Input/output interface circuit for digital and/or analog signals

Abstract: The invention relates to an integrated input/output interface for low and/or high voltage signals of the digital and/or analog type. It comprises essentially a power amplification circuit block (2) having at least one low voltage input (A) and at least one high voltage output (B), and a second amplification circuit block (3) including active power components and having a high voltage input connected to said high voltage output (B) and at least one low voltage output (D). A conventional circuit block (4) prevents a high voltage signal being input to (B) from propagating through the block (2), so that it only affects the block (3). This interface is implemented in mixed high voltage BCD technology.

Description: This invention relates to an interface input/output integrated circuit for low and/or high voltage signals of the digital and/or analog type. As is well known, it is often necessary to interface devices which operate with widely different voltages. A typical example of that requirement is to be found in the field of industrial control systems where an intelligent unit, typically a microcontroller, is employed to process input information and output electric signals for driving a variety of actuators. The input information may either be very small analog voltages from sensors arranged to monitor a process to be controlled or digital signals. The microcontroller is normally operated at a much lower voltage than the operating voltage of an actuator, and accordingly, the microcontroller output would be uncapable to drive the actuator directly. Thus, a need exists for appropriate interfaces effective to provide electric and functional links between such devices. The interface circuits proposed heretofore, as employed in similar or comparable situations to that just outlined, exhibit a low degree of applicational flexibility. In fact, they can admit of but a narrow range of variability in the amplitude of the signals (voltages or currents) which may be input thereto. Also, just as limited is the range of variability of the signals to be obtained at the output. In other words, prior art interfaces can only be input low voltage (hence, low power) signals to output high voltage signals, or be input high voltage (high power) signals to output low voltage signals; and this within a fairly narrow range of variability of the values involved. The technical problem of this invention is to provide an input/output interface, of a type integrated to a semiconductor, which can operate on both low and high voltage input signals to correspondingly output high or low voltage signals. Another object of the invention is to provide an interface which can drive loads requiring working voltages or power levels which may vary within a very broad range, and can also handle analog signals of small amplitude as well as digital signals that may have high voltages. The technical problem is solved by an input/output interface as indicated above and defined in the characterizing parts of the claims appended hereto. The features and advantages of an interface circuit according to the invention will be apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. In the drawings: Figure 1 is a block diagram of an interface circuit according to the invention; and Figure 2 shows a possible circuit embodiment of the block diagram in Figure 1. With reference to the drawing views, generally and schematically shown at 1 is an interface circuit according to the invention. Also shown in Figure 1 are the main circuit blocks which make up the interface 1. The number of these main circuit blocks may narrow down to the following two: a first, power amplification circuit block 2 connected to a high voltage supply terminal Vdd and having at least a first, low voltage input terminal A, second input terminal A', and high voltage output terminal B; and a second amplification circuit block 3, being implemented with active power components, connected to a low voltage supply terminal Vee and having a first, high voltage input terminal E connected to the output terminal B of the first block 2, a second, low voltage input C, and at least one low voltage output terminal D. Also shown in phantom lines on the drawing is a cutoff circuit block 4 intended for inhibiting conduction by the circuit block 2 from the output terminal B to the input; in this way, any high voltage signal applied to B would only affect the circuit block 3. The internal construction of the cutoff block 4 is known per se. By having the circuit block 3 comprised of power elements which can operate with high voltages, it can be "tolerant" of high voltage inputs, to eventually output low voltage signals. In relation to its application to industrial control systems, the high voltage circuit block 2 would operate in association with the actuator, while the low voltage circuit block 3 would operate in association with the microcontroller. The circuit block 2 may be supplied directly from the same source as the actuator, and the circuit block 3 may share the low voltage supply with the microcontroller. The implementation of both low voltage parts and high voltage or large current parts in the same semiconductor integrated circuit is made possible by the availability of a mixed high voltage technology. Specifically, recourse is had to the so-called BCD (Bipolar-CMOS-DMOS) technology, which allows integrated active components of the bipolar, CMOS, or DMOS types to be provided. Figure 2 shows a preferred embodiment in BCD technology of the interface depicted schematically in Figure 1. The circuit block 2 includes an output stage of the push/pull type