High voltage tolerance output stage

Abstract: An output stage (1) for electronic circuits (2) with high voltage tolerance and of the type comprising an output buffer made up of a complementary transistor pair (Pu,Nu) comprising a P-channel MOS pull-up transistor (Pu) and an N-channel MOS pull-down transistor. The transistors are connected together to make up an output terminal (U) of the stage which comprises in addition a switch (6) having an input (8) connected to the output terminal (U) of the stage and an output (9) connected to the control terminal of the pull-up transistor to drive said control terminal in a state of extinction of the output buffer.

Description: Field of application The present invention relates to an output stage for high voltage tolerance electronic circuits. Specifically but not exclusively the present invention relates to an output stage for high voltage tolerance electronic circuits with protection device against undesired starting up and of the type comprising an output buffer made up of a complementary pair of transistors comprising a P-channel MOS pull-up transistor and an N-channel MOS pull-down transistor with these transistors being connected mutually to form an output terminal of the stage. As known to those skilled in the art, some electronic devices integrated on semiconductor and supplied with low supply voltage, e.g. 3 V, must sometimes be interconnected with other electronic devices operating with higher supply voltages, e.g. 5 V. This fact involves some problems in the use and management of resources common to both types of device. A typical example is represented by the so-called data bus which is shared by different digital systems incorporating electronic circuits operating at 3 V and electronic circuits operating at 5 V. In digital systems of this type there is normally present a logical control unit, e.g. a CPU (Central Processing Unit), which emits enable and disable signals for the various circuits sharing the data bus. Specifically each circuit has control inputs and comprises a final output stage incorporating an output buffer made up essentially of a complementary pair of pull-up and pull-down circuits connected to the data bus. When a given circuit has to use the data bus the CPU acts on the OE# (Output Enable) or CE (Chip Enable) signal inputs of the unused circuits to force the corresponding output buffers into an off state, or 'tristate'. A diagram of this situation is shown in the annexed FIG 1 illustrating a group of electronic circuits sharing the same data bus even though operating with different supply voltages. In FIG 1 are also indicated the bias voltages applied to the various output buffers when one of the circuits (in the example, the one with high power supply) is using the data bus. From FIG 1 it can be seen that the pull-up transistors of the output buffer turned off have their respective gate and body terminals powered by a voltage Vcc of 3 V. When the high power device (5 V) is operative the voltage on the data bus cannot reach the high logical level, i.e. 5 V, but is blocked at a Vcc value (3V) + min [|Vthp|, Vγ], where the latter are the threshold voltage of a PMOS transistor and the drain-bulk voltage drop respectively. In the case where |Vthp| < Vγ, the PMOS transistor turns on. Otherwise the NMOS transistor turns on. This involves an undesired consumption by the active device, which is forced to power the currents I1 and I2. For example, FIG 2 shows in enlarged scale and vertical cross section the internal structure of a P-channel MOS transistor provided in an N well. The symbols S, G and D indicate respectively the terminals of the source, gate and drain regions. B indicates the body or bulk terminal of the transistor. Prior art The prior art already offers a solution seeking to obviate the above mentioned shortcomings. This solution consists of providing a diode connected in parallel to the P-channel pull-up transistor of the output buffer. This diode is directly biased like the drain-bulk junction but turns on first because of an expected lower threshold voltage. An output stage including a pair of complementary transistors in configuration pull-up/pull-down is known from the US Patent No. 5,451,889 to Heim et al. Such an output stage comprises a further transistor connected between the output terminal of the stage and the control terminal of the pull-up transistor to drive said control terminal when the output voltage exceeds a predetermined range. Other circuit solutions of known type do not protect the drain-bulk junction without causing consumption of