United States Patent [is>]
Date of Patent:
 SUBSCRIBER LINE INTERFACE CIRCUIT WITH REDUCED ON-CHIP POWER DISSIPATION
 Inventor: Russel J. Apfel, Austin, Tex.
 Assignee: Advanced Micro Devices, Inc., Sunnyvale, Calif.
 Appl. No.: 31,785
 Filed: Mar. 12,1993
 Int. a.6 H04M 19/00
 U.S. a 379/413; 379/398;
 Field of Search 379/399, 413, 398
 References Cited
U.S. PATENT DOCUMENTS
4,677,667 6/1987 Burns 379/399
4,677,669 6/1987 Kawami et al 379/399
4,866,768 9/1989 Sinberg 379/413
4,879,746 11/1989 Young et al 379/399
5,046,089 9/1991 Pariani et al 379/399
Electronic Design, Jul. 11, 1994 vol. 42, No. 14, article entitled "High-Voltage Bonded-Wafer ICs Create First SLIC to Switch The 93-V RMS Phone-Line Ring Voltage on-Chip, Eliminating PABX Relays", written by Frank Goodenough, pp. 55-64, Cover Page, Table of Contents.
HC-5506 Specification for Dual Subscriber Line Inter
face Circuit (DSLIC), Harris Semiconductor, Advance Information, Jun. 1994, 16 pages.
Primary Examiner—James L. Dwyer
Assistant Examiner—Jacques M. Saint-Surin
Attorney, Agent, or Firm—Skjerven, Morrill,
MacPherson, Franklin & Friel
A subscriber line interface circuit is provided that includes an output stage having a pair of differential transconductance amplifiers to drive the tip and ring lines of a telephone exchange. The output stage includes circuitry that advantageously reduces the power dissipated within the integrated circuit when the subscriber loop impedance is relatively low. As a result, heat generation within the integrated circuit is reduced. Circuit reliability may therefore be enhanced and fabrication and packaging costs may be reduced. In one embodiment, an off-chip resistor is coupled from an output line of the ring line amplifier to the negative voltage rail. When the loop impedance is low, a relatively large voltage drop is established across the resistor which therefore absorbs a greater percentage of the loop current. Since the resistor is provided off-chip and diverts current that would otherwise flow through the ring-line amplifier, the overall heat dissipated within the integrated circuit is reduced.
42 Claims, 4 Drawing Sheets