller software.

- Low numberof externalcomponents.

- High reliabilityand lowcost.

In order to satisfy the above requirements a com-

plex circuit structure is necessary. Such a struc-

ture cannot be integratedin asingle chip because

high voltage technology is required to interface

the subscriber line. Single chip solutions based

on presentH.V. Technologyare possibleonly giv-

ing up most of the above requirements or increas-

ing too much the device size and the external

components number. The SGS-THOMSON SLIC

family actually in production and based on a two

chip approach is able to satisfy the above require-

ments. In fact only the circuit directly connected to

the line is realized in High Voltage Technology

while all the control and signal management func-

tions are performed in the control chip realized in

high density, Low Voltage Technology. (15V). In

addition this approach allows an easy adaptation

to different applicative situation only selecting the

proper low voltage control chip. In the following

the L3000N/L3092kit isdeeply described.

2. L3000N-L3092GENERAL DESCRIPTION

The SLIC KIT L3000N/L3092 integrate all the

functions, except the overvoltage protection,

needed to interface a subscriber line. As shown

in fig.1 the L3000N chip is a directly connected to

the telephone line. On the L3092 chip two inter-

faces are present. The first is an analog interface

(4W) connected to the analog input/output of a

Codec/Filter. The second one is a parallel digital

interface that allows to set different operating

modes of the kit and to transferinformation about

line status. Because of the internal latches circuit

this interface can be connected to a SLIC com-

mon control bus driven by the Line Card Control-

ler (see fig. 1). In alternative this interface can be

directly connected to the I/Ocontrol port of a sec-

ond generation COMBO as SGS-THOMSON

TS5070/TS5071(see fig. 2). The main features of

the L3000N/L3092are the following :

- Programmable DC feeding resistance and limit-

ing current (25/40/60mA)

- Low power dissipation (50mW) in on hook con-

dition (off-hookdetector active).

- Loop extension / Integrated message waiting

function. 1/10

- Signaling function (ON-HOOK/OFF-HOOK;

GROUND-KEY/GROUND-START detection)

- Line lengthevaluation

- Hybrid function

- Line impedance synthesis(real/complex)

- Balanced ringing signal generation with zero

crossinginjection

- Automatic ringing stop with zero crossing when

OFF-HOOK is detected.

- Parallel latched digital interface for direct con-

nection with 2nd generation COMBO or Card

Controller

-Low number of externalstandardtolerancecom-

ponents.

- Integrated thermal protection that disable out-

put stages when junction temperature exceeds

140°C

Figure1: The latched paralleldigital interfaceof L3092 allows aneasy commonbus controlstructure. The

card controllerselectthe properSLIC via the corresponding CSpin.

L3092 L3000N

LINE

INTERFACE

LINE

INTERFACE

APPLICATION NOTE

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Figure2: The digitalinterface of L3092 is directly connectedto the COMBOII I/O ports.The card controller

manage this I/O digital interface via a 2Mbit serial controlport.

3. OPERATING MODES

Three input and two output pins represent the

parallel digital interface. In addition a CS pin al-

lows to connect the digital interfaces of all the

SLICs on the Line Card to the same control bus;

the digital datas applied to this interface selects

one of the followingoperating modes:

- Conversationor Active Mode

- Stand-byor On-Hook Mode

- Power Down or Disable Mode

- RingingMode

4. CONVERSATION OR ACTIVE MODE

The SLIC is setted in this mode when the off-hook

conditionhas been detected and the communica-

tion must be activated; the main functions per-

formed in this modeare:

1) DC currentfeeding into subscriber loop.

2) Signaling recognition (OFF-HOOK; GND-KEY;

DIAL PULSES)

3) Bidirectional transfer of speech band signals

between line (2wire) and COMBO (4wire) inter-

faces.

4.1 DC CHARACTERISTIC

One of the main SLIC function is to provide a DC

current to the subscriber loop in order to feed

properlythe connectedtelephone set. In fig. 3 dif-

ferent DC characteristics Line Current (Il) versus

loop resistance (Rl) are shown. The (a) curverep-

resents the DC characteristic obtained with tradi-

tional line interface based on transformer; as you

can see it is a purely resistive characteristic

(Il=VBAT/(2xRFS)) where typical RFS values are

200ohm and 400ohm. The (b), (c), (d) curves

represent the three different programmable DC

characteristics that can be obtained with the

L3000N/L3092 SLIC kit in conversation mode.

