Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Utility Patent
1998-09-29
2001-01-02
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S050000, C327S051000
Utility Patent
active
06169425
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a voltage sensing switch circuit, particularly to a voltage dependent current limiting circuit, and more particularly to a voltage dependent current limiter for sensing the voltage across the terminals of a bi-directional switch.
2. Description of the Prior Art
The solid-state switches of the prior art which are connected to Tip and Ring circuits in telecommunications applications have traditionally required an external protection device to limit the maximum switch voltage to less than the minimum breakdown voltage rating of the switch. This is due to the fact that the Tip and Ring pair is often exposed to high power fault conditions, such as lightning or a power cross.
In such systems, the current limiting aspect of the switch is typically set by an internal current sense resistor. In this way, the power dissipation of the entire integrated circuit is determined from this switch current limit and the protector clamp voltage of the external device. The current limit of these prior art switches is typically set above about 120 mA to avoid any interference with normal audio and ringing signals.
Because of wide variation in protector clamp voltages, and the need to prevent the clipping of ringing voltage above about 200 v, the current limiting switch is normally required to withstand over 300 v in both the ON and OFF states. Since the current through the circuit is not dependent on the switch voltage in such circuits, the output DMOS transistors in these prior art systems are sized to survive the peak power associated with both the internal switch current limit and the external maximum protector voltage. Thus, these switches are typically current-limited independent of the switch differential voltage, and the power dissipation of the circuit is determined as a function of the external protector clamp voltage.
These prior art switches have the significant disadvantage that they are very costly in terms of the silicon area needed. However, if the transistors are undersized to reduce the silicon area, then extremely high temperatures can be reached in the active regions of the device, greatly reducing the life of the switch and greatly increasing the possibility of damage to or failure of the switch itself. These high temperatures significantly degrade device performance and reliability.
Accordingly, an improved circuit is needed which is capable of protecting the switch circuit while simultaneously maintaining proper sizing of the transistors and reducing the cost of the circuit in terms of the silicon area needed.
SUMMARY OF THE INVENTION
To achieve this objective, the present invention is directed to an integrated circuit comprising a voltage source and a voltage sensing circuit having a switch circuit for sensing a switch voltage and a switch current; a voltage comparison circuit connected to the switch circuit for comparing the switch voltage to a limiting voltage; and a current limiting circuit connected to the switch circuit and the voltage comparison circuit, which is capable of limiting the switch current when the switch voltage reaches or exceeds the limiting voltage. In a preferred embodiment of the invention the switch circuit is a bi-directional switch circuit and the voltage comparison circuit incorporates a series of Zener diodes, wherein the current limiting circuit is a current foldback circuit.
REFERENCES:
patent: 4873453 (1989-10-01), Schmerda et al.
patent: 5361008 (1994-11-01), Saijo
patent: 5376831 (1994-12-01), Chen
patent: 5396117 (1995-03-01), Housen et al.
patent: 5844440 (1998-12-01), Lenk et al.
Spires Dewayne Alan
Umberger Dean Michael
Callahan Timothy P.
Lucent Technologies - Inc.
Nguyen Hai L.
Schnader Harrison Segal & Lewis LLP
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