A floating, low-power subscriber line interface circuit [bipolar IC technology]

A subscriber line interface circuit is described which is smaller than and dissipates substantially less heat than conventional circuits which perform similar functions. Worst case heat dissipation in the circuit is only 0.65 W as compared to conventional arrangements which dissipate up to 4 W. The battery feed function is accomplished by a small 1.5 W DC-to-DC converter which also includes circuitry for detecting loop supervisory functions. The floating nature of the output circuitry renders it immune to power line induction and common mode power crosses. Primary protection from lightning and other induced surges utilizes conventional carbon block type protectors. Secondary circuit protection is accomplished with a high voltage diode and simple carbon composition resistor. Circuit disconnect is accomplished with a medium voltage SCR for ringing and loop testing the subscriber line. The battery feed control and loop supervisory functions have been realized in a low voltage, linear integrated circuitry using CBIC technology.