Each curve is composed by two regions the first

withconstant line current and the second resistive

were the RFS value is programmed by means of

one external resistor. The constant current region

allows a big power dissipation reduction with

short lines if compared with the transformer solu-

tion. In addition the possibilityto program by soft-

ware different limiting currents (25,40,60mA)

make the system more flexible allowing an easy

adaptation to the different administration require-

ments and the possibility of the line length meas-

urement. In fact when TEST MODE function is

activated one of the digital interface output will

show if the actual line current is equal or lower

than the programmed limiting current value. The

possibility to program three different limiting cur-

rent values allows to distinguishfour different line

resistances ranges as shown in fig. 3; this infor-

mation is very useful in order to optimize the

transmission performances for each line length.

L3092 L3000N

LINE

INTERFACE

L3092 L3000N

LINE

INTERFACE

APPLICATION NOTE

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Figure 3: The (a) curve represents the DC characteristicobtained with traditionalline interface based on

transformer.The(b), (c), (d) curvesrepresentthe three differentprogrammable DCcharacteristicsthat can

be obtainedwith the L3000N/L3092SLIC kit in conversationmode.

In particular if a second generation COMBO, as

SGS-THOMSON TS5070/71 is adopted the line

length can be used to select by software proper

transmit/receive gains and balance impedances

obtaining in this way an autoadaptive system at

four states. Another important feature consists in

the balanced line feedingcircuit; it means that line

current variations will produce equal and opposite

voltage variationson the A, B, or TIP, RING SLIC

line terminations. This characteristic is very im-

portant because during dial pulse selection the

line current change from zero to the maximum

value producing large line voltage variations. The

balanced structure avoid high common mode sig-

nals generation with very short rise/fall time. It

should be noted that this problem is always pre-

sent when monolithic SLICs with integrated

DC/DC converter are adopted in fact in this case

doesn’t exist voltage symmetry between the two

line terminations. This kind of common mode sig-

nals produce serious transmission problems on

the ISDN line present in the same system; in ad-

dition they are also noise sources for the other

analog lines. The L3000N/L3092 SLIC kit has

been studied not only in order to avoid common

mode noise on the line, but also to be less sensi-

tive to the noise present on the battery voltage

typically generated by dial pulse selection and

ringinginjection operations.In particular the noise

on the battery voltage is transferred to the line

with an attenuationhigher than 20dB for low fre-

quency components (10Hz) and with more than

40dBfor speechband components(typ. 1kHz).

4.2 LONG LINES FEEDING.

The L3000N/L3092 SLIC kit is also suitable for

public central office, since in this application is

sometimes required to feed very long lines a par-

ticular operating mode (Boost Battery) is pro-

vided. When the boost battery mode is activated

the DC characteristic is modified allowing 20mA

of feeding current for 4Kohm of loop resistance

equivalentto more than 10Km (see fig.4). Consid-

ering PABX dedicated to Hotel applications this

operating mode can be used to perform the

”MESSAGE WAITING” function. In fact when the

Boost Battery mode is selected and the line cur-

rent is zero the line voltage is greater than 100V;

this voltage is high enough to switch on the neon

lamp on the telephoneset typically used to inform

the subscriber to call back the operator. During

Boost Battery operation the L3000N line interface

circuit fed its internal output stages between the

negative battery (typ -48V) and the positive bat-

L3000N/3092DC Characteristic.

feeding resistance: 2 x 200Ω;

batt. voltage 48V

APPLICATION NOTE

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Figure 4: DC characteristic in Boost Battery mode allows to feed very long lines (20mA/4Kohm)and to

perform the ”message waiting” function. For line current close to zero the TIP/RINGvoltageis greater than

100V.

tery voltage (typ. +72V) already present for the

ringing operation.

4.3 SIGNALING.

When the Conversation operating mode is se-

lected on the two output pins of the digital inter-

face the following informationsare provided:

1) ON-HOOK/OFF-HOOK detection

2) GROUND KEY / GROUND START detection.

In addition the off-hookdetection delay is very low

allowing dial pulse detection with a distortion

lower than 1% . On the contrary the ground key

information is filtered with a time constant in the

rangeof 100ms.

4.4 AC CHARACTERISTIC.

The L3000N/L3092SLIC kit provide excellent AC

characteristics, in addition its internal structure al-

lows an easy programming of the line and bal-

ance impedances both real or complex by means

of external scaled (by 25) components. It means

that in case of complex impedances the external

capacitors values will be 25 time lower than the

synthetized values. Another peculiarity of the

L3000N/L3092 SLIC kit is the possibility to re-

cover the insertion loss due to the external pro-

tection resistors connectedbetween the SLIC TIP

and RING(A, B) output pins and the line; such re-

sistors cannot be avoided being necessary to limit

the line current when surges or other causes pro-

duces overvoltageson the line. The major part of

the monolithic SLICs available on the market re-

quires low values for the external protectionresis-

tors ( < 20ohm) being not able to recover the at-

tenuationproducedby such components.