which can drive loads requiring both high and low voltages. This output stage comprises a P-channel MOS transistor M1 whose source terminal is connected to the high voltage supply terminal Vdd, and a DMOS transistor M2 having its source terminal connected to a signal ground. A circuit portion 6 for driving the transistor M1 comprises a pair of low power MOS transistors M3 and M4, an inverter INV, a pair of MOS transistors M5 and M6, a pair of Zener diodes DZ1 and DZ2, and bipolar transistors T3 and T4. The respective drive signals are low voltage signals which are applied to the gate terminal of the transistor M3 forming the low voltage input A' of the interface 1. The two transistors M3 and M4 have their respective gate terminals connected together through the inverter INV. Furthermore, the source terminals are also connected together, as well as to ground, via a current generator I'g. The MOS transistors M5 and M6 are cross-coupled, with their source terminals connected to the supply terminal Vdd and their drain terminals connected to the anodes of the diodes DZ2 and DZ1, respectively. The latter have a Zener voltage adequate to respectively turn M5 and M6 on. The bipolar transistors T3 and T4 are instead coupled through their respective emitter and base terminals, with the collector of T3 being connected to the voltage Vdd, the collector of T4 connected to ground, the common emitter connected to the gate terminal of M1, and the common base connected to the anode of DZ2, that is to the drain terminal of the transistor M6. The transistors M5 and M6 are cross-coupled to provide a positive feedback that will quicken the upward and downward sloping edges of potential variation at the node P or common base of the bipolars T3 and T4. The operation of the drive circuit 6 will now be described. When the low voltage signal applied to the input terminal A' of the interface, that is to the gate of the transistor M3, has such an amplitude as to turn on the transistor M3, the other transistor M4 will be cut off because its gate terminal is connected to the input A' through the inverter INV. With M3 in the on state, the current from the generator I'g is allowed to flow through the diode DZ1, which will be operating in reverse conduction. Thus, its Zener voltage will be applied between the source and the gate of the transistor M6, thereby turning the latter on. The transistor M6, in turn, will turn on the bipolar T3 to drive the final power transistor M1 off conduction. Conversely, when the amplitude of the signal applied to the terminal A' is insufficient to turn on the transistor M3, the other transistor M4 will be turned on, and the diode DZ2 will conduct the current from the generator I'g in the reverse direction. The Zener voltage of DZ2 will turn on the second bipolar T4, and through the latter, M1 as well. The DMOS transistor M2, by having its source connected to ground, requires no drive circuit like that described for M1. M2 will be driven directly by the signal received on its gate terminal, that is the second low voltage input A'' of the interface 1. Therefore, this signal can turn the transistor M2 on or off, according to its amplitude. A second circuit portion 7 of the interface 1 comprises two bipolar transistors T1 and T2 of the pnp type which are coupled through their respective emitters. These transistors are for high voltages and constitute, together with a current generator I''g connected between said emitters amd the low voltage supply Vee, the practical implementation of the circuit block 3 in Figure 1. The base of T2 is connected to the drain terminals of both transistors M1 and M2. This connection between terminals constitutes the input/output terminal B for high voltage signals in the diagram of Figure 1. Accordingly, the base of T2 will be applied high voltage signals, and since T2 is a transistor for high voltages, its emitter-base junction can comfortably accomodate the large potential difference which develops across it. The configuration adopted to implement the circuit block 3 is highly flexible in use. That block may actually be used in an analog manner both as operational amplifier and differential amplifier, where a low voltage signal can be applied to the base of T1 which forms the other low voltage input C of the interface 1. The block 3 may also function as a comparator, in particular a digital comparator. Assuming a constant voltage of 2.5 volts applied to the base of T1 and Vee=5 volts, the T1-T2 pair can convert a signal applied to the terminal B (i.e., to the base of T2), and as high as 60 volts, to an output signal of 0.5 volt on the collector of T1, that is a standard signal in the instance of CMOS circuits. The collector of T1 constitutes, therefore, a low voltage output D of the entire interface. As for the cutoff circuit block 4, its construction does not appear in detail on the drawing because conventional. It is intended for inhibiting both transistors M1 and M2 from conducting upon a high voltage signal being applied to the high voltage input/output terminal B, so that this signal will only be conveyed to the base of T2. The push/pull stage M1-M2 would, therefore, operate as a "tristate", since both MOS transistors are made to conduct by an enable signal from said circuit block 4 as well as by the driving just described. The interface 1 may be provided with a further control circuit for the stage M1-M2 operative to ensure that only one of said MOS power transistors would be conducting at any one time. This prevents a large current from flowing between the supply Vdd and ground. The implementation of the two last-mentioned circuits is well known and presents no difficulty for the skilled one.