the circuit. These meet the High Voltage Tolerance specification but consume current during stand-by operation. The technical problem underlying the present invention is to conceive an electronic circuit output stage having high voltage tolerance and also having structural and functional characteristics allowing protection against undesired turning on which would also make current consumption virtually null when the output buffer of the stage is off. This would allow overcoming the limits and shortcomings of the present solutions offered by the prior art. Summary of the invention The technical problem is solved by an output stage of the type indicated above and defined in the characterizing part of claims 1 and following. The characteristics and advantages of the stage in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings. Brief description of the drawings FIG 1 shows a diagrammatic view of a group of electronic circuits having respective output stages connected to the same data bus, FIG 2 shows a diagrammatic view in enlarged scale and vertical cross section of the internal structure of a P-channel MOS transistor provided in an N well of a semiconductor substrate, FIG 3 shows a voltage-voltage diagram representative of the transfer function of a component of the output stage in accordance with the present invention, FIGS 4A, 4B and 4C show respective diagrammatic views of an output stage provided in accordance with the present invention and in different operating conditions, FIG 5 shows a diagrammatic view of a circuit detail of a detail of the stage of FIG 4A, FIG 6A shows a diagrammatic view of a detail of the circuit of FIG 5, and FIG 6B shows a chart of voltage signals present in the circuit of FIG 5. Detailed description With reference to the above FIGS and in particular to the example of FIG 4, reference number 1 indicates as a whole and schematically a final output stage provided in accordance with the present invention for an electronic circuit 2. The circuit 2 is represented only partially and by way of indication in FIG 4 because it may be of varying type such as e.g. a memory device, a reading circuit or some other. What counts for the purposes of the present invention is that the circuit 2 have at least one enable input, an operational portion whose operation is interlocked with the signal present on that input, and a final output stage which in our case corresponds to the stage 1. The stage 1 comprises a CMOS complementary transistor pair made up of a first P-channel pull-up transistor Pu connected in series with a second N-channel pull-down transistor Nu. These two transistors Pu and Nu are connected between a first reference supply voltage Vcc and a second reference voltage, e.g a signal ground. The point of connection between the transistors Pu and Nu corresponds to the output terminal U of the stage 1. The circuit 2 calls for a drive portion 3 of the pull-up transistor Pu and a like portion 4 for driving the pull-down transistor Nu. The first 3 of these portions comprises essentially an inverter 5 having its output connected to the control terminal of the transistor Pu through a pass-transistor P1. The second portion 4 calls for a direct connection between the output of an inverter 10 and the control terminal of the pull-down transistor Nu. A pass-transistor is virtually an enable circuit element having two inputs on which are applied mutually complementary signals. One of the inputs of the pass-transistor P1 is connected to the output of a CMOS inverter 7 made up of a complementary transistor pair receiving in parallel on respective control terminals the same enable signal OE. The other input of the pass-transistor P1 receives this signal OE directly. Advantageously in accordance with the present invention the stage 1 is equipped with a protection device against undesired turning on. This protection device comprises a switch 6 having an input 8 and an output 9. The input 8 of the switch 6 is connected to the output node U through a pass-transistor P2 receiving the signals OE and OE# denied. The output 9 of the switch 6 is connected to power the inverter 7. The switch 6 has a threshold and switching voltage Vs which was appropriately selected within the following range: Vcc ≤ Vs ≤ Vcc + min [ Vthp , Vγ] Preferably, for reasons clarified below, the voltage Vs was chosen equal to a Vcc + Vtnat where Vtnat is the threshold voltage of a natural transistor and its value is less than min [|Vthp|, Vγ]. The structure of the stage 1 in accordance with the present invention is completed by a first P-channel MOS transistor