Method and corresponding circuit to prevent a parasitic transistor turn on in an output stage of an electronic circuit

Abstract: The invention relates to a method, and related circuit, for preventing the triggering of a parasitic transistor in an output stage (2) of an electronic circuit, said stage (2) comprising a transistor pair (M1,M2) with at least one transistor (M2) of the pull-up PMOS type having respective source, gate and drain terminals and a body terminal, and a parasitic bipolar transistor (3) having a terminal connected to said body terminal, characterized in that it comprises the steps of: providing a capacitor (C1) connected between the body and source terminals of the PMOS transistor; using a control circuit (5) to suppress the body effect of the pull-up PMOS transistor.

Description: This invention relates to a method and related auxiliary circuit for preventing the triggering of a parasitic transistor in an output stage of an electronic circuit. More particularly, the invention relates to a method for preventing the triggering of a parasitic transistor in an output stage of an electronic circuit, said stage comprising a transistor pair with at least one transistor of the PMOS pull-up type having respective source, gate and drain terminals and a body terminal, and a parasitic bipolar transistor having a terminal connected to said body terminal. Background Art As is well known, a final output stage of a generic electronic circuit, of any degree of complexity, basically comprises a driver portion arranged to deliver electric power to a load whatever. Therefore, the stage must be capable of draining or drawing current from the load according to necessity and the type of application. Countless configurations have been devised for state-of-art output stages. Shown schematically in the accompanying Figure 1 is one of the most common output stage configurations. The stage of Figure 1 comprises a complementary pair of MOS transistors connected in series between a first voltage reference Vdd and a second voltage reference Vss, wherein the latter may be a negative supply or ground. The first transistor M1 in the complementary pair is a pull-down transistor of either the NMOS or the DMOS type, and has its body terminal connected to the source terminal. The second transistor M2 in the complementary pair is a pull-up transistor of the PMOS type, and has its body terminal connected to the source terminal. Respective driving stages have respective outputs connected to the gate terminals of the transistors M1 and M2. The two transistors, M1 and M2, are connected together by their drain terminals, which are coincident with an output node OUT. An electric load LOAD is connected between the output node OUT and a ground. The load LOAD is driven alternatively by the PMOS transistor M2 or the NMOS transistor M1, depending on the different sourcing or sinking operational steps. The complementary pairs of MOS transistors are those most widely used for implementing output stages on account of the clear advantages that they can offer as concerns the control logics; however, the considerations made hereinafter can also apply to output stage configurations which incorporate bipolar transistors or other pairs of MOS transistors. Figure 2 shows, for example, an output stage incorporating a pair of transistors which are both of either the NMOS or the DMOS type. These transistors M3, M4 are connected together at the output node OUT. Some applications require that the current be looped back from the load to the positive and/or negative supplies. This is necessary, for instance, in the field of flat panels, where a high voltage pulser generates a pulsed supply. Figure 4 shows schematically an embodiment wherein two, respectively positive and negative pulsers supplying respective voltage references are associated with the circuit of Figure 1. During the rising edge of the supply signal, energy is delivered to the load, whereas during the signal falling edge, the load returns part of its energy to the supply line. Figure 3 shows schematically the scalene trapezoid pattern of the supply voltage during the periods when such energy transferrments occur. The current loop-back is allowed by the bi-directional operability of the components, namely the PMOS; NMOS or DMOS transistors, employed. However, each MOS component has a parasitic diode associated therewith which affects operation in the reverse range. As to the PMOS transistor, also present beside the parasitic diode formed by the drain-body p-n junction is a parasitic bipolar transistor of the vertical pnp type which is formed by the drain-body-substrate junctions. Unfortunately, this parasitic bipolar transistor is also active during the operation of its associated MOS transistor, and instead of contributing to the current loop-back, will contribute to dissipating the current flowing through it to the substrate. The above situation, additionally to being unfavorable from the viewpoint of energy recovery, may harm the output stage because the substrate potential can rise locally to the point of triggering more parasitic elements. Thus, it becomes necessary to limit the gain of the parasitic transistor, e.g. by keeping its beta coefficient low using suitable techniques. Another solution could be that of "freezing" the areas around the PMOS transistor by means of appropriate substrate taps. Other, more radical solutions provide for the use of SOI or dielectric insulation technologies. EP-A-0 346 898 discloses a solution of preventing turn-on of a parasitic diode in the pull-up stage by using a level shifter which raises the gate-source voltage of the pull-up MOS transistor and an additional MOS transistor. US-A- 5 422 591 discloses CMOS output driver with body bias control. The underlying technical problem of this invention is to provide a method and related circuit, which have such functional and structural features as to prevent the triggering of parasitic transistors during the driving of an output stage. Summary of the Invention The solution idea behind the present invention is to prevent activation of the drain-body diode, or at least reduce substantially the current carried thereby. The idea is materialized by leaving the body