A TELEPHONE LINE INTERFACE CIRCUIT WITH AC AND DC LINE ADJUSTMENT

Abstract: Disclosed is an interface circuit for a telephone line comprising integrated circuit means and switchable coupling means for coupling the telephone line and the integrated circuit means, the switchable coupling means being embodied as regulating means for regulating the AC and/or DC telephone line properties of the interface circuit. This reduces maximum voltage to the integrated circuit and shields the circuit of against high line voltages. Conversely take over distortion in the interface circuit is not propagated to the telephone line. Additionally there is no parallel current leaking away.

Description: A Telephone Line Interface Circuit with AC and DC Line Adjustment The present invention relates to an interface circuit for a telephone line comprising integrated circuit means and switchable coupling means for coupling the telephone line and the integrated circuit means. The present invention also relates to a telephone and to a communication device provided with a telephone. Such an interface circuit is known from EP-A-0 581 372 disclosing an end station type interface circuit connected to telephone line terminals. The known interface circuit is provided with integrated circuit (IC) means and switchable coupling means having a transistor, referred to as Tl. Transistor Tl has a control input to which a control signal DP is being applied for pulse dialling and other signalling (e.g. flash/ timed loop break) purposes. Transistor Tl functions as a switch for periodically interrupting the line current. This means that the integrated part of the remaining circuit has to withstand the relatively high line voltage. Not only does this limit the miniaturisation of the remaining integrated part of the interface circuit but in addition this limits the choices of the integration processes. It is an object of the present invention of alleviating these problems in order to clear obstacles to further miniaturisation and IC process choices. To this end the interface circuit according to the invention is characterised in that the switchable coupling means are embodied as regulating means for regulating the AC and/or DC telephone line properties of the interface circuit. It is an advantage of the interface circuit according to the invention that the AC/DC telephone line properties are regulated before being applied to the remaining part of the interface circuit i.e. the part behind the regulating means, which thus no longer have to withstand the relatively high line voltage. Thus the blockade towards a further reduction of IC chip area and a further integration is lowered and the maximum voltages associated with integration processes can be decreased allowing a further reduction of IC power consumption. Furthermore it is an advantage that the regulating means apart from providing modulation during transmission across the telephone line, perform a kind of shielding function as the regulation provides some form of increased impedance against spectral impurities on the telephone line, which could propagate to in particular the IC, or vice versa. In an embodiment of the interface circuit according to the invention the regulating means have a regulating input and the integrated circuit means are provided with control means having inputs for telephone line condition signals and an output coupled to the regulating input. Various DC voltage-current characteristics including the particular French characteristic can be realised simultaneously with the adjustment of nation dependent AC set impedance by means of the measured relevant telephone line condition signals. Also transmitting line current modulation is being performed. In a further embodiment of the interface circuit according to the invention the switchable coupling means comprises a first controllable semiconductor having a mean stream path to which the telephone line is coupled in series. Depending on the send/receive mode of the interface circuit according to the invention an AC component can easily be superimposed on the DC voltage over the first controllable semiconductor, which DC voltage is generally large enough to keep said semiconductor out of saturation. In that state the semiconductor has a high output impedance and accordingly the shielding function mentioned earlier is high too, whereas its required base current is low, thus reducing power consump- tion. In another embodiment of the interface circuit according to invention the first controllable semiconductor has a control input, and the switchable coupling means comprises a second controllable semiconductor having a mean stream path coupled to the control input of the first controllable semiconductor and a control input coupled to the regulating input of the regulating means. The second controllable semiconductor is also not driven in saturation so both semiconductors shield the IC part of the interface circuit from high voltage pulses on the telephone line, and vice versa. During pulse dialling or another line break action (flash) a pulse signal e.g. coming from an I 2 C bus can easily switch off the first semiconductor by means of the second semiconductor, which semiconductors may be bipolar transistors or the like. In still another embodiment of the interface circuit according to the invention the integrated circuit means comprises current sourcing/shunting means coupled to the mean stream path of the controllable semiconductor for preventing unwanted saturation thereof. In cases, such as when the current drawn from the telephone line momentarily exceeds the internally taken current a surplus current is sunk or shunted by means of sourcing the surplus through the current sourcing means. In such cases the taking over of the surplus current might cause cross-over distortion, which will however not propagate to the telephone line as this distortion is shielded by the high output impedance of the regulating means, in particular the first controllable semiconductor. In still a further embodiment of the interface circuit according to the invention the interface circuit comprises a semiconductor junction coupled to the telephone line for conducting line current to a supply output terminal of the interface circuit. The junction prevents current from flowing back into the IC during the current sinking of said surplus current above. In an alternative embodiment the semiconductor junction is a controllable semiconductor, which is coupled to a negative supply terminal. In this alternative there is no voltage loss across a junction, which contains a diode. The telephone device and the communication device according to the invention have the features outlined in claims 8 and 9 respectively. At present the interface circuit and telephone and communication devices according to the invention will be elucidated further together with their additional advantages while reference is being made to the appended drawing, wherein similar components are being referred to by means of the same reference numerals. In the drawing: Fig. 1 shows a preferred embodiment of the interface circuit according to the invention, and Fig. 2 shows a telecommunication system having telephone and communication devices containing the interface circuit of fig. 1. Fig. 1 shows an interface circuit 1, which can be used in a telecommuni- cation system 2 such as the one shown in fig. 2. The telecommunication system 2 comprises for example a central telephone network generally referred to as 3, and several telephone devices and/or facsimiles 4-1, 4-2...4-n. The telecommunication system 2 is connected to the devices 4-1...4-n through a telephone line 5. At each telephone device referenced 4 the telephone line 5 ends at line terminals Line and VEE. The interface circuit 1 comprises integrated circuit means 6, wherein various microprocessor controlled telephone, speech, facsimile and data functions μ are implemented and integrated, mainly on chip. The interface circuit 1 further comprises switchable coupling means 7 coupling the telephone line 5 with the integrated circuit means 6. The switchable coupling means 7 are switchable in the sense that they are at least capable of performing pulse dialling or other line break functions as well as electronic hookswitching functions. In the particular embodiment of fig. 1 the switchable coupling means 7 comprises regulating means in the form of semiconductors 8 and 9 for performing the functions mentioned above and in addition performing realisation of various nation dependent DC voltage-current characteristics, including the French character- istic. Apart from the transmitting of information across the telephone line 5 also the adjustment of nation dependent AC set impedance is realised in a way to be described further. As shown the main stream path of the first semiconductor 8 is connected in series with the telephone line 5. The semiconductor 8 is being driven in its non saturation working area. Herein line voltage is reduced, such that the integrated circuit means 6 only have to withstand a lower voltage, generally in the order of 3,5-6 volt. As a consequence thereof the choice of the integration process is not determined by this lower voltage. The base 10 of the first semiconductor 8, generally a bipolar transistor is controlled by the main stream path of the second semiconductor 9 which is connected as shown. The integrated circuit means 6 are provided with control means 11 having inputs 12-1, 12-2, 12-3, and 12-4 for telephone line condition signals and an output 13 coupled to a regulating input 14 in the form of the control input or base of the semiconductor 9, which as semiconductor 8 is off-chip. The on-chip part 15 of the integrated circuit means 6 generates an internal supply voltage on terminal VCC, which terminal is coupled to the collector of semiconductor 8 through a current sensing device 16, which provides a line current representative signal on input 12-1 of control means 11, and through a semiconductor junction D, which allows the line current to flow to terminal VCC only and not back. At wish junction D may be replaced by a controllable semiconductor (not shown) whose control input (base) is to be coupled to terminal VEE to overcome the voltage drop across the junction D. The current sensing device 16 may contain a known current controlled current source (CCCS). Further condition voltage representative signals are taken from both sides of junction D and directly from line terminal Line through a high ohmic resistance R2 and fed to inputs 12-2, 12-3, and 12-4 respectively. These condition signals are being processed in control means 11 resulting in the required DC voltage and current and AC sending and receiving voltage and current characteristics on telephone line 5. A possible embodiment of in particular the control means 11 is described in EP-A-0 581 372 referred to above, whose disclosure is included herein by reference thereto. The integrated circuit means 6 furthermore comprises current sourcing or shunting means for example in the form of a further semiconductor 17 coupled to the mean stream path of the controllable semiconductor 8 for preventing saturation thereof. Once an approaching saturation of semiconductor 8 is detected by the control means 11, base 18 thereof is controlled to sink the surplus current to terminal VEE. The take over by semiconductor 17 is hardly noticeable on telephone line 5 since semiconductor 8 is controlled to shield by having a high output impedance. Rl and R2 are high ohmic resistances so their currents can be neglected relative to the line current itself and in addition they practically do not deteriorate the balance return loss, i.e. the impedance seen by the telephone line 5. Of course various modifications are within the skill of the man skilled in the relevant art. Instead of transistor 8 connected to terminal Line the transistor 8 could at wish be replaced by a NPN transistor connected to terminal VEE, whereas transistor 9 would then become a PNP transistor. Sensing device 16 would then be included in the VEE terminal connection as well. In view of the foregoing it will thus be evident to a person skilled in the art that various modifications may be made within the spirit and the scope of the present invention as hereinafter defined by the appended claims and that the present invention is thus not limited to the examples provided.