This attenuation become a problem in presence

of complex line impedance because its value de-

pends also on the frequency. If protection resis-

tors higher than 20ohm are adopted the corre-

spondent amplitude distortion with frequency will

not be acceptable and external complex equaliz-

ing circuit required.

On the contrary the L3000N/L3092 SLIC kit al-

lows to use protection resistors in the range from

30 to 100ohm increasing in this way the system

protection level. In this condition the guaranteed

insertion loss flatness (including external protec-

tion resistors) between 2W (line) and 4W

(COMBO) interfaces is +/- 0.1dB for both real or

complexline impedances.

Another important L3000N/L3092SLIC kit feature

for the overall system performances is the capa-

bility to workalso in presenceof high inducedlon-

L3000N/3092DC Characteristic.

feeding resistance: 2 x 200Ω;

Boost Battery mode (tot. batt.: 120V)

APPLICATION NOTE

5/10

gitudinalline current. The longitudinal current can

be generatedby the followingcauses:

1) Magnetic/capacitive coupling between the

subscriber line and the AC main power distribu-

tion network (110/220V;50/60Hz).

2) Magnetic/capacitive coupling with adjacent

subscriber line on which large voltage/current

variations are present (ex: dial pulse selection;

ringing injection..)

3) Magnetic/capacitive coupling with high fre-

quency electromagneticwaves.

For a proper SLIC operation in presence of the

above conditions the following characteristics are

necessary:

1) High value of Longitudinalto Transversal Con-

version (LTC)

2) High value of Longitudinalto Transversal Con-

version also in presence of high common mode

current.

3) High value of Longitudinalto Transversal Con-

version also in presence of high frequency com-

mon mode current.

The L3000N/L3092 SLIC kit is able to satisfy all

theaboverequirements;in particular the LTC guar-

anteed value is 52dB; the typical is 60dB. This

value remain nearly unchangedalso for high com-

mon mode current (ex. 65mA peak on each wire

with 25 mA of transversalDC line current or 50mA

peakwith 40mAof transversal DC line current).

In addition the high gain-bandwidthproduct of the

L3000N integrated operational amplifiers allows

to keep the LTC value very high also for common

mode current frequencies up to 200/300kHz.

Higher frequenciescommon mode currentcan be

easily attenuatedconnecting betweenthe line ter-

minations and ground small capacitors (10/20nF).

The low effect of such capacitors on the SLIC AC

parameters (gains, impedances, two to four wire

conversion....) can be easily recovered acting on

the SLIC external components thanks to the high

flexibility of the two chiparchitecture.

In fig.5 are shown the test circuit used for EMI

evaluation up to 10MHz and the results obtained

with and without the filtering capacitors on line

terminations. This excellent result show that the

L3000N/L3092 SLIC kit provide very high rejec-

tion to EMI signals without requiring expensive

H.F. coils.

Figure5a: Test circuit for EMI evaluation.

BASIC TEST CIRCUIT CONFIGURATION

FOR EMI MEASUREMENTS

L3092 L3000N

APPLICATION NOTE

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Figure5b: The high bandwidthand high current capability of L3000N output operational amplifier allows

an excellent rejection to longitudinal EMI signals up to 200kHz (a) curve. This good performances are

maintainedfor higherfrequenciesusing two inexpensive small capacitors(Crf=10/20nF).

5. STAND-BY OR ON-HOOK MODE

The L3000N/L3092 SLIC kit is usually pro-

grammed in Stand-By mode when the connected

telephone set is in on-hook condition; the only

functionsperformedin this mode are:

1) DC line feeding with low limiting current value

(10mA)

2) OFF-HOOK detection This second function is

performed by a sophisticate circuit that allows a

properoff-hook detectionalso in presenceof high

common mode currentsinduced on the line there-

fore the off-hook information provided by the

SLIC doesn’t need additional processing by the

card controller. The reduced number of functions

provided in this mode allows a significative SLIC

power dissipation reduction(150mW).

6. POWER DOWN MODE

This mode is used in all those condition in which it

is necessary to reduce to zero the power deliv-

ered to the line (power failure, emergency, line

not connected ....). In power down mode the line

interfacecircuit L3000N is completelyswitched off

therefore not able to detect OFF-HOOK, its im-

pedance at line terminations is 1Mohm and the

current sinked from the batteryis reduced to zero.

