Negative overvoltage protection circuit, in particular for output stages

Abstract: A negative overvoltage protection circuit for stages having a transistor with collector output and with the emitter connected to a reference voltage line; a diode (D₁) for protection against negative overvoltages present on the output (V out ) is arranged between the collector and the output of the stage. In order to give the output of the stage a presettable minimum voltage level, the reference voltage line is set to a preset voltage which differs from the ground voltage. For this purpose, the circuit comprises an operational amplifier (O p ) in a voltage-follower configuration, the output whereof is connected to the reference voltage line, a diode (D₂) which is connected between the ground and the non-inverting terminal of the operational amplifier (O p ), and a current source which is connected between the non-inverting input of the operational amplifier (O p ) and a negative supply line (-V EE ).

Description: The present invention relates to a negative overvoltage protection circuit for a circuit stage which has a transistor with a collector connected to an output of the stage and an emitter connected to a reference voltage line, in particular for a digital output stage, for example for a line driver for transmitting data on a line. A known driving stage of the indicated type is schematically shown in figure 1, wherein Q indicates an output transistor, the collector whereof defines the output on which the voltage V OUT is present and the emitter whereof is connected to the ground. In detail, the stage comprises an input comparator C which has an inverting input connected to a reference voltage V REF and a non-inverting input to which a control voltage V IN is supplied. The output of the comparator C controls the open or closed state of a switch T which is connected on one side to a source of current I and on the other side to the base of Q; a resistor R is furthermore provided between the base of Q and the ground, whereas a load resistor R L is provided between the collector of Q and a positive supply voltage V CC . Consequently, when the voltage V IN at the non-inverting terminal of the comparator C exceeds the value of the reference voltage V REF , the switch T is closed, injecting the current I in the base of the transistor Q, which starts to conduct and saturates, so as to bring the output to a voltage which corresponds to a low logical state. Instead, when the voltage V IN is lower than the reference voltage, the switch T opens, Q is switched off and the output reaches the value of V CC due to the presence of R L . Therefore the low output level is equal to the saturation voltage of the transistor with respect to the ground, whereas the high output level is equal to V CC , as required by the specifications. However, though it operates satisfactorily, this stage cannot be used in all applications, in particular when it is required that a negative voltage which is higher, in absolute value, than the negative supply voltage -V EE (i.e. when V OUT < -V EE ) can be applied to the output of the stage without damaging the stage. The output transistor Q is in fact an integrated bipolar transistor executed with junction insulation, as illustrated in the transverse sectional view of figure 2. In this figure, the reference numeral 1 indicates the P-type substrate, 2 indicates the epitaxial layer and 3 indicates the P⁺-type insulation ring which insulates the well 3′, in which the transistor is integrated, from the other structures which are integrated in the same device. The reference numeral 4 indicates the N⁺-type buried layer and 5 indicates the N⁺-type sinker layer which forms, together with the well 3′, the collector of the transistor. The P-type diffusion 6 defines the base, whereas the N⁺-type diffusion 7 forms the emitter of Q. The diffusions 5, 6 and 7 are connected to the collector contact C, to the base contact B and to the emitter contact E respectively. As pointed out in the figure in broken lines, the P-type substrate 1 forms, together with the collector structure 4, 5, a parasitic diode DP₁. Said diode is thus connected with its cathode to the collector of Q and with its anode to the negative supply voltage -V EE of the stage, since the substrate is connected to said voltage. The parasitic diode DP₁ consequently starts to conduct when the voltage applied to the collector terminal is more negative than the voltage applied to the substrate (i.e., indeed, V OUT < -V EE ), conducting a high current which compromises the integrity of the stage. In order to solve this problem, the arrangement of a diode in series to the collector of Q, between said collector and the output, has already been proposed. This solution is shown by way of example in figure 3, wherein the protection diode has been indicated by D₁ and the rest of the structure is identical to figure 1. With this known solution, when a negative overvoltage is applied to the output terminal, the diode D₁ remains blocked and the current flows exclusively across R L and is limited thereby. However, even this solution is not optimum, since the output voltage of the circuit in the low state becomes equal to the sum of the saturation voltage between the collector and the emitter of Q and the drop on D₁, i.e.: V OUT,low = V CEsat,Q + V D1 ≃ 0.1V + 0.7V = 0.8 V. This value is unacceptable, since the output voltage at the low level must never exceed 0.4 V, as required for the outputs of all digital circuits. The use of a Schottky diode with a typical V D of 250 mV would lead to a low output voltage of approximately 350 mV which would therefore be acceptable. The problem with this solution is that in the bipolar technology used for this stage Schottky diodes are not available. Consequently, in a stage of the indicated type there is the problem of providing a protection which on one hand is capable of ensuring the integrity of the protected stage even when the output is subjected to a voltage which is lower than the negative supply voltage and on the other hand provides output voltage levels which are compatible with current specifications of digital