Driver interface for a telephone line switch enabling a speech circuit under any polarity conditions

Abstract: A telephone subscriber system comprising a speech circuit (SPC) and a ring circuit (RIN) adapted to be integrated monolithically to a semiconductor material substrate (G) and being coupled to a telephone subscriber line (SL), respectively through first (PP) and second (PR) circuit means of the diode bridge type, further comprises a first circuit changeover means (SW1), under control by the user, which drives at least second (SW2) and third (SW3) circuit changeover means connected between the first diode bridge circuit (PP) and the speech circuit (SPC). The third circuit changeover means (SW3) is connected to the speech and ring circuits at a circuit communication node (G) which forms the system point of reference, physically coincident with the substrate. The system includes a circuit interface means (CL) for controlling the second (SW2) and third (SW3) circuit changeover means by external electrical signals which are related to the circuit communication node (G). Said circuit interface means (CL) comprises a diode (D1) connected with its cathode to the circuit communication node (G), in parallel with the third circuit changeover means (SW3).

Description: This invention relates to telephone devices for subscriber systems, in particular telephone subscriber devices adapted to be monolithically integrated. In telephone circuits of the one-chip type which serve multiple functions normally performed by discrete integrated circuits such as modern speech, ring, and tone circuits -- the system common point of reference, both physically and electrically, is an individual one coinciding with the substrate of semiconductor material whereto the whole one-chip circuit is integrated. In a telephone subscriber system, the speech circuit and ring circuit are both coupled to, and energized through, the telephone line. During the ring phase, however, any coupling of the two circuits together must be prevented, as by disconnecting the speech circuit through switches, whereas during the speech phase, the ring circuit must be disabled. Current telephone company standards, moreover, call for the ring circuit to be DC uncoupled from the telephone line for any line polarity. This requires, as is well recognized by the skilled ones in the art, either the provision of a dual RC network, or the inclusion of a traditional electric switch which can be readily integrated. Understandably, the last-mentioned solution is the one which would normally be utilized wherever the telephone circuits involved are of the one-chip type. In a telephone subscriber system, the speech phase is initiated by clamping the DC line to the speech circuit as the handset is picked up or, in more sophisticated designs, on a pulse being issued from a keyboard. A single-chip telephone line interface of this type is for example described in the article entitled: "A single-chip telephone line interface in BCD technology", by Consiglio et al., IEEE International Solid-State Circuits Conference, February 1992, New York, US. This telephone interface circuit includes a speech circuits discoupled by the telephone line by means of switches driven by a keyboard switch. But, if one wishes to clamp the line to the speech circuit without picking up the handset and without acting physically on the telephone apparatus as by keying in -- operation from an external electric control being preferred instead -- problems may arise from that the line polarity is undefined and the line own reference potential is other than the reference potential of the one-chip circuit, i.e. its substrate. In fact, an input terminal of the device -- namely that connected to the terminal of the polarity bridge, which would usually be a diode bridge enabling the speech circuit to be coupled to the line -- might take a negative potential with respect to the system reference, i.e. the device substrate. To obviate the difficulties brought about by the different electrical references, it has been common practice to employ in most cases either electromechanical devices or optoelectronic components, which are effective but cost-intensive. The underlying technical problem of this invention is to provide a telephone subscriber system with a monolithically integrated line switch, for driving by external electrical signals, which can enable the speech circuit even under a condition of differing telephone line and subscriber system reference potentials. This problem is solved by a telephone subscriber system as defined in Claim 1, specifically in the characterizing part thereof. The features and advantages of a telephone subscriber system according to the invention can be appreciated from the following description of an embodiment thereof, to be taken by way of example and not of limitation in conjunction with the accompanying drawings. In the drawings: Figure 1 is a circuit diagram, partly in block form, of a telephone subscriber system according to the invention; and Figure 2 is a circuit diagram of a possible embodiment of that portion of the telephone subscriber system which characterizes the invention. The circuit illustrated by the diagram in Figure 1 comprises first, PP, and second, PR, diode bridges adapted to respectively couple a speech circuit SPC and a ring circuit RIN to a telephone subscriber line represented by a block SL. Both bridge circuits have a first terminal and second terminal for coupling to the line. The bridge circuit PR is, however, DC uncoupled from the line by means of a resistor R and a capacitor C. It functions as a rectifying bridge for the ring signals, whereby the ring circuit is activated to operate the bell of the subscriber set. On the other hand, the bridge circuit PP functions to maintain, on two terminals ("+" and "-") thereof for coupling to the speech circuit, a predetermined polarity irrespective of the line polarity. Connected across such terminals is a first circuit changeover