AN585 APPLICATION NOTE ISDN INTERFACE PROTECTION 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. STMicroelectronics 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. TRIBALANCED PROTECTION = TPIXX SERIES The use of TRIBALANCED protection is mandatory under the following conditions: – 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 Figure 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 U INTERFACE TPIxx OR S INTERFACE (a) April 2004 FLOATING GROUND OR S INTERFACE (b) ABSOLUTE GROUND REFERENCE REV. D2A - 3597 1/5 AN585 APPLICATION NOTE ISDN PROTECTION - PRODUCT RANGE STMicroelectronics offers specific protection devices for ISDN interface protection. The product range is given in Table 1, 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. Figure 2 illustrates the typical application schematic for TRIBALANCED PROTECTION. When the discrete solution is used, three components per line are necessary. Table 1. Tribalanced protection - product range DISCRETE SOLUTION (3 devices per line) MONOLITHIC DEVICES SOD15 / CB429 SO8 / DIL8 HIGH SURGE CAPABILITY 75A 10/1000 msec MEDIUM SURGE CAPABILITY 30A 10/1000 msec SMTHDT58 / TPU58 SMTHDT80 / TPU80 SMTHDT120 / TPU120 TPI8011P / TPI8012P TPI12011P / TPI12012P Figure 2. Tribalanced protection - functional schematic DISCRETE SOLUTION 3 x SMTHDTXX / 3 x TPUxx LOW CAPACITANCE LINE/GND MONOLITHIC SOLUTION TPIxx NO SIGNAL ATTENUATION GOOD CAPACITANCE BALANCE A/LINE B GOOD LONGITUDINAL BALANCE ON THE LINE APPLICATION SCHEMATICS Figure 3 and 4 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 a nd performance can be traded in a variety of combinations. All of these components are innovative and ideal for use in high-speed transmission lines. 2/5 AN585 APPLICATION NOTE Figure 3. Central office / PABX - U-Interface protection +V PTC BAT42 –V TPIxx or SMTHDTxx TPUxx +V LINE U PTC SMPS –V ABSOLUTE GND REFERENCE Figure 4. NT1 = S-Interface protection U INTERFACE S INTERFACE BAT42 TPIxx or SMTHDTxx TPUxx CENTRAL OFFICE BAT42 TE TPIxx or SMTHDTxx TPUxx FLOATING GND SMPS ABSOLUTE GND REFERENCE 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. ST offers a wide range of devices designed specifically for ISDN protection, enabling this requirement to be satisfied. 3/5 AN585 APPLICATION NOTE REVISION HISTORY Table 2. Revision History 4/5 Date Revision Description of Changes March-1993 1 First Issue 16-Apr-2004 2 Stylesheet update. No content change. AN585 APPLICATION NOTE Information furnished is believed to be accurate and reliable. However, STMicroelectronics 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 STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners © 2004 STMicroelectronics - All rights reserved STMicroelectronics GROUP OF COMPANIES Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States www.st.com 5/5