Subscriber line interface circuit with reduced on-chip power dissipation

A subscriber line interface circuit comprising:

• an input circuit for receiving signals from a telephone exchange;
• a first output driver circuit coupled to said input circuit for driving a tip line of a subscriber loop;
• a second output driver circuit coupled to said input circuit for driving a ring line of said subscriber loop;
• and
• a first resistor coupled between said first output driver circuit and a first voltage terminal so that direct current flow in said subscriber loop is independent of direct current flow through said first resistor and power dissipation is moderated in said first output driver circuit when an off-hook condition exists between said tip line and said ring line.

A subscriber line interface circuit comprising:

• an input circuit having an input for receiving signals from a telephone exchange;
• a first resistor having a first terminal coupled to a tip line terminal, and further having a second terminal;
• a first operational amplifier having an input coupled to an output of said input circuit and an output coupled to the second terminal of said first resistor;
• a first feedback circuit coupled to said first operational amplifier, said first feedback circuit configuring said first operational amplifier to provide a regulated DC current through said first resistor when the subscriber line is in an off-hook condition;
• a second resistor having a first terminal coupled to a ring line terminal, and further having a second terminal;
• a second operational amplifier having an input coupled to an output of said input circuit and an output coupled to the second terminal of said second resistor;
• a second feedback circle coupled to said second operational amplifier, said second feedback circuit configuring said second operational amplifier to provide the regulated DC current through said second resistor when the subscriber line is in an off-hook condition;
• and
• a third resistor having a first terminal coupled between the output of said second operational amplifier and the second terminal of said second resistor, and a second terminal coupled to a first voltage terminal.

A subscriber line interface circuit comprising:

• an input circuit receiving signals from a telephone exchange;
• a first output driver circuit coupled to said input circuit and having an output driving a tip line signal onto a subscriber loop;
• a second output driver circuit coupled to said input circuit and having an output driving a ring line signal with a substantially constant DC current component onto the subscriber loop, said second output driver circuit including a first current sensing element having a first terminal coupled to the subscriber loop and a second terminal coupled to the output of said second output driver to convey the ring line signal thereto;
• and
• a first resistive element external to said second output driver circuit and having a first terminal coupled between the second terminal of said current sensing element and the output of said second output driver, and a second terminal coupled to a first voltage terminal so that power dissipation in said second output driver circuit is moderated.

A subscriber line interface integrated circuit having a tip node and a ring node, the subscriber line interface circuit comprising:

• a first transconductance amplifier having an output coupled to the tip node, an input, and a feedback path coupled across the output and input;
• a second transconductance amplifier having an output coupled to the ring node, an input, and a feedback path coupled across the output and input;
• an off-chip resistor node in the feedback path of the second transconductance amplifier;
• and
• a system interface having a plurality of I/O nodes, a first node coupled to the input of the first transconductance amplifier, and a second node coupled to the input of the second transconductance amplifier.

A subscriber line interface circuit coupled to a first voltage reference terminal and a second voltage reference terminal and having a tip line terminal and a ring line terminal for coupling the subscriber line interface circuit to a subscriber loop, the subscriber loop having a subscriber loop impedance defined in part by a tip line, a ring line, and a telephone connected between the tip line and ring line, the subscriber loop interface circuit comprising:

• a first amplifier circuit having an output coupled to the tip line terminal and an input;
• a second amplifier circuit having an output coupled to the ring line terminal and an input;
• an input and monitor circuit having an input coupled to the system interface for receiving signals, a first node coupled to the tip line terminal and a second node coupled to the ring line terminal to detect a telephone off-hook condition, the input and monitor circuit further having a third node coupled to the first amplifier circuit input and a fourth node coupled to the second amplifier circuit input, wherein when the input and monitor circuit is coupled to the subscriber loop the input and monitor circuit responds to the telephone off-hook condition and the subscriber loop impedance and controls the first and second amplifier circuits to establish an operating voltage across the subscriber loop and a loop current in the subscriber loop;
• a first resistor coupled between the first voltage reference terminal and the first amplifier circuit output;
• and
• a second resistor coupled between the second voltage reference terminal and the second amplifier circuit output, wherein the first amplifier circuit delivers a first portion of the loop current to the subscriber loop and the first resistor delivers a second portion of the loop current, the second amplifier circuit sinks a third portion of the loop current and the second resistor sinks a fourth portion of the loop current.