means, SW1, adapted to be actuated by the user picking up the handset or causing a pulse to be issued from a keyboard. It drives two more circuit changeover means, SW2 and SW3, whereby the speech circuit is coupled, over a terminal pair, to the "+" and "-" terminals of the bridge circuit PP. The above three changeover circuit means act as switches effective to disconnect the speech circuit from the line during the ring phase. A fourth circuit changeover means, SW4, driven by SW1 acts as a switch effective to disable the ring circuit by breaking the connection between this circuit and the second bridge circuit PR upon SW1 making to drive SW2 and SW3 and accordingly enable the speech circuit. Also shown in the Figure is an actuator circuit represented by a block ACT. It comprises the required circuit parts to actually control the circuit changeover means SW2, and is connected between the "+" terminal of the first bridge circuit PP and a circuit communication node G establishing a link between the speech circuit, the ring circuit and the third circuit changeover means SW3. In the instance of telephone devices of the one-chip type, said circuit communication node G is a representation circuit-wise of the semiconductor material substrate, whereto the speech and ring circuits are integrated monolithically, constituting the reference potential for the telephone subscriber system. Unlike the third circuit changeover means SW3, which is also connected to the node G, the circuit changeover means SW2 is connected to no reference potential and, therefore, it is convenient to provide, with current circuit techniques, an actuator circuit having as its reference potential that of the substrate of the single chip. In accordance with this invention, the circuit in Figure 1 comprises a circuit interface means CL having an input terminal E for the application of external electrical signals in relation to the circuit node G, and output terminals for driving the second circuit changeover means SW2 through the actuator circuit ACT, as well as driving the third circuit changeover means SW3 directly. An essential feature of the invention is that such circuit changeover means include a circuit unidirectional current flow element, shown as a diode, connected, in parallel with the third circuit changeover means, between the "-" terminal of the bridge circuit PP and the circuit node G. In a telephone subscriber system according to the invention, an electrical command related to the substrate potential and the making of the high-voltage electronic switch SW3 related to the negative potential of the polarity bridge circuit PP are properly interfaced in quite a simple manner which allows of full integration. In fact, the making of switch SW3 is ensured by the provision of diode D1 between the bridge circuit PP and the substrate. The drive currents produced in the interface circuit by the external electrical command can be clamped to the substrate, via the diode, under any polarity conditions. The electronic switch SW3 may be implemented, in a known manner to the skilled ones, using a single NPN transistor in a Darlington configuration, or alternatively, a transistor of the MOS type, depending on the integration technology employed. Shown in Figure 2 is a preferred embodiment of the electronic switch SW3 and the interface circuit for driving by external electrical commands related to the system common node. The switch is implemented by an N-channel DMOS component, and the interface circuit is also made up of components of the MOS type. Of course, bipolar components would be employed in embodiments incorporating a bipolar type of electronic switch. Figure 2 shows a resistor R1 and a Zener diode Z1 connected across the terminals A and B of the switch SW3; the resistor R1 ensures that the electronic switch SW3 normally breaks when no commands (by picking up the handset or keying in) are applied to the terminal B, whilst the Zener Z1 between the gate and the source of the N-channel DMOS transistor functions as a gate voltage protection. The interface circuit comprises a current generator consisting of a current mirror circuit whose output leg D drives both the actuator circuit ACT and the electronic switch SW3 through terminal B. Provided between the input leg of the current mirror circuit and the system communication node G is a DMOS transistor whose gate terminal is connected, via a diode, to the current mirror circuit as well as to the input terminal to which external electrical commands related to the common node G, i.e. the substrate, can be applied. In this embodiment, a diffusion of the P type is "seen" at the output D of the interface circuit: this is important because whenever line polarity conditions are encountered, in the absence of any command, whereby the potential at point A is lower than that at point G -- which is the system common point (substrate) -- the junction is reverse biased with respect to the substrate and, therefore, the telephone set isolation resistance kept high. The output current flows into R1 from point D, and loops to the substrate through the diode D1. The voltage drop across R1 drives the transistor of switch SW3 into conduction, and allows the current to flow between the "+" and "-" terminals of the bridge circuit PP. It also causes the switch SW2 to make through the actuator circuit, thereby enabling the speech circuit and the single chip as a whole to be energized and operated. In this way, the operability of the telephone circuit connected to the line can be ensured by means of electrical commands which have for their reference the system common point and may be supplied, for example, from an automatic answering set, as connected or incorporated to the telephone subscriber system. It should be noted that the diode D1 is intrinsic to the monolithically integrated N-channel DMOS component, so that this embodiment of the invention, additionally to being uniquely reliable, is far less expensive than prior art solutions.