Claims: CLAIMS: 1. An interface circuit for a telephone line comprising integrated circuit means and switchable coupling means for coupling the telephone line and the integrated circuit means, characterised in that the switchable coupling means are embodied as regulating means for regulating the AC and/or DC telephone line properties of the interface circuit. 2. The interface circuit according to claim 1, characterised in that the regulating means have a regulating input and that the integrated circuit means are provided with control means having inputs for telephone line condition signals and an output coupled to the regulating input. 3. The interface circuit according to one of the claims 1 and 2, characterised in that the switchable coupling means comprises a first controllable semiconductor having a mean stream path to which the telephone line is coupled in series. 4. The interface circuit according to claim 3, characterised in that the first controllable semiconductor has a control input, and that the switchable coupling means comprises a second controllable semiconductor having a mean stream path coupled to the control input of the first controllable semiconductor and a control input coupled to the regulating input of the regulating means. 5. The interface circuit according to claims 3 or 4, characterised in that the integrated circuit means comprises current sourcing or shunting means coupled to the mean stream path of the controllable semiconductor for preventing saturation thereof. 6. The interface circuit according to one of the claims 1-5, characterised in that the interface circuit comprises a semiconductor junction coupled to the telephone line for conducting line current to a supply output terminal of the interface circuit. 7. The interface circuit according to claim 6, characterised in that the semiconductor junction is a controllable semiconductor, which is coupled to a negative supply terminal. 8. A telephone device provided with an interface circuit according to one of the claims 1-7, the interface circuit comprising integrated circuit means and switchable coupling means for coupling a telephone line and the integrated circuit means, characterised in that the switchable coupling means are embodied as regulating means for regulating the AC and/or DC properties of the interface circuit. 9. A communication device provided with a telephone device according to claim 8, the telephone device having an interface circuit comprising integrated circuit means and switchable coupling means for coupling a telephone line and the integrated circuit means, characterised in that the switchable coupling means are embodied as regulating means for regulating the AC and/or DC properties of the interface circuit.

Inventors: DERKS HENK, MULDER JACOB, DERKS, HENK, MULDER, JACOB
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1999
Main classification: H04M
IPC codes: H04M1, H04M9