ST AN585 Application note

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

2. Good capacitance balance between line a and line b

→ No signal attenuation.

→ Good longitudinal balance on the line.

Figure 1. ISDN interface protection

U INTERFACE

U INTERFACE

S INTERFACE

S INTERFACE

(a) FLOATING GROUND

OR

OR

U INTERFACE

U INTERFACE

TPIxx

(b) ABSOLUTE GROUND REFERENCE

OR

OR

S INTERFACE

S INTERFACE

REV. D2A - 3597

1/5April 2004

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)

SOD15 / CB429 SO8 / DIL8

HIGH SURGE CAPABILITY

75A 10/1000 msec

SMTHDT58 / TPU58
SMTHDT80 / TPU80

SMTHDT120 / TPU120

Figure 2. Tribalanced protection - functional schematic

MONOLITHIC DEVICES

MEDIUM SURGE CAPABILITY

30A 10/1000 msec

TPI8011P / TPI8012P

TPI12011P / TPI12012P

DISCRETE
SOLUTION

3 x SMTHDTXX / 3 x TPUxx

LOW CAPACITANCE LINE/GND

GOOD CAPACITANCE BALANCE A/LINE B GOOD LONGITUDINAL BALANCE ON THE LINE

NO SIGNAL ATTENUATION

MONOLITHIC

SOLUTION

TPIxx

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.

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