Low voltage sensing and control of battery referenced transistors in subscriber loop applications

Abstract

A subscriber line interface circuit apparatus includes tip/ring sense circuitry generating a tip sense signal and a ring sense signal from three sensed currents, wherein the tip sense signal and the ring sense signal correspond to subscriber loop tip and ring currents, respectively. In one embodiment, the tip/ring sense circuitry includes a current mirror generating first and second mirrored sense currents from a first sense current proportional to a battery feed node voltage of a subscriber loop. Current differencing circuitry provides the tip sense signal from a difference between the first mirrored sense current and a second sense current associated with a tip line of the subscriber loop. The current differencing circuitry provides the ring sense signal from a difference between the second mirrored sense current and a third sense current associated with a ring line of the subscriber loop.