M1 having its control terminal connected downstream of the output of the inverter 7. The source terminal of the transistor M1 is connected to the reference supply voltage Vcc while the drain terminal and the body terminal of this transistor are coupled to the control terminal of the pull-up transistor Pu through a MOS transistor M2. A second M2 and a third M3 transistors, also of P-channel type, are inserted between the control terminal of the pull-up Pu and the output 9 of the switch 6. On the control terminal of both these transistors M2 and M3 is applied the signal OE. The interconnection point between them is indicated as node A and is connected to the drain terminal of the first transistor M1. There is now briefly described the operation of the stage 1 in accordance with the present invention and of the circuitry associated therewith. When the electronic circuit 2 is active to perform e.g. reading of the information transmitted on the data bus the corresponding output buffer is on. The enable signal OE has high logical value and the transistors M2 and M3 are off. Under these conditions the pass-transistor P2 is also off and the switch 6 is deactivated. The transistor M1 and the pass-transistor P1 are on and the circuit shown in FIG 4A is equivalent to an operational structure as shown in FIG 4B. Essentially when the output buffer is on, the respective inverters 3 and 10 directly drive the control terminals of the pull-up Pu and pull-down Nu. When the CPU switches the logical value of the signal OE to take it to a low value, the state of the output buffer is forced into extinction, also called tristate. Under these conditions the transistors M2 and M3 are turned on as is the pass-transistor P2. The transistor M1 and the pass-transistor P1 are turned off. FIG 4C diagrams this operational situation. In this FIG it can be seen that the output 9 of the switch 6 is directly connected to the control terminal and the bulk of the pull-up transistor Pu. Accordingly, the stage in accordance with the present invention has the special characteristic of allowing direct connection between the output of the switch 6 and the gate and bulk regions of the output buffer pull-up. This connection is active only when the output buffer is in tristate. During normal operation of the circuit 2 the switch 6 is disconnected. The table below shows the various components of the switching circuit of the switch 6. Buffer OE M1 M2 M3 P1 P2 SW6 ON 1 ON OFF OFF ON OFF Deact OFF 0 OFF ON ON OFF ON Active FIG 3 shows a transfer function Vin/Vout representing the output voltage Vout of the switch 6 for variations in the voltage Vin applied at input. The switch 6 is capable of comparing the voltage present on the buffer output U with its own threshold voltage Vs and supplying at output the greater of the two values. The result of this behavior is given in the graph shown in FIG 3 by a broken line corresponding to the values of the following table. Vin Vout ≤ Vin ≤ Vcc + Vtnat Vcc Vin ≥ Vcc + Vtnat Vin The threshold voltage Vs for switching of the switch 6 was chosen in the range defined by the (1) to avoid consumption of the active device. Now with particular reference to the example of FIG 5 there is described a preferred embodiment of the switch 6 associated with the output stage in accordance with the present invention. The switch 6 has an input terminal 8 and an output terminal 9. The input terminal is connected to the control terminal G1 of a P-channel MOS transistor M10 inserted between the reference supply voltage Vcc and the output terminal 9. A second P-channel MOS transistor M20 is in turn connected between the reference supply voltage Vcc and the output terminal 9 of the switch 6. The control terminal G2 of the second transistor M20 is connected to the input 8 through a transistor Dp in diode configuration and biased directly to said control terminal G2. A transistor pair Dp1 and Dnat1 in mutual series is connected in parallel with the above mentioned diode Dp but with opposite bias. A third P-channel MOS transistor M30 is inserted in turn between the input 8 and the output 9. This third transistor M30 has its control terminal G3 connected to a capacitive divider 11. Specifically the divider 11 comprises a capacitor C2 connected to a capacitor C1 between the power supply reference and the second ground reference. The point of interconnection between the capacitors C2 and C1 is connected to the terminal G3. In addition a transistor Dnat in diode configuration is connected in parallel with the capacitor C2 with bias towards the supply voltage Vcc. The