terminal of the PMOS transistor in the complementary pair that forms the output stage disconnected from the source terminal, and connecting it to a "pull-up" capacitor connected between the body and source terminals. Based on this solution idea, the technical problem is solved by a driving method as previously indicated and defined in the characterizing portion of Claim 1. The technical problem is also solved by an auxiliary circuit as defined in Claims 4. The features and advantages of the method and the circuit of this invention will be apparent from the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figures 1, 2 and 4 are respective schematic views of final output stages according to the prior art; Figure 3 shows a waveform of a signal present within the stage of Figure 2; Figure 5 is a schematic view of a part of an auxiliary circuit used in this invention; Figure 6 is a schematic detail view of the driving stage shown in Figure 5; Figure 7 shows an exemplary application of the method and the circuit of this invention; Figure 8 shows an exemplary application of the invention to a plurality of output stages. Detailed Description Referring to the drawings, in particular to the example in Figure 5, generally and schematically shown at 1 is an auxiliary or body circuit embodying a part of this invention, for a final output stage 2 of an electronic control circuit. The stage 2 is adapted to drive an electric load LOAD connected between an output node OUT of the stage 2 and a ground. The stage 2 preferably comprises a complementary pair of MOS transistors, M1 and M2, which are substantially identical with those discussed in the preamble to this Application with reference to Figure 1. Accordingly, no detailed description of the stage 2 will be given hereinafter and reference will be made to the description of Figure 1 given hereinabove. In particular, attention is drawn on that schematically shown at 3 in Figure 5 is the parasitic bipolar transistor associated with the PMOS transistor M2 in the complementary pair of the stage 2. This transistor 3 of the pnp type has its emitter terminal connected to the drain terminals of the complementary pair, base terminal connected to the body terminal of the PMOS transistor M2, and collector terminal connected to ground. The auxiliary circuit 1 is connected to the stage 2, and comprises a capacitor C1 connected between the body and source terminals of the PMOS transistor M2 in the complementary pair incorporated to the stage 2. The circuit 1 further comprises a diode DJ connected with one end to the body terminal of the transistor M2, and hence to the base terminal of the parasitic transistor 3. The diode DJ has the other end free to receive a voltage signal denoted by Vbody. Advantageously in this invention, the construction of the circuit 1 leaves the body terminal of the PMOS transistor M2 substantially disconnected from the source terminal. This body terminal is led to the "pull-up" capacitor C1, which is connected between the body and source terminals of the transistor M2. This capacitor C1 is precharged, that is charged before the pulse, through the diode DJ whenever the high voltage supply happens to be low. In fact, the diode is connected between one end of the capacitor C1 and a DC potential line at a low voltage of predetermined value. As the pulsive supply rises, the body terminal of the transistor M2 is also brought up to maintain a permanent body effect on the PMOS transistor. This effect may be regarded as disadvantageous, but can be accommodated by a higher driving overvoltage; that is, by an appropriate drop in the voltage Vgate-source across the gate and source terminals. On the falling edge of the pulsive signal, the voltage at the drain terminal of the PMOS transistor M2, acting as the source terminal, can instead be held below the voltage at the body terminal by suitably dimensioning the PMOS transistor M2 and applying a preselected suitable voltage Vbody. In this way, the junction would not become activated, or draw any significant amount of current if somehow turned on. As previously mentioned, the auxiliary circuit of this invention, while solving the technical problem, has the disadvantage of imposing a body effect on the PMOS transistor during the rising edge of the supply signal. This raises the threshold voltage (Vth) of the PMOS transistor, and lacking the boosting feature, would require that the PMOS transistor be over-designed to supply the same current. To obviate this drawback, a body control circuit 5 has been introduced for shorting the body terminal of the transistor M2 to the source terminal thereof during the rising edge. This suppresses the body effect. Figure 6 shows schematically the general layout of the interconnections between the stage 2 and driving and control circuitry according to this invention. A level shifter and driving stage 4 has at least one output connected to the control terminal of the transistor M2 in the stage 2. The body circuit 1 has an output connected to the body terminal of the transistor M2, and the body control circuit 5 has an output connected to the body circuit 1. In Figure 5, the stage 4 is shown duplicated in a stage 8 for driving the transistor M1. Figure 7 shows in greater detail the internal construction of the body control circuit 5. The shifter stage 4 (e.g. in Fig. 6) holds M1 (e.g. in Fig. 6) on and M2 off throughout the period of the positive supply pulse Vdd. The circuit stage 4 can be classed substantially as a high voltage level shifter transferring logic information to the two MOS switches, M1 and M2. The body control circuit 5 comprises a differential cell level shifter which includes a pair of input DMOS transistors, M3 and M4. An input signal CON is applied directly to the transistor M3, and in inverted form through an inverter 6 to the transistor M4. The differential cell of the circuit 5 has on its output a pair of NMOS transistors M5, M6 whose gate terminals are connected together and to a voltage reference Vref. These transistors are voltage-effective and can ensure shifting of the signal from a relatively low voltage V5 to a higher voltage related to Vdd+Vbody. In addition, a further pair of transistors M7, M8 are connected to form a flip-flop