Patent ID: 8249247

Claim:
An apparatus comprising: a first resistance coupled between a first lead of a subscriber loop and a first node, and a first capacitance coupled between the first node and a second node, the second node coupled to a second lead of the subscriber loop; and a circuit to convert a voltage across the first capacitance into a proportional absolute value signal used to provide a control signal to a switching regulator for a subscriber line interface circuit (SLIC) coupled to the subscriber loop, wherein the circuit comprises: a first operational amplifier having a first input terminal coupled to the first node and a second input terminal coupled to receive a reference voltage; a pair of metal oxide semiconductor field effect transistors (MOSFETs) having gate terminals coupled to an output of the first operational amplifier, the MOSFET pair having first terminals coupled to provide a feedback signal to the first input terminal of the first operational amplifier; and a current minor having an input node coupled to a second terminal of the first MOSFET and an output node coupled to a second terminal of the second MOSFET, wherein the second terminal of the second MOSFET is to provide the control signal to the switching regulator.