structure of the switch 6 is completed by a fourth P-channel MOS transistor M40 having its drain terminal connected to the input 8, its source terminal connected to the control terminal G2 of the second transistor M20 and its bulk terminal connected to the output 9. Even the control terminal G4 of the fourth transistor M40 is connected to the same capacitive divider 11. We shall now discuss the specific operation of the switch 6 starting from an initial state in which the voltage Vin on the input 8 rises from the value = to the value Vcc - |Vtp|. Under these conditions the transistor M10 is on and holds the output voltage Vout at the value of the supply voltage Vcc. The series of the transistors Dp1 and Dnat1 causes a voltage drop on the terminal G2 which reaches a potential voltage Vg2 equal to Vin - (|Vtp1| + Vtnat). The second transistor M20 is accordingly turned on until the input voltage Vin reaches the value Vcc + Vtnat. The output terminal 9 is thus held at Vcc. The transistors M30 and M40 turn on when on their drain terminals, connected to the input 8, the voltage Vin reaches the value Vcc-Vtnat+|Vtp|. This causes switching of the output 9 to a potential value equal to Vin. Even the voltage Vg2 takes on this value. With present technology it is possible to define the value of Vtnat in such a manner as to determine the simultaneous turning on of the third transistor M30 and turning off of the second transistor M20. The value of Vin at which the switching takes place corresponds exactly to the threshold voltage Vs defined above. The function of the fourth transistor M40 is to equalize the control terminal G3 of the third transistor M30 at the potential present on the drain terminal, all this for values of input voltage Vin higher than the sum of Vcc + Vtnat. This allows keeping the third transistor M30 turned off even for values of Vin = Vdrain higher than Vcc + |Vtp|. Indeed, if the third transistor were driven with supply voltage Vcc the output voltage Vout could not exceed the value of Vcc + |Vtp| because of the third transistor M30 which would be in diode configuration. The presence of the MOS transistor Dp allows lowering the potential on the terminal G2 of the second transistor M20 in the falling phase of the input voltage Vin. The following table diagrams the above described operation. Vin M10 M20 M30 M40 Vout 0 - Vcc-Vnat ON ON OFF OFF Vcc Vcc-Vtp - Vcc-Vnat+Vtp OFF ON OFF OFF Vcc ≥ Vcc-Vnat+Vtp OFF OFF ON ON Vin Essentially the output stage in accordance with the present invention achieves important advantages with respect to the prior art. The first of these advantages is permitting reaching of the full 5 V level on the bus side when the high voltage device is active and thus reducing to virtually zero consumption of the electronic circuits with high power supply connected to the data bus and made active when the low power supply circuits are deactivated. In the case where |Vγ| < |Vthp| direct biasing of the drain-bulk junction of the pull-up transistor is avoided and, lastly, the output stage in accordance with the present invention can be made operative with zero consumption. Now for the sake of completeness of the description there is discussed an aspect of the present invention which is correlated with the possibility that the supply voltage Vcc might undergo variations in a range between a Vmin and a Vmax during operation of the output stage. Reference is made to FIGS 6A and 6B to discuss the answer of the present invention to this variation in the power supply. The switch 6 functions correctly if the control terminal of the third transistor M30 manages to follow the value of the supply voltage Vcc in the above mentioned interval, unless there is a voltage drop equal to the threshold of a natural MOS. The capacitive divider 11 was especially provided for this purpose. When the input voltage Vin equal to Vcc rises, the capacitor C1 is charged by the MOS in diode configuration Dnat and thus the output voltage Vout moves to Vcc - Vtnat. Contrariwise, during fall of Vcc, the diode Dnat is biased inversely and the discharge takes place through the capacitor C2. The two capacitors C1 and C2 must be sized in such a manner that the voltage to which their common node would move in the absence of the diode Dnat would be less than Vcc - Vtnat. This means that Vcc * C2/(C1+C2) must be less than Vcc-Vtnat and the output voltage Vout will be constrained to a minimum value equal to Vcc-Vtnat. The output stage in accordance with the present invention accordingly functions correctly even with variations in the supply voltage Vcc.