Claims: A telephone subscriber system, comprising: at least one speech circuit (SPC) having at least first and second terminals; a telephone ring signal detection circuit (RIN) having at least first and second terminals; a first circuit means in bridge (PP) form having first and second terminals for coupling to a telephone subscriber line (SL) and third and fourth terminals for coupling to the first and the second terminals, respectively, of the speech circuit; a second circuit means in bridge (PR) form having first and second terminals for coupling to the line through at least one DC uncoupling circuit means and third and fourth terminals for coupling to the first and the second terminals of the ring signal detection circuit; and a first circuit changeover means (SW1) connected between the third and the fourth terminals of the first bridge circuit means and adapted to be controlled through an input terminal actuated by a user and to drive at least second (SW2) and third (SW3) circuit changeover means, respectively connected between the third terminal of the first bridge circuit means and the first terminal of the speech circuit, and between the fourth terminal of the first bridge circuit means and a circuit common node (G) whereto the fourth terminal of the second bridge circuit means (PR), the second terminal of the speech circuit (SPC) and the second terminal of the ring signal detection circuit (RIN) are connected, characterized in that it comprises: a circuit interface means (CL) connected to the circuit common node (G) and operative to drive at least the second and third circuit changeover means (SW2,SW3); said circuit interface means having an input terminal (E) for the application of external electrical signals in relation to the circuit common node (G) and including at least one unidirectional current flow circuit element (D1) in parallel with a third circuit changeover means (SW3) and coupled to the circuit common node (G). A telephone subscriber system according to Claim 1, wherein the third circuit changeover means (SW3) comprises a first transistor having a first terminal (C) connected to the circuit common node (G), a second terminal (A) connected to the fourth terminal of the first bridge circuit means (PP), and a control terminal (B) connected to a first circuit changeover means (SW1), characterized in that the unidirectional current flow circuit element (D1) is connected between the first (C) and the second (A) terminals of the first transistor. A telephone subscriber system according to Claim 1, characterized in that the unidirectional current flow circuit element comprises a first diode (D1) having its cathode connected to the circuit common node (G) the anode of said first diode (Dl) being connected to the control terminal (B) of the first transistor by means of a resistive element (R1) and a second diode (Z1). A telephone subscriber system according to Claim 2, characterized in that the unidirectional current flow circuit element comprises a first diode (D1) having its cathode connected to the circuit common node (G) the anode of said first diode (D1) being connected to the control terminal (B) of the first transistor by means of a resistive element (R1) and a second diode (Z1). A telephone subscriber system according to any of Claims 2 and 3, characterized in that the circuit interface means (CL) comprises a current mirror circuit having an input leg connected to the circuit common node (G) through a second transistor and an output leg coupled to the control terminal of the first transistor (B) and the second circuit changeover means (SW2), the second transistor having a control terminal for coupling to the input terminal (E) of the circuit interface means. A telephone subscriber system according to any of Claims 1, 2, 3 and 4, characterized in that it comprises an actuator circuit (ATC) for controlling at least the second circuit changeover means (SW2), said actuator circuit being connected between the third terminal of the first bridge circuit means (PP) and the circuit common node (G), as well as between the first circuit changeover means (SW1), the circuit interface means (CL) and the second circuit changeover means (SW2). A telephone subscriber system according to Claim 4, wherein at least the speech (SPC) and ring (RIN) circuits are integrated monolithically to a semiconductor material substrate, characterized in that said substrate is the circuit common node (G), and that the first transistor is an n-channel D-MOS type and the first diode (D1) is an intrinsic diode to the integrated structure of said first transistor.

Inventors: ANTONINI CARLO, CONSIGLIO PIETRO, VANALLI GIANPIETRO, ANTONINI, CARLO, CONSIGLIO, PIETRO, VANALLI, GIANPIETRO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H04M1
IPC codes: H04M1