High voltage level shifter for driving an output stage

Abstract: The invention relates to an electronic level shifter circuit (1) for driving a high-voltage output stage (2). This output stage (2) comprises a complementary pair (3) of transistors (I36,I32) connected between first (Vdd) and second (Vss) supply voltage references, and at least one PMOS pull-up transistor (I32) connected in series with an NMOS pull-down transistor (I36). An additional transistor (I34) is connected in parallel with the pull-up transistor (I32), and the driver circuit (1) has a first output (A) connected to the control terminal of the pull-up transistor (I32) and a second output connected to the control terminal of the additional transistor (I34).

Description: This invention relates to an electronic level shifter circuit for driving a high-voltage output power stage. More particularly, the invention relates to an electronic level shifter circuit for driving a high-voltage output stage, said stage comprising a complementary pair of transistors connected between first and second supply voltage references, and including at least one PMOS pull-up transistor connected in series to an NMOS pull-down transistor. Background Art As is well known, a final output stage of a generic electronic circuit basically includes a drive portion intended for powering a load whichever. Thus, the output stage should be capable of draining or taking up current from the load, according to necessity and the type of application involved. A buffer circuit for a semiconductor integrated circuit is known from the EP application No. 0 238 358 in the name of Toshiba. Countless configurations have been proposed in the prior art for the output stage. Figure 1 attached hereto shows schematically one of the commonest of such output stage configurations. The stage shown in Figure 1 comprises a complementary pair of MOS transistors connected in series with each other, between a first voltage reference Vdd and a second voltage reference Vss, wherein the latter may either be a negative supply or a ground. Complementary pairs of MOS transistors are those more often employed in output stages on account of the definite advantages that they afford in the respect of control logics; however, the considerations made herein below would also apply to output stage configurations incorporating bipolar transistors or any other pairs of MOS transistors. The first transistor M7 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 M6 in the complementary pair is a pull-up transistor of the thin oxide PMOS type, and has its body terminal connected to the source terminal. The transistors M6 and M7 are connected to each other through their respective drain terminals, which terminals coincide with an output node OUT. An electric load, not shown, is connected between the output node OUT and ground. The electric load is driven alternately by the PMOS transistor M6 or the NMOS transistor M7, according to whether the operation phase is one of sourcing or of sinking. Respective driving stages have their respective outputs connected to the gate terminals of the transistors M6 and M7. Only the driving stage of the PMOS transistor M6 is shown in Figure 1. Figure 2 shows a second example of an output stage incorporating a pair of NMOS and PMOS transistors M15 and M23. These transistors are again connected together at the output node OUT, and a driving stage of greater complexity than in the previous example of Figure 1 is provided for the PMOS transistor M23. Figure 3 shows schematically a further embodiment wherein all the PMOS transistors shown in Figure 2 are replaced with similar thick oxide PMOS power transistors. For many applications, output stages which can operate in both low driving voltage and high driving voltage conditions would be desirable. However, for the time being, the state of the art provides no solutions which can efficiently fill the demand in terms of performance as well as of reduced occupation of circuit area. The underlying technical problem of this invention is to provide an output stage and associated level shifting driver circuit with such respective functional and structural features as to be usable in low- and high-voltage applications and to have reduced circuit area requirements. Summary of the Invention The solving idea behind the present invention is one of providing an output stage which incorporates both a thin gate oxide PMOS transistor and a thick field oxide PMOS transistor, and of having both transistors driven from the same level shifting driver circuit. Based on this solving idea, the technical problem is solved by a circuit as previously indicated and defined in the characterizing portion of Claim 1. The features and advantages of the circuit according to 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. In the drawings: Brief Description of the Drawings Figures 1, 2 and 3 are respective schematic views of final output stages according to the prior art, as driven by respective driver circuits; Figure 4 is a schematic view of a driver circuit according to this invention; Figure 5 is a schematic view of an output stage and associated driver circuit according to this invention. Detailed Description Referring to the drawing figures, in particular to the example of Figure 5, generally and schematically shown at 1 is a level shifter circuit for use, in accordance with this invention, with a final output stage 2 of an electronic circuit. The stage 2 is a power stage intended for driving an electric load connected between an output node OUT of the stage 2 and ground, for example. The stage 2 is shown in detail in Figure 5 as comprising preferably a complementary