Claims: An interface input/output integrated circuit (1) for low and/or high voltage signals of the digital and/or analog type, characterized in that it comprises: a first, power amplification circuit block (2) connected to a high voltage supply terminal (Vdd) and having at least one low voltage input terminal (A') and one high voltage output terminal (B); and a second amplification circuit block (3) comprising active power components and being connected to a low voltage supply terminal (Vee) which has a high voltage input terminal connected to said high voltage output terminal (B) of the first circuit block (2), a second low voltage input (C), and at least one low voltage output terminal (D). An interface circuit according to Claim 1, characterized in that said circuit blocks (2) and (3) are implemented in BCD technology. An interface circuit according to Claim 2, characterized in that said first circuit block (2) comprises: a push/pull output stage consisting of a MOS power transistor (M1) and a DMOS transistor (M2) connected through their respective drain terminals to said high voltage output terminal (B), the source terminal of the power transistor (M1) being connected to said high voltage supply terminal (Vdd) and the source terminal of the DMOS transistor (M2) being connected to ground; and a drive circuit (6) for driving said power transistor (M1) and being connected between said at least one low voltage input terminal (A') and the gate terminal of the power transistor (M1). An interface circuit according to Claim 3, characterized in that said drive circuit (6) for driving the power MOS (M1) comprises: a pair of low power MOS transistors (M3,M4) coupled together through their respective source terminals and connected to ground through a current generator (I'g), the gate terminal of the former (M3) of said transistors being connected to said input terminal (A') and to the gate terminal of the latter (M4) of said transistors through an inverter (INV); a pair of Zener diodes (D1,D2) having respective cathodes connected to said high voltage supply terminal (Vdd) and respective anodes connected to the drain terminals of said MOS transistor pair (M3,M4); and a second pair of MOS transistors (M5,M6) having respective source terminals connected to said high voltage supply terminal (Vdd) and the gate terminal of one transistor (M5) connected to the drain terminal of the other transistor (M6); the drain terminal of one transistor (M5) being connected to the anode of said first diode (DZ1) and the drain terminal of the other transistor (M6) being connected to the anode of said second diode (DZ2); and a complementary symmetry pre-amplification stage comprising a first bipolar transistor (T3) of the npn type and second bipolar transistor (T4) of the pnp type, the respective emitter terminals of said transistors being connected together and to the gate terminal of said power transistor (M1), the respective base terminals of said transistors (T3,T4) being connected together and to the anode of said second diode (DZ2), the collector terminal of the first bipolar (T3) being connected to said high voltage supply terminal (Vdd), and the collector terminal of the second bipolar (T4) being connected to the ground terminal. An interface circuit according to Claim 2, characterized in that said second amplification circuit block (3) comprises a pair of bipolar transistors (T1,T2) for high voltage applications which are coupled together through their respective emitter terminals and to the low voltage supply terminal (Vee) through a current generator (I''g); the base terminal of the first transistor (T1) being connected to the second low voltage input (C); the base terminal of the second bipolar (T2) being connected to said high voltage terminal (B); and the collector terminal of either the first (T1) or the second (T2) bipolar being the low voltage output terminal (D).

Inventors: CORDINI PAOLO, PEDRAZZINI GIORGIO, ROSSI DOMENICO, CORDINI, PAOLO, PEDRAZZINI, GIORGIO, ROSSI, DOMENICO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H03F3
IPC codes: H03K19, H03F3, G06J1