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APPLICATION NOTE
ISDN INTERFACE PROTECTION
C. Politano
I. INTRODUCTION The choice of a suitable protection device for an ISDN line interface requires consideration of a parameter which is not critical in analogue line applications : the parasitic capacitance that the device introduces. Because of the high data rates used, parasitic capacitances must be minimized in order to ensure correct signal transmission. In particular, attention must be paid to the capacitance imbalance in the line which can cause considerable signal degradation. Such imbalance most frequently results from the presence of common-mode protection, in which the capacitance introduced between each line and earth is frequently unequal. SGS-THOMSON has developed a complete range of specific protection devices for ISDN applications : the " TRIBALANCED PROTECTION " TPIxx series.
These devices introduce only a minimum of capacitance imbalance (30pF), which does not affect the transmission performance of the line. 2. TRIBALANCED PROTECTION = TPIxx SERIES The use of TRIBALANCED protection mandatory under the following conditions : is
- The bias voltage on line A and line B is different (line A = GND, Line B = -Vbat) - The protection is realized in common mode, as illustrated in fig.1 (b). In this case, two conditions must be satisfied : 1) Low capacitance from line to ground No signal attenuation 2) Good capacitance balance between line a and line b Good longitudinal balance on the line.
Figure 1 : ISDN interface protection
U INTERFACE OR S INTERFACE
U INTERFAC E
TPIxx
OR S INTE RFACE
(a)
FLOATING GROUND
(b)
ABSOLUTE GROUND REFERENCE
AN585/0393
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APPLICATION NOTE
3. ISDN PROTECTION - PRODUCT RANGE : SGS-THOMSON offers specific protection devices for ISDN interface protection. The product range is given in fig.2, which shows that this function is available with different package versions. Discrete and monolithic versions are available, in order to provide a wide choice of cost/performance compromises. Depending on the solution chosen, different recommendations apply with regard to the optimum configuration to use. Fig.3 illustrates the typical application schematic for TRIBALANCED PROTECTION. When the discrete solution is used, three components per line are necessary. MONOLITHIC DEVICES
Figure 2 : Tribalanced protection - product range DISCRETE SOLUTION (3 devices per line) SOD15 / CB429
SO8 / DIL8
HIGH SURGE CAPABILITY 75A 10/1000 msec
MEDIUM SURGE CAPABILITY 30A 10/1000 msec
SMTHDT58 / TPU58 SMTHDT80 / TPU80 SMTHDT120 / TPU120 Figure 3 : Tribalanced protection - functional schematic
TPI8011P / TPI8012P TPI12011P / TPI12012P
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APPLICATION NOTE
4. APPLICATION SCHEMATICS Fig.4 and 5 illustrate the use of tribalanced protection in a u-interface and an s-interface respectively. In each case there is the choice of a discrete solution (SMTHDTxx or TPUxx) where a high surge capability is required, or a single-chip solution for low cost (TPIxx). Thus cost and performance can be traded in a variety of combinations. All of these components are innovative and ideal for use in high-speed transmission lines.
Figure 4 : Central office / PABX - U-Interface protection
Figure 5 : NT1 = S-Interface protection
5. CONCLUSION Due to the sensitivity of ISDN to capacitance imbalance on the lines, tribalanced protection has to be used where common-mode protection
of lines with different bias voltages is required. SGS-THOMSON offers a wide range of devices designed specifically for ISDN protection, enabling this requirement to be satisfied.
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APPLICATION NOTE
Information furnished is believed to be accurate and reliable. However, SGS-THOM SON Microelectronics assumes no responsability 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 mentioned 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 lif e support devices or systems without express written approval of SGS-THOMSON Microelectronics.
(c) 1995 SGS-THOMSON Microelectronics - Printed in Italy - All rights reserved. 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 - Thailand - United Kingdom - U.S.A. 4/4
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