pair 3 of MOS transistors I32, I36 and a third MOS transistor I34. The transistors in the pair 3 are connected in series with each other, between a first voltage reference Vdd and a second voltage reference Vss, it being possible for the latter to either be a negative supply or ground. The first transistor I36 in the complementary pair 3 is a pull-down transistor of the NMOS type having its source terminal connected to the second supply voltage reference Vss and its gate terminal arranged to receive a signal PD1. The second transistor I32 in the complementary pair 3 is a pull-up transistor of the thin oxide PMOS type. This transistor I32 is connected in parallel with the third transistor I34 and has its source terminal connected to the first supply voltage reference Vdd and its body terminal connected to the body terminal of the third transistor I34. In a preferred embodiment, these two body terminals can be formed in the semiconductor, in common with the transistors I32 and I34 inserted into the the same well in the semiconductor, with the source and drain terminals in common. The transistors I36 and I32 are connected to each other through their respective drain terminals being coincident with the output node OUT. Advantageously, the third transistor I34 is a thick oxide PMOS power transistor, and has its source and drain terminals connected to the first voltage reference Vdd and the output node OUT, respectively. The gate terminal of the first PMOS transistor I32 is connected to a first output A of the driver circuit 1, and the gate terminal of the third PMOS transistor I34 is connected to a second output of the driver circuit 1. The driver circuit 1 can be classed essentially as a high-voltage level shifter which transfers the logic information to the three MOS switches I32, I34 and I36. Figure 4 is a more detailed view of the internal construction of the driver circuit 1, which comprises a shifter circuit stage equipped with a differential cell level shifter comprising a pair of DMOS input transistors I44 and I51. An input signal PU1 is applied to the transistor I44 directly, and in inverted form to the transistor I51 via an inverter INV. The differential cell of the circuit 1 has on its output a pair of thick oxide PMOS power transistors. A first transistor I30 is connected in series with the input transistor I44 and forms a complementary pair therewith. The transistor I30 has its drain terminal connected to the drain of the NMOS transistor I44, to form the first output A of the circuit 1, which output is connected to the gate of the transistor I32 in the stage 2. A parallel of a Zener diode D1 and a resistor R1 are provided between this output A and the first supply reference Vdd. The PMOS transistor I30 has its source terminal connected to the first supply reference Vdd and its body terminal connected to the source terminal. The gate of the transistor I30 is connected to the drain terminal of a second output transistor I31 forming a second complementary pair with the second NMOS input transistor I51. The transistor I31 is a thick oxide PMOS transistor, and has its drain terminal connected to the drain of the transistor I51, its source terminal connected to the first supply reference Vdd, and its body terminal connected to the source terminal. The construction of the circuit 1 is completed by an additional transistor I29 of the thick oxide PMOS type, which transistor has its source terminal connected to the first supply reference Vdd and its body terminal connected to the source terminal. The gate terminal of the transistor I29 is referenced Gthi in the figure and is connected to the gate terminal of the transistor I30, as well as to the drain terminal of the transistor I31. The drain terminal of the transistor I29 is connected to the drain terminal of an NMOS transistor I5 to form a second output B of the circuit 1 for driving the control terminal of the thick oxide transistor I34 in the output stage. The signal produced on the output B will be referenced GATE_TH hereinafter. The output node B is also connected to the gate terminal of the transistor I31. In addition, the source terminal of the transistor I5 is in common with the source terminals of the transistors I44 and I51, and receives the second reference potential Vss. The gate terminal of the transistor I5 receives directly the same control signal PU1 as the transistor I44. The foregoing structure is but a non-limitative example given to illustrate a possible application. This structure is not binding for the invention operation. The operation of the driver circuit 1 according to the invention will now be described. Advantageously, the circuit 1 can operate on either a constant supply or a pulsive type of supply, that is with the voltage Vdd oscillating between the ground value and a relatively high positive value, and with the voltage Vss oscillating between the ground value and a relatively high negative value. In the constant supply case, the voltages are kept stable, and the output stage 2 allows the load to be applied either the voltage Vdd or the voltage Vss. The PMOS transistors I32 and I34 in the output stage 2, once enabled, allow a voltage to be applied to the node OUT for powering the load. When the signal PU1 is at a low logic value, the transistor I44 is in the off state, and the transistor I51 is turned on. This situation allows the gate terminal Gthi to fall as much as possible toward the voltage value Vss, thereby allowing the transistor I30 to be turned on with a very strong overdrive on the threshold voltage. At this point, the gate terminal of the PMOS transistor I32 will go to a lower voltage than the threshold voltage, and