VOLTAGE SELECTOR AND COMPARATOR USED THEREWITH

Abstract: A voltage selector uses a comparator (COMP) in order to clamp the potential (VSUB) of a p-doped substrate of a telecommunication Subscriber Line Integrated Circuit to the most negative voltage appearing at two input terminals (V1/2), one being supplied with the battery voltage and the other with a synthesized voltage, both varying within a relatively wide range with either one being able to be more negative than the other. With V1/2 connected to VSUB through respective DMOS transistors (N1/2), V1/2 also constitute the inputs of COMP whose outputs (C1/2) are connected to the gates of the DMOS transistors. The comparator is so designed as to allow a fast change-over of the opposite conductivity states of N1 and N2 whenever there is an inversion in the relative magnitudes of the V1 and V2 voltages, the voltage drop over a conductive N1/2 transistor being much lower than in the case of clamping diodes.

Description: VOLTAGE SELECTOR AND COMPARATOR USED THEREWITH —-_.._ _—-____ _ ______ ^ The invention relates to a voltage selector including first and second input terminals connected via respective switching means to an output terminal with the first or the second switching means being made conductive in accordance with the relative amplitude of the voltages at the two input terminals. Such a circuit is already known and reference can be maαe for instance to European patent application published under No 0284979. Therein, an integrated circuit for driving inductive loads is described in which the substrate can be clamped through respective diodes either to the output ground or to the live terminal of the inductive load . Quite apart from the problem posed by this particular type of load, in many applica ions, particularly those involving a CMOS technology, it is desirable that the substrate be at a well defined potential which in the case of CMOS integrated circuits avoids the problem of latch-up. This occurs due to the triggering of parasitic Silicon-Controlled Rectifiers, a 4-layer PNPN structure inherent in CMOS. This may lead to unwanted trigger currents turning on back-to-back bipolar transistors that regenerat lvely turn each other on more strongly, with the possibility in time of shorting the power supply to ground and causing the collector supply aluminium line to vaporize. Substrate bias voltage generators are therefore commonly used to counteract such effects and a well known technique for this purpose is the charge pump of which a particular example is to be found in the European patent application published under No 0293045 which is especially concerned in further problems which arise when such a substrate bias voltage generator is driven by an external clock. Moreover, such a charge pump cannot be used in all circums ances since this principle can only deal with relatively small voltage differences between the supply potential and that of the substrate. In some telecommunication line circuits, for instance of the type disclosed in the European patent application published under No 0201635, such voltage differences can reach 15 volts to which one must still add the amplitude of voltage transients. Also, the frequency of such a charge pump can lead to problems of psophometric noise if it is low and at higher frequency, the clock pulse transitions can generate undesirable sub-harmonics and in turn intermodulation products . As is clear from the above European application 0284979, transient effects cannot always be solved by a charge pump alone and just as it uses clamping this type of circuit is added to a charge pump generator in the French patent application published under No 2614724. In telecommunication line circuits of the type envisaged in the above European application 0201635, an efficient feed of the telecommunication lines is obtained by using synthesized impedances (AC and DC) and voltage CDC). In particular, the latter can vary widely between a minimum and a maximum value and with the normal DC telephone battery supply varying also between some limits, the synthesized DC feed voltage can be either lower or higher than the tiattery voltage at any moment. With negative values for both the battery and the synthesized voltage, in the case of an integrated circuit with junction isolation and with a p-doped substrate, the latter must be kept at the lowest, i.e. most negative potential. In principle, the clamping solution of the type disclosed in the above European application 0284979 could be used but available diode switches are not satisfactory for the purpose, since the voltage drop over a conductive diode would be too high . The invention is based on the insight that it is possible to select one out of two distinct DC voltages by a circuit which does not rely on clamping diodes but on appropriately controlled DMOS transistors. In accordance with a first feature of the invention, a voltage selector as initially defined is characterized in that each switching means has a control terminal distinct from its input and output terminals, with the two control terminals connected to respective outputs of a voltage comparator whose inputs are respectively coupled to the first and to the second input terminal Following a further feature of the invention, both towards the corresponding input terminal, the input voltage and the control voltage for the switching means, e.g. DMOS transistors, are oppositely poled. In such a way, it has been found possible to use transistor switches such as DMOS devices having an ON resistance of about 10 ohms and over which the potential difference is adequately low when conductive, i.e. small steady state current as opposed to surge current when becoming conductive. The invention also relates to a voltage comparator including a current source feeding two parallel branches each of which includes the main path of a transistor, with one input terminal of the comparator coupled to the