circuits. More generally, an aim of the present invention is to provide a negative overvoltage protection circuit for a circuit stage having a transistor with a collector connected to an output of the stage and an emitter connected to a reference voltage line, which protection circuit is able to effectively protect the stage, ensuring at the same time that the output thereof has a preset minimum voltage level. Within the scope of this aim, a particular object of the present invention is to provide a protection circuit which can be completely integrated and does not require the use of external components. Another object of the present invention is to provide a protection circuit by virtue of which the currents which circulate in the protected stage and in the protection circuit itself are limited. Not least object of the present invention is to provide a protection circuit which is highly reliable and flexible. This aim, the objects mentioned and others which will become apparent hereinafter are achieved by a negative overvoltage protection circuit for protecting a circuit stage, in particular an output stage of a digital circuit, against the effects of negative overvoltages at an output thereof, said circuit stage having a transistor with a collector connected to said output of the circuit stage and an emitter connected to a reference voltage line, said protection circuit including a first diode connected in series between the collector of said transistor and the output of the protected circuit stage, characterized in that the protection circuit comprises means for generating a voltage having a preset value with respect to ground and opposite to the voltage drop on the first diode when the transistor conducts, said means being connected to apply said voltage to said reference voltage line of the protected circuit stage. A further aim of the present invention is to provide a circuit for generating a reference voltage with a presettable value for a protection circuit for a digital stage. This aim is achieved by a protection circuit comprising an operational amplifier in a voltage-follower configuration, a diode connected between a first reference voltage line and the non-inverting terminal of said operational amplifier, a voltage source of presettable value connected in series to said diode, and a current source connected between said non-inverting terminal of said operational amplifier and a second reference voltage line. The characteristics and advantages of the invention will become apparent from the description of a preferred embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is an electric diagram of a known output stage without protection; figure 2 is a transverse sectional view taken across a chip of semiconductor material which integrates the output transistor Q of figure 1; figure 3 is an electric diagram of the known output stage of figure 1 with overvoltage protection of a known type; and figure 4 is an electric diagram of the protection circuit according to the invention applied to the known output stage of figure 1. Figures 1-3 are not described hereinafter; for these figures, reference is made to the preceding description. According to the invention, in order to protect an output stage of the indicated type, with a transistor which has a collector output and has its emitter connected to a reference voltage, a diode is inserted between the collector and the output of the stage, as in the known solution of figure 3, and the emitter of the output transistor is furthermore no longer connected to the ground of the integrated device comprising the output stage, but is connected to an appropriate voltage, in particular a negative one, the value of which is preferably equal to that of a drop across a diode. An example of embodiment of the circuit suitable for generating the variable reference voltage according to the invention is illustrated in figure 4, wherein said circuit has been generally indicated by the reference numeral 10. The stage to be protected is, in the specific case, a line driving output stage of the type shown in figures 1 and 3, and therefore the components of figure 4 which are in common with the known solutions have been given the same reference signs. Therefore, with reference to figure 4, the output stage comprises the transistor Q, the collector whereof is connected, across the diode D₁, to the output of the stage, whereas its emitter is connected to the reference voltage, indicated here by V₂. This voltage is generated at the output of an operational amplifier Op, which is connected as a voltage follower, i.e. with its inverting terminal connected to the output. The operational amplifier Op has the specific function of absorbing the current delivered by the transistor Q and by the source I. The non-inverting terminal of Op (on which the voltage V₁ is present) is connected (across the voltage source V R ) to the cathode of a diode D₂, the anode whereof is connected to the ground. The diode D₂ is physically executed like the diode D₁, so as to have in practice the same voltage drop across its terminals. A current source I₁ is furthermore connected between the non-inverting input of Op and the negative supply voltage -V EE and has the function of keeping D₂ conducting. Figure 4 also illustrates, in broken lines, the parasitic diodes which are the result of the integration of the stage and of the protection circuit in a silicon chip with junction insulation. Since the transistor Q is executed as shown in figure 2, the parasitic diode DP₁ is in fact associated therewith, whereas the parasitic diodes DP₂ and DP₃ are associated with the diodes D₁ and D₂, which are executed as base-collector diodes with a structure which is similar to that of figure 2 but in which the emitter diffusion 7 is not provided, whereas the diffusions 5 and 6 constitute the cathode and the anode respectively. Ignoring the voltage source V R for the time being, the circuit of figure 4 operates as follows. Since the amplifier Op is voltage-follower connected, its output voltage is equal to the voltage on its non-inverting terminal and is therefore equal, in absolute value, to the drop across