turn off this transistor. The transistor I30 of the circuit 1 is suitably sized to provide a voltage drop from Vdd effective to turn off the transistor I32. The third transistor I34 in the output stage 2 receives the control signal GATE_TH, which is pulled to a high logic value toward the supply Vdd by the action of the transistor I29 driven by the signal Gthi. It is interesting to observe that the output stage 2 contains no protection elements, such as are instead provided between the gate terminal of the transistor I32 and the supply Vdd as parts of the circuit 1. The zener Z1 and resistor R1 allow the maximum variation of the voltage applied to the gate terminal of the transistor I32 to be clamped. As the signal PU1 goes to a high logic value, the transistors I44 and I5 are activated, and the transistor I51 is turned off. Presented to the gate terminal Gthi is a high logic signal impressed by the transistor I31, which transistor has its gate terminal subjected to a rapid fall of potential GATE_TH due to the action of the transistor I5, suitably sized for this purpose. As the signal Gthi rises, the transistor I30 tends to go off, thereby forcing the first output A to a value of potential which is dependent essentially on the presence of the zener Z1. The value of potential of the second output B will also be changed, because of the signal GATE_TH being brought close to the value Vss. During the power-off phase, in order to discharge voltage from the output load, or to connect the load to a negative voltage, it will be sufficient that the PMOS transistor of the circuit 1 be deactivated to bring the signal PU1 to a low logic value and activate the power transistor I36 in the stage 2 by pulling the signal PD1 high. The circuit 1 may also be operated in a pulsed mode, by activating the signal PU1 before the raising of the supply Vdd pulse. In this way, both PMOS transistors I32 and I34 will be activated upon the supply voltage Vdd being raised over their threshold voltage. During this first operation phase, the body terminals of both transistors would be connected to the supply Vdd, e.g. as described in European Patent Application No. 97830327.9 by this Applicant. In view of the different physical constructions of the transistors I32 and I34 in parallel, their threshold voltages are different due to the oxide thickness, and only one of them will be activated initially. But upon the value of the supply Vdd reaching its maximum, the voltage at the body terminal may be brought over the maximum supply voltage such that a possible current loop-back is enabled without the parasitic elements of either transistors becoming activated. This arrangement is particularly advantageous in that some of the current from the load can be recovered and redirected to the supply. During the negative pulse half-wave, the signal PD1 is activated with the transistors I32 and I34 held in the off state. Consequently, the DMOS transistor I36 will be turned on, thereby allowing the output to follow the negative voltage Vss.

Claims: An electronic level shifter circuit (1) for driving a high-voltage output stage (2), which stage (2) comprises a complementary pair (3) of transistors (I36,I32) connected between first (Vdd) and second (Vss) supply voltage references, and including at least one PMOS pull-up transistor (I32) connected in series with an NMOS pull-down transistor (I36), characterized in that said output stage (2) comprises an additional transistor (I34) connected in parallel with the pull-up transistor (I32), and that said level shifter circuit for driving the output stage (1) has a first output (A) connected to the control terminal of the pull-up transistor (I32) and a second output connected to the control terminal of said additional transistor (I34), said level shifter circuit for driving the output stage (1) comprising a differential cell level shifting stage, in turn including a pair of MOS input transistors (I44,I51) receiving an input signal (PU1) and the inverted input signal, respectively, said differential cell having an output portion with a set of thick oxide PMOS power transistors. A circuit according to Claim 1, characterized in that a parallel of a Zener diode (D1) and a resistor (R1) are provided between said first output (A) and the first supply voltage reference (Vdd). A circuit according to Claim 1, characterized in that said output portion comprises a first PMOS transistor (I30) connected in series with a first input transistor (I44) to form a complementary pair therewith and, at the interconnection node, the first output (A) of the level shifter circuit for driving the output stage (1), which output is connected to the pull-up transistor (I32) in the output stage (2). A circuit according to Claim 1, characterized in that said output portion further comprises a second power transistor (I31) connected in series with the second input transistor (I51) to form a second complementary pair, a third power transistor (I29) being arranged with its control terminal connected to the interconnection node between the transistors (I31,I51) of said second pair and a conduction terminal forming said second output (B) of the level shifter circuit for driving the output stage (1) connected to said additional transistor (I34) in the output stage (2). A circuit according to Claim 1, characterized in that the power transistors (I30,I31,I29) of said output portion are all thick oxide PMOS transistors. A circuit according to Claim 5, characterized in that the control terminal of said second power transistor (I30) is connected to said second output (B).

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: H03F1
IPC codes: H03K17, H03K19