control terminal of one transistor and the other input terminal coupled to the control terminal of the other transistor. one or the other transistor becoming conductive depending on the relative magnitudes of the voltages at the two input terminals . A voltage comparator of this type is generally known and can be found for instance in the European patent application published under 0086397 disclosing a differential current distributing circuit with the output currents being the collector currents of the two bipolar transistors whose bases constitute the input terminals. In some applications, particularly for a voltage selector of the type discussed above, it is desirable to use a comparator in an integrated circuit which is able to work with relatively high voltages. For this purpose, PMOS transistors with a drain extension allowing a voltage difference of up to 120 volts between drain and source are available but the maximum voltage between gate and source must not exceed 15 volts. A second object of the invention is therefore to realize a comparator in which one can control the voltage between the gate and the source of such transistors to avoid excessive amplitudes. In accordance with a second feature of the invention, the control terminals of the transistors part of the comparator are biassed by a second current source identical to the first and a decoupling diode is used in each coupling between an input terminal and the corresponding control terminal of the transistor. In this manner, with identical current sources for driving the sources of the two transistors as well as, through individual biassing resistances, the two gates of these transistors, one ensures that the maximum gate-to-source voltage for the transistors is that produced by the constant current for the gates flowing through the biassing resistance and this can be designed to remain below the maximum acceptable value for the transistor. The invention also relates to a voltage comparator including a current source feeding two parallel branches each of which includes the main path of a transistor in series with a resistance, with one input terminal of the comparator coupled to the control terminal of one transistor and the other input terminal coupled to the control terminal of the other transistor, one or the other transistor becoming conductive depending on the relative magnitudes of the voltages at the two input terminals. Such a circuit is also disclosed in the above European application 0086397 on a differential current distributing circuit. A third object of the invention is to use the voltage drop across the comparator resistances in such a way as to improve operations. In accordance with a third feature of the invention, each comparator resistance is in series with a diode connected transistor forming a current mirror with a further transistor whose main path is connected to the corresponding input terminal. Following a further feature of the invention, in a voltage selector as previously characterized and using a voltage comparator with resistances as characterized immediately above, the further transistors of the comparator constitute the switching means of the voltage selector and are made conductive as a result of the potential developed across the associated serial combination of a resistance with a diode connected transistor . In this manner, not only can the current amplification produced by the mirror circuits be used advantageously to render the switching means of the voltage selector conductive but one can ensure a design for the integrated circuit comparator which is not affected by some production tolerances. This is due to the diode connected transistor part of the current mirror constituting an adaptive load, in series with the associated resistance. When current flows through the diode connected transistor, this produces a gate-to-source voltage which can correspond to the gate-to-source voltage of the further or switching transistor when the same integrated circuit technology is used for all such transistors. Thus, with the additional drop of voltage across the resistance when the serially connected transistor with its interconnected gate and drain is conductive, this will ensure that the gate-to-source voltage of the further or switching transistor exceeds the threshold for conduction. It is to be noted that in the British patent 2135846, one already has a circuit using two current sources, one for feeding the commoned emitters of two transistors in separate branches and one coupled to the bases of such bipolar transistors through individual resistances. However, no indication is given regarding the relative characteristics of the two current sources and the circuit of the British patent is not a comparator reacting to the relative amplitudes of two input voltages but it is a current splitter distributing two output currents in accordance with the relative magnitudes of the two resistances and this independently of the load impedances. The same British patent also includes a current mirror coupled to the collectors of the above two transistors fed by the two current sources. But this single current mirror linking the two parallel branches cannot therefore be compared with the two current mirrors now proposed for a voltage comparator, with one in each of the two parallel branches. In accordance with yet another feature of the invention, a voltage comparator using two resistances as previously characterized includes a regenerative feedback circuit between the two resistances in the two parallel branches and constituted by a pair of regeneratively coupled transistors one with its main path across one resistance and the other with its main path across the other resistance, the control terminals of these regeneratively coupled transistors being respectively connected to the control terminal of the further or switching transistor in the other branch. In this manner, a faster transition time