Patent ID: 6507654
Filing Date: 2003-01-14
Classification: H04M

Abstract:
A line interface circuit for supplying energizing current from power terminals to a two wire communication line and for coupling communications signals between the communication line and a telephone facility via a hybrid circuit means, comprising:tip and ring terminals for connection to the two wire communication line; hybrid transmit and receive terminals for connection to the hybrid circuit means; a tip and ring signal voltage detector, being responsive to differential signals appearing across the tip and ring terminals and being of at least a voice band frequency, and being response to signals appearing at the receive terminal, for generating a composite signal; a loop driver circuit having an input for receiving a control signal and a pair of output terminals, the loop driver circuit being responsive to the control signal for driving alternating current signals on the pair of output terminals; a loop current detector having a first pair of terminals connected to the tip and ring terminals respectively, a second pair of terminals and a third terminal, a tip feed resistor and a ring feed resistor connected between the first and second pairs of terminals wherein the loop current detector generates a supervision signal at the third terminal in response to current flow in the communication line; a network having a first port being connected to receive the composite signal from the tip and ring signal voltage detector, a second port being connected to receive the supervision signal from the loop current detector, and a third port being connected to the input of the voltage amplifier for providing the control signal; a tip positive temperature coefficient (PTC) resistor and a ring PTC resistor electrically coupled between the second pair of terminals of the loop current detector and the pair of output terminals of the loop driver circuit, the tip and ring PTC resistors being electrically connected in series respectively with the tip and ring feed resistors and being thermally coupled respectively to the tip and ring feed resistors; whereby an increase in current through the tip and ring feed resistors causes an increase in temperature of the tip and ring feed resistors hence increasing the temperature and consequently the impedance of the PTC resistors.