the diode D₂ (V D2 ) which, as mentioned, is kept conducting by the source I₁. Therefore V₁ = V₂ = -V D2 . Consequently, the high output voltage V OUT is still equal to V CC , whereas the low output voltage, indicating the collector-emitter saturation voltage of Q with V CEsat,Q and indicating the drop across D₁ with V D1 , is equal to: V OUT,sat = V₂ + V CEsat,Q + V D1 ≃ V CEsat,Q since V D1 is equal to V D2 in absolute value. The presence of the parasitic diodes, in particular of DP₁ and of DP₂, produces no damage to the output stage even in the presence of negative overvoltages, since the diodes DP₁ and DP₂, associated with Q and with D₁, can never start conducting due to the presence of D₁. The output voltage in the low logical state is furthermore perfectly within the specifications. The fact should furthermore be stressed that due to their structure the base-collector parasitic diodes have a high breakdown voltage (>50V), so that the structure is completely reliable. The voltage source V R offers an extension to what has been described above. In fact it allows to vary the reference voltage of the emitter of Q with respect to the ground according to the requirements. In particular, in the presence of V R the minimum output voltage is equal to: V OUT,low = -V D2 - V R + V CEsat,Q + V D1 ≃ V CEsat,Q - V R . In practice, by inserting a voltage source in series to D₂ it is possible to make the output voltage in the low logical state assume any value according to the sign and value of V R . As can be seen from the above description, the invention achieves the intended aim and objects. A protection circuit has in fact been provided which on one hand ensures an excellent protection of the associated stage in the presence of overvoltages and on the other hand is compatible with the specifications of digital circuits. The described structure is furthermore easily integratable and no external components need to be added. The fact is furthermore stressed that the reference voltage V₂ is available on the line 11, to which all the stages or devices which are present on the same integrated circuit and require said reference, in particular all the line driving devices, can be connected. This therefore allows to provide a single circuit 10 for the entire integrated circuit even when a plurality of stages of the type shown in figure 1 is provided. The circuit according to the invention is furthermore particularly flexible and adaptable to the various requirements, by virtue of the possibility of varying the reference voltage V₂ by means of the source V R . Said source is in turn easily integratable. The invention is furthermore circuitally simple and does not entail a large use of silicon area. The invention thus conceived, besides the possibility of inserting the source V R or not, is susceptible to numerous modifications and variations within the scope of the inventive concept as defined by the appended claims. In particular details of the invention may be replaced with other technically equivalent ones. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: Protection circuit for protecting a circuit stage, in particular an output stage of a digital circuit, against the effects of negative overvoltages at an output (Vout) thereof, said circuit stage having a transistor (Q) with a collector connected to said output (Vout) of the circuit stage and an emitter connected to a reference voltage line (11), said protection circuit including a first diode (D1) connected in series between the collector of said transistor (Q) and the output (Vout) of the protected circuit stage, characterized in that the protection circuit comprises means (10) for generating a voltage having a preset value with respect to ground and opposite to the voltage drop on the first diode (D1) when the transistor (Q) conducts, said means being connected to apply said voltage to said reference voltage line (11) of the protected circuit stage. Circuit according to claim 1, characterized in that said voltage generating means (10) comprise a second diode (D2) which is connected between the ground of the stage and a negative supply voltage line (-V EE ). Circuit according to claim 2, characterized in that said transistor (Q) is of the NPN type and said first diode (D1) has its anode connected to the output (Vout) of the protected circuit stage and its cathode connected to the collector of said transistor (Q), and wherein said second diode (D2) is arranged with its anode connected to the ground and its cathode connected to said negative supply voltage line (-V EE ). Circuit according to claim 2 or 3, characterized in that said voltage generating means (10) furthermore comprise an operational amplifier (O P ) in a voltage-follower configuration interposed between said second diode (D2) and said reference voltage line (11), said operational amplifier (O P ) having its inverting terminal connected to its own output and its non-inverting terminal connected to said second diode (D2), said voltage generating means (10) furthermore comprising a current source means (I₁) which is connected between said non-inverting terminal of said operational amplifier (O P ) and said negative supply line (-V EE ). Circuit according to one or more of claims 2 to 4, characterized in that said voltage generating means comprise a voltage source (V R ) with a preset value arranged in series to said second diode (D2). Circuit for generating a reference voltage with a presettable value for a protection circuits for a digital stage, characterized in that it comprises an operational (O p ) amplifier in a voltage-follower configuration, a diode (D₂) connected between a first reference voltage line and the non-inverting terminal of said operational amplifier (O p ), a voltage source (V R ) of presettable value connected in series to said diode (D₂), and a current source (I₁) connected between said non-inverting terminal of said operational amplifier and a second reference voltage line (-V EE ).

Inventors: GARIBOLDI ROBERTO, GOLA ALBERTO, GARIBOLDI, ROBERTO, GOLA, ALBERTO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1990
Main classification: H02H9
IPC codes: H02H9