can be achieved for the voltage selector or comparator when the relative magnitudes of the input voltages is inversed. It is to be noted that in the above European application 0086397 on a differential current distributing circuit, the two branches are also coupled by two additional bipolar transistors having their collectors and gates cross coupled in regenerative fashion. Such transistors have however their main paths in series and not in parallel with the resistances in the two branches. The above mentioned and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawing which represents the voltage selector and the comparator of the invention. The circuit shown uses a BCD (Bipolar Cmos Dmos) tecnnology and essentially consists in the two DMOS selector transistors Nl and N2 operated so that their sources are both connected to the substrate, i.e. terminal VSUB, while their drains are connected to the input terminals VI and V2 respectively. The rest of the circuit shown constitutes the comparator COMP, as indicated by the interrupted lines, being fed between ground terminal GND and substrate * terminal VSUB as well as from the input terminals VI and V2, and COMP supplying output control signals at terminals Cl and C2 connected to the gates of transistors Nl and N2. While the actual values of the input DC potentials applied at terminals VI and V2 are application dependent, each may for instance independently vary within a range of the order of -20 to -60 volts. The switching transistors Nl and N2 are used in order to always maintain the potential at VSUB very close to the lowest negative value present either at terminal VI or V2. The description of the circuit can be simplified by noting that it is entirely symmetrical so that apart from the current source IS feeding the source-to-drain paths of the transistors PI and P2 in the two parallel branches and from current source 16 feeding the gates of these two transistors through biassing resistances Rl and R2 in the two parallel branches, all the other elements of COMP are paired, just as this is true for the transistor switches Nl and N2, odd reference numbers being used for the elements on the lefthand side and one being added to each such number for the corresponding even numbered element on the righthand side. Thus, apart from the current sources IG and IS constituted by identical PMOS transistors with a drain extension operated with their commoned sources connected to ground terminal GND and their commoned gates fed from reference current terminal I, another pair of identical transistors Pl/2 of the same type as IG and IS forms an essential part of COMP. The gate of P1/P2 is connected to terminal Vl/2 through decoupling diode Dl/2 which is in s.eries with resistor R3/4 in order to improve the matching of the two sides of the comparator. Control terminal Cl/2 at which the terminal of Pl/2 acting as drain and the gate of Nl/2 are connected is further coupled to terminal VSUB through diode connected DMOS transistor N3/4 acting as adaptive load in series with resistor R5/6. A further DMOS transistor N5/6 poled as Nl/2/3/4 has its drain-to-source path shunted across R5/6 and its gate cross-coupled to control terminal C2/1 in the other branch, N5/N6 providing positive feedback by a regenerative action in order to improve the gain and the speed of the circuit. Hence, the circuit can work at a relatively high voltage, i.e. up to 100 volts, in view of the nature of the potentials present at terminals VI and V2 while having a fast response time to transients and with the voltage drop across the switches Nl and N2 being always kept under the value corresponding to a forward biassed diode junction in the associated circuits. Considering now the operation of the circuit and assuming for instance that the potential at VI is more negative than at V2, this will lead to output terminal Cl of the comparator being activated in such a way as to saturate transistor switch Nl with the result that the most negative voltage at terminal VI will appear at output terminal VSUB. In a switching transition starting with the voltage at VI only slightly more negative than that at V2, more current will flow from ground at GND and IG through resistor Rl and which is much larger than equalizing resistor R3, than through resistor R2 on the other side, with the result that the gat -to-source voltage of PI is larger than the corresponding potential for P2, recalling that the floating ends of the two current sources IG and IS are at the same potential when they are identical. This action will entail more current flowing through the main path of PI than through the corresponding part of P2 and the ensueing -higher voltage at Cl, developed over R5, as compared to C2 will be amplified due to the cross-couplings involving N5/6. Indeed, more potential will be developed across resistor R5 on the left side so that N6 on the right side will be more conductive than N5 thereby effectively reducing the load impedance at C2 in view of resistance R6 being shunted by transistor N6 , Thus, this cumulative effect will rapidly ensure saturation of Nl, the full currents from the two sources IG and IS flowing through he left side of the circuit. With the identical constant current sources SG and SS, one ensures that the maximum voltage between the gate and the source of such a transistor as Pl/2 can never exceed the potential across such a biassing resistor as R2, isolated from the input at V2 through decoupling diode D2, and one may resort for PI and P2, as well as for IG and IS, to the use of PMOS transistors with a drain extension which allow a potential between source and drain of up to 120 volts provided the maximum voltage between gate and source does not exceed 15 volts. The diode-connected transistor N3/4 constitutes an adaptive load ensuring, when current flows through the transistor, a drain-to-source voltage slightly above the threshold voltage of the transistor. In a given