3/5

AN585 APPLICATION NOTE

Figure 3. Central office / PABX - U-Interface protection

Figure 4. NT1 = S-Interface protection

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.

+V

LINE U

– V

BAT42

PTC

ABSOLUTE GND REFERENCE

SMPS

+V

– V

PTC

TPIxx

or

SMTHDTxx

TPUxx

U INTERFACE

S INTERFACE

TE

ABSOLUTE GND

REFERENCE

TPIxx

or

SMTHDTxx

TPUxx

TPIxx

or

SMTHDTxx

TPUxx

SMPS

FLOATING

GND

BAT42

BAT42