In this mode the power dissipation from the +/-5V

supplies is typ. 50mW. In case a very low power

dissipation is requested also when the telephone

is in on-hook and in such condition it doesn’t sink

more than 500µA the Automatic Stand-by Mode

can be selected. The Automatic Stand-by Mode

combine the advantages of the two previous op-

eratingmodes, in particular:

1) Accurate OFF-HOOK detection

2) Low power consumption (50mW from +/-5V

and <10mW from battery).

When thismode is selectedtheL3000Nis normally

in Power-Down Mode. A dedicated circuitry inside

L3092 will sense the line voltage and will automat-

ically program in Stand-by Mode the L3000N if a

possible off-hookconditionis detected.

7. RINGING MODE

One of the L3000N/L3092 SLIC kit main charac-

teristic is the possibility to inject directly the ring-

ing signal on the line without request of external

electromechanical relay and high level ringing

L3000N/3092EMI measurements

Test circuit: see FTZ 12 TR1 Teil 21

APPLICATION NOTE

7/10

generator.In order to obtainfrom the L3000Nthe

proper level for the ringing signal an additional

positive voltage source is requested (typically

+72V).

The ringing signal is obtained amplifying a low

level signal (1.5Vrms) applied to the SLIC, this

low level oscillation can be obtained either from a

local oscillator or from the COMBO RX path. The

ringing signal is injected on the line in balanced

mode with a nominal amplitude of 60Vrms that

canbe increasedup to 70Vrms.

During the ringing injection the SLIC output im-

pedance is just equal to the two series protection

resistors (dohm); this characteristic allows to use

lower ringing signal amplitude compared with the

standard solutions where the ringing generator

series impedance is typically 800ohm. In fig.6 is

represented the effect of the low output imped-

ance with different ringer load and compared with

the traditional solution with a source voltage of

75Vrmsand 800ohm of output impedance.

A dedicated circuitry guarantee that the ringing

signal is injected and disconnected from the line

always in presence of the zero voltage crossing

transition, avoiding the generation of very fast

voltage transition on the line.

During ringing injection a sophisticate circuit will

perform a complete ring trip detection; when off-

hook condition is recognized the SLIC will auto-

matically stop the ringing signal without waiting

for the card controller command.

In conclusion the L3000N/L3092 SLIC kit allows

to save:

- The centralizedH.V. ringinggenerator

-all the ringingrelays

-all the zerocrossing circuitry

- all the software for ring trip detection.

Figure 6: Comparison between L3000N/L3092 SLIC kit and standard solutions ringing performances.

The voltage across the ringer load after 1Km line (280ohm) is shown for different REN (Ringer Equiva-

lent Number). The L3000N low output impedance in ringing mode allows to recover the lower source

voltage (60Vrmsversus75Vrms) and to be betterthan standard solution when the REN increase.

8. CONCLUSIONS

The complete circuitry needed to interface the

PCM system highways to the subscriber line is

shown in fig.7. The SLIC two chip approach al-

lows to reduce drastically the number of external

components, to save the centralized ringing gen-

erator, the ringing relay, the zero crossing ringing

control and to implement the ”message waiting”

function. In addition the combination of the

L30XX/3000NSLIC KITs

EFFECT OF LOW OUTPUT IMPEDANCE DURING

RINGING INJECTION (after280Ωline)

APPLICATION NOTE

8/10

Figure 7: Complete application circuit neededto interfacea subscriber line with system PCM highways. Very

low numberof externalcomponentsandhigh flexibilityare achieveddue tothe SLICtwo chip architecture.

L3000N/L3092 SLIC with COMBOII increase the

system flexibility and performances. In particular

the card controller can select by software:

- DC feeding currents (25/40/60mA)

- TX and RX gains (25dB range; 0.1dB step) -

Balancenetworks

- Time slot assignment (up to 64x2 slots)

In addition the Test Mode implemented on L3092

in conjunction with COMBOII allows to realize a

fourstep autoadaptivebalancesystem.

L3092 L3000N

APPLICATION NOTE

9/10

Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the

consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No

license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications men-

tioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied.

SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without ex-

press written approval of SGS-THOMSON Microelectronics.

SGS-THOMSON Microelectronics GROUP OF COMPANIES

Australia - Brazil -France - Germany - Hong Kong - Italy- Japan - Korea -Malaysia - Malta - Morocco - The Netherlands -Singapore - Spain

- Sweden - Switzerland- Taiwan - Thaliand- United Kingdom - U.S.A.

APPLICATION NOTE

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