integrated circuit, the threshold voltages of MOS transistors using the same technology are almost equal so that such a diode connected transistor in series with a resistance presents a gate voltage to the transistors Nl/2 and N6/5 which is adequately above the threshold voltage, regardless of production tolerances. Apart from the advantages of the circuit already recited previously, the configuration is also * of particular advantage with regard to the parasitic diodes. Indeed, the essential parasitic elements of this type will be constituted by the reverse diodes across the source-to-drain path as well as by the reverse diodes across the source-to-gate junction of Nl/2 and in such a way that both types have no detrimental but on the contrary favourable effects. The first mentioned spurious diodes are poled as clamping diodes so that any failure in the circuit will still leave the substrate clamped to the more negative potential present at Vl/2 but with a more significant potential difference between VSUB and VI or V2 than that explained above. The second mentioned spurious diodes also have a beneficial effect since during a switching transition, on a break before make basis, any transient voltage peak at VSUB cannot create an undesirably high potential difference across the gate-to-source junction of Nl/2 since the spurious diodes will act as short circuits for this positive transient pulse. Simulation results for the described circuit have indicated that the gate-to-source voltages of transistors Pl/2 can be inversed in about 20 milliseconds for a transition caused by a synthesized line circuit voltage at on of the terminals Vl/2 which varies linearly from 0 to -60 vϋlta in 100 milliseconds while the battery potential is maintained at -50 volts at the other terminal V2/1, the load between substrate terminal VSUB and ground terminal GND being 1 milliampere. While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

Claims: CLAIMS 1) Voltage selector including first and second input terminals (Vl/2) connected via respective switching means (Nl/2) to an output terminal (VSUB) with the first or the second switching means being made conductive in accordance with the relative amplitude of the voltages at the two input terminals, characterized in that each switching means has a control terminal distinct from its input and output terminals, with the two control terminals connected to respective outputs (Cl/2) of a voltage comparator whose inputs are respectively coupled to the first and to the second input terminal 2) Voltage selector as claimed in claim 1, characterized in that, both towards the corresponding input terminal, the input voltage and the control voltage for the switching means, e.g. DMOS transistors, are oppositely poled. 3) Voltage comparator including a current source CIS) feeding two parallel branches each of which includes the main path of a transistor CP1/2), with one input terminal (VI) of the comparator coupled to the control terminal of one transistor and the other input terminal CV2) coupled to the control terminal of the other transistor, one or the other transistor becoming conductive depending on the relative magnitudes of the voltages at the two input terminals, characterized in that the control terminals are biassed by second current source (IG) identical to the first and a decoupling diode (Dl/2) is used in each coupling between an input terminal and the corresponding control terminal of the transistor. 4) Voltage comparator including a current source CIS) feeding two parallel branches each of which includes the main path of a transistor CP1/2) in series with a resistance CR5/6), with one input terminal CV1) of the comparator coupled to the control terminal of one transistor and the other input terminal CV2) coupled to the control terminal of the other transistor, one or the other transistor becoming conductive depending on the relative magnitudes of the voltages at the two input terminals, characterized in that each comparator resistance is in series with a diode connected transistor CN3/4) forming a current mirror with a further transistor CN1/2) whose main path is connected to the corresponding input terminal. 5) Voltage selector as claimed in claim 1 using a voltage comparator as claimed in claim 4, characterized in that the further transistors of the comparator constitute the switching means of the voltage selector and are made conductive as a result of the potential developed across the associated serial combination of a resistance with a diode connected transistor. 6) Voltage comparator as claimed in claim 4, charaterized in that it includes a regenerative feedback circuit circuit between the two resistances in the two parallel branches and constituted by a pair of regeneratively coupled transistors CN5/6), one with its main path across one resistance and the other with its main path across the other resistance, the control terminals of these regenerat ively coupled transistors being respectively connected to the control terminal CC2/C1) of the further of switching transistor in the other branch. 7) Voltage comparator as claimed in claim 3, characterized in that the transistors biassed by the second current source are of the PMOS type with a drain extension. 8] Voltage comparator as claimed in claim 3, characterized in that the identical current sources are constituted by PMOS transistors with a drain extension. 9) Voltage comparator as claimed in claim 4, characte ized in that the diode connected transistor and the further transistor are of the DMOS type. 10) Voltage comparator as claimed in claims 6 and 9, characterized in that the regeneratively coupled transistors are of the DMOS type.

Inventors: WILLOCX EDDIE LOUIS MARIE, WILLOCX, EDDIE, LOUIS, MARIE
Applicants: ALCATEL NV, BELL TELEPHONE MFG, ALCATEL N.V., BELL TELEPHONE MANUFACTURING COMPANY, NAAMLOZE VEN
Application year: 1989
Main classification: H03K5
IPC codes: G01R19, H03K5, H03K17