TELEPHONE LINE INTERFACE CIRCUIT WITH VOLTAGE SWITCHING

摘要

A two-wire telephone line interface circuit comprises a driver circuit (10) for supplying direct current on the line (13), a controlled voltage generator (14), a sensing circuit (12) for monitoring the line current, a control circuit (15) for controlling an output voltage (CV) of the voltage generator, and a switching circuit (11). The control circuit controls the switching circuit to supply, as a supply voltage (DV) for the driver circuit, a battery voltage (BV) in an on-hook state of the line or the controlled output voltage of the voltage generator in an off-hook state of the line, this output voltage being controlled to provide off-hook current limiting. The control circuit also controls the switching circuit to selectively supply the controlled output voltage of the voltage generator as a signaling voltage to at least one wire (TV, RV) of the line for high voltage signalling, e.g. ringing, on the line. Desirable forms of the switching circuit are described.