Claims: Output stage (1) for electronic circuits (2) with high voltage tolerance and of the type comprising an output buffer made up of a complementary transistor pair (Pu,Nu) comprising a pull-up P-channel MOS transistor and a pull-down N-channel MOS transistor with said transistors being connected together to form an output node (U) of the stage and comprising a switch (6) having an input terminal (8) connected to said output node (U) and an output terminal (9) connected to the control and bulk terminals of said pull-up transistor to drive said control terminal when the output buffer is disabled, characterised in that it further comprises: a first P-channel MOS transistor (M1) having its source terminal connected to the supply voltage reference (Vcc), its drain and bulk terminals coupled together and to the control and bulk terminals of the pull-up transistor (Pu) and its control terminal connected to the output terminal (9) of said switch (6) by means of an inverter (7), said inverter (7) being driven by an output enable signal (OE) in such a way to enable a direct connection between said output terminal (9) and the control and bulk terminals of said pull-up transistor (Pu), said switch (6) having a switching threshold voltage (Vs) chosen in the following range: Vcc ≤ Vs ≤ Vcc + min [|Vtp|, Vγ] where Vcc is the value of a supply voltage reference (Vcc) and Vtp is the value of the threshold voltage of a P-channel transistor and Vγ is the value of the threshold voltage of a drain-bulk junction. Output stage according to claim 1 and characterized in that said input terminal (8) of said switch (6) is connected to said output node (U) of the stage through a pass-transistor (P2) receiving said output enable signal (OE) and its opposite logic signal (OE#). Output stage according to claim 1 and characterized in that said switching threshold voltage (Vs) is preferably equal to Vcc + Vtnat where Vtnat is the value of the threshold voltage of a natural transistor. Output stage according to claim 1 and characterized in that it further comprises a second (M2) and a third (M3) P-channel MOS transistors inserted between said control terminal of said pull-up transistor (Pu) and said output terminal (9) of said switch (6), said output enable signal (OE) being applied to the control terminals of said second and third P-channel MOS transistors (M2, M3) and the interconnection point (A) between said second and third P-cannel MOS transistors (M2, M3) being connected to the drain terminal of said first P-channel MOS transistor (M1). Output stage according to claim 1 and characterized in that said switch (6) comprises the following: a first P-channel MOS transistor (M10) inserted between said supply voltage reference (Vcc) and said output terminal (9) and having its control terminal connected to said input terminal (8), a second P-channel MOS transistor (M20) and in turn connected between said supply voltage reference (Vcc) and said output terminal (9) and having its control terminal coupled to said input terminal (8), a pair of transistors (Dp1,Dnat1) in mutual series in diode configuration and connected between the series in diode configuration and connected between the supply voltage reference (Vcc) and said control terminal of said second P-channel MOS transistor (M20); and a third P-channel MOS transistor (M30) inserted between said input (8) and the output (9) terminals and having its control terminal connected to a control terminal of a fourth P-channel MOS transistor (M40), in turn inserted between said input (8) and output (9) terminals. Output stage according to claim 5 and characterized in that said switch (6) further comprises a capacitive divider (11), in turn including a first (C2) and a second (C1) capacitors connected between the supply voltage reference (Vcc) and a ground reference (GND), an interconnection point between said first and second capacitors (C2, C1) being connected to said control terminals of said third (M30) and fourth (M40) P-channel MOS transistors. Output stage according to claim 5 and characterized in that said switch (6) further comprises a biasing transistor (Dp) inserted, in diode configuration, between said input terminal (8) and said control terminal of said second P-channel MOS transistor (M20), in parallel with said pair of transistors (Dp1,Dnat1) and directly biased toward said control terminal . Output stage according to claim 6 and characterized in that it further comprises a diode (Dnat) connected in parallel to said first capacitor (C2), said diode (Dnat) being biased toward said power supply reference (Vcc).

Inventors: CAMPARDO GIOVANNI, GOLLA CARLA, ZANARDI STEFANO, CAMPARDO, GIOVANNI, GOLLA, CARLA, ZANARDI, STEFANO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: H03K17
IPC codes: H03K19