storage element 7, with the control terminal of the transistor M7 being connected to the drain terminal of the transistor M8, and vice versa. This storage element is effective to store On/Off information. The On/Off information is also passed to two switches Ia and Ib operated in phase opposition, that is one of them will be open when the other is closed. The connection between the storage element 7 and the switches Ia and Ib is established through two inverters INV1 and INV2, each comprising a transistor pair. MOS transistors M10 and M11 are provided in the inverter INV1, and MOS transistors M12 and M13 in the inverter INV2. The inverters INV1 and INV2 are cascade connected and biased by the voltage across the pull-up capacitor C1 of the body circuit 1. During the rising edge of the supply signal Vdd, the switch Ib is in the on state and shorts together the body and source terminals of the transistor M2. During the falling edge of the supply signal Vss, the switch Ia is in the on state and the body terminal of the transistor M2 is connected to the positive potential terminal of the pull-up capacitor C1. Thus, the body terminal is biased to a voltage equal to the voltage drop across the previously charged capacitor C1, and the body circuit 1 is allowed to perform its function of preventing the triggering of the parasitic transistor in the manner described above. The arrangement just described represents but one exemplary application, it being non-limitative as concerns the operation of this invention. A noteworthy advantage of this invention is that in multiple output driving circuits, such as those used for flat panels where plural output stages are utilized as drivers for rows and columns of cell arrays having 40 or 64 outputs, for example, the body terminal locates in a common well to all of the PMOS transistors in the output stages. In this way, a single control circuit 5 can be used for the whole integrated circuit incorporating the plurality of stages. Figure 8 illustrates schematically this possible situation where plural output stages, three stages in the example, are arranged in parallel with one another, connected to a single pull-up capacitor C1, and employ a single control circuit 5. The capacitor C1 could even be shared by a number of integrated circuits. Similar considerations would also apply to the diode DJ, which can be of either the integrated or the discrete type for charging the capacitor C1. This capacitor should be designed to deliver a sufficient charge to all the wells connected thereto, where the latter must be charged during the supply pulse.

Claims: A method for preventing the triggering of a parasitic transistor in an output stage (2) of an electronic circuit, said stage (2) comprising a transistor pair (M1,M2) with at least one transistor (M2) of the pull-up PMOS type having respective source, gate and drain terminals and a body terminal, and a parasitic bipolar transistor (3) formed by the drain - body junction, characterized in that it comprises the steps of: providing a capacitor (C1) connected between the body and source terminals of the PMOS transistor; using a control circuit (5) to suppress the body effect of the pull-up PMOS transistor by shorting the body terminal and the source terminal of the PMOS transistor during a rising edge of each supply voltage pulse supplied to the source terminal; and further comprising connecting one end of a diode (DJ) to the terminal of the capacitor (C1) which is connected to the body terminal, and having the other end free to receive a voltage potential (Vbody). A method according to Claim 1, characterized in that said control circuit (5) comprises a level shifter (M3, M4, M5, M6) associated with a storage element (M7, M8) and connected to a pair of switches (Ia, Ib) operated in phase opposition, with one switch (Ib) being connected between said source and body terminals, and the other switch (Ia) being connected between said body terminal and the connection node of said capacitor and said diode. A method according to Claim 1, characterized in proving a further control circuit (4, 8) implementing the structure of said control circuit such that a pair of outputs is connected to drive the respective control terminals of said transistors (M1,M2). An auxiliary circuit for preventing the triggering of a parasitic transistor in an output stage of an electronic circuit, said stage (2) comprising a transistor pair (M1,M2), wherein at least one transistor (M2) is of the pull-up PMOS type having respective source, gate and drain terminals and a body terminal, and a parasitic bipolar transistor (3) formed by the drain - body junction, characterized in that it comprises a capacitor (C1) connected between the body and source terminals of the PMOS transistor, a diode (DJ) having one end connected the terminal of the capacitor (C1) which is connected to the body terminal, and the other end free to receive a voltage potential (Vbody), and a control circuit (5) for suppressing the body effect of the pull-up PMOS transistor, in that the control circuit comprises a switch (Ib) being connected to short the body terminal and the source terminal of the PMOS transistor during a rising edge of each supply voltage pulse supplied to the source terminal. A circuit according to Claim 5, characterized in that said control circuit (5) comprises a level shifter (M3, M4, M5, M6) associated with a storage element (7) and connected to a pair of switches (Ia,Ib) operated in phase opposition, with one switch (Ib) being connected between said source and body terminals, and the other switch (Ia) being connected between said body terminal and the connection node of said capacitor and said diode. A circuit according to Claim 5, characterized in that said storage element (7) incorporated to the control circuit (5) is a MOS transistor flip-flop (M7, M8). A circuit according to Claim 5, characterized in that said control circuit (5) is connectable to a plurality of output stages in an integrated circuit.

Inventors: DEPETRO RICCARDO, MARTIGNONI FABRIZIO, SCIAN ENRICO, DEPETRO, RICCARDO, MARTIGNONI, FABRIZIO, SCIAN, ENRICO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: H03K17
IPC codes: H01L21, H03K19, H01L27, G11C11, H03K17, G05F3