INTEGRATED CIRCUIT FOR DRIVING INDUCTIVE LOADS

Abstract: An integrated circuit for driving inductive loads comprises at least one substrate (30) and a plurality of separate epitaxial wells (31-35), and has at least one output terminal (14) for connection to an inductive load (16) and a reference terminal (15) for connection to a reference voltage (17). To reliably isolate the different epitaxial wells in each operating state, the circuit comprises diodes interposed between the substrate (30) on one side and the output and reference terminals (14,15) on the other to set the substrate (30) to the reference potential when the potential on the output terminal (14) is greater than the reference potential and to set the substrate to the output potential when the latter becomes smaller than the reference potential.

Description: The present invention relates to an integrated circuit for driving inductive loads. As is known, integrated circuits which use a P-type substrate require that substrate be biased at the lowest voltage of the circuit so that the different epitaxial walls or regions which contain the active and passive components of the circuit are isolated from one another. In fact, the P-N junctions which form between the substrate and the different epitaxial wells are inversely biased and ensure an effective electrical isolation of said various wells. It is furthermore known that in integrated circuits for driving inductive loads having a terminal connected to the earth (and thus hereinafter also defined as "loads" referred to earth), when the current supplied to the load is suddenly interrupted, a negative overvoltage is created across the latter. Accordingly, if the substrate, as occurs presently, is clamped to the ground, the epitaxial well or tubs connected to the load reaches a voltage lower than that of the substrate. In this condition said substrate injects current in the epitaxial well entailing a whole series of problems, such as for example the undesired activation of other components of the circuit. To solve this problem, different solutions have already been studied, in particular to create preferential current paths to collect the current injected by the epitaxial well at negative voltage. However such solutions are not free from disadvantages, for example they are not applicable in all the required cases. Another solution including an integrated circuit with protection means against parasitic injection currents in a telephone system has been proposed by EP-A-289 431. The known integrated circuit includes switching means formed in the circuit substrate and connected to the lines leading from an exchange to the telephone apparatus. A similar protection means for integrated circuits is also known from DE-A-35 07 181. The protection means according to DE-A-35 07 181 includes switching means connected to a substrate for lowering its potential when a negative voltage occurs at the output of the integrated circuit. Given this situation, the aim of the present invention is to provide an integrated circuit for driving inductive loads referred to earth, capable of eliminating the disadvantages of the prior art, and in particular of keeping the substrate at an appropriate voltage for an effective isolation of the different epitaxial wells. Within this aim, a particular object of the present invention is to provide an integrated circuit which has a simple structure, can be manufactured with the methods currently used in the integrated electronics industry and therefore has modest manufacturing costs. Not least object of the present invention is to provide an integrated circuit which has a reduced bulk, and thus does not entail a high waste of area and operates reliably in all operating conditions. This aim, as well as the objects mentioned and others which will become apparent hereinafter, are achieved by an integrated circuit for driving inductive loads according to the invention, as defined in the accompanying claims. In practice, according to the invention, the substrate of the circuit is no longer rigidly clamped to the external reference voltage (earth) of the circuit, but is set at a voltage varying according to the operating state of said circuit: in particular when the voltage on the load (output voltage of the circuit) is positive with respect to the earth, the substrate is substantially at ground voltage, closely linked to the earth voltage, while when the output voltage drops below the external earth the substrate is biased at said output. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a cross sectional view taken through a silicon wafer, in which a known integrated circuit is provided; figure 2 is an equivalent electric diagram of a known integrated circuit; figure 3 illustrates the plot of the output voltage on the load; figure 4 is an equivalent electric diagram of the integrated circuit according to the invention; and figure 5 is a cross sectional view taken through a semiconductor wafer in which the circuit according to the invention has been provided. For a better understanding of the invention, reference should initially be made to figures 1 to 3, illustrating a known driving circuit. Figure 1 illustrates the cross section of a semiconductor wafer in which a circuit for driving inductive loads, with P-type substrate, is provided. In detail, 1 indicates the P-type substrate, and 2-5 indicate different epitaxial wells of N⁻-type conductivity, mutually separated and isolated by means of the P-type isolation regions 6. According to the prior art, the P substrate is clamped to the earth and forms with the epitaxial wells P-N junctions 2-5 which must be inversely biased to keep the various epitaxial wells isolated from one another. Said wells then accommodate the various layers and the various regions of semiconductor material with alternate conductivity type so as to obtain a complete driving integrated circuit, such as for example illustrated in figure 2 and generally indicated at 10. With reference thus to figure 2, the driving integrated circuit 10 essentially comprises a stage 11 which can be configured differently, according to the required functions, to drive a power element 13 in turn intended to drive an external load. The integrated circuit 10 furthermore comprises a power supply, here represented by a voltage source 12, a terminal whereof (connected to a circuit pad 15, constituting the reference output of the circuit) represents the ground of the circuit, and the other terminal whereof is connected to the different components of the driving stage 11 to supply them. In turn, the power transistor 13 is connected with its collector to a supply voltage V cc and with its emitter to a contact pad 14 constituting the output terminal of the driving circuit. Said output terminal is connected to a terminal of the inductive load 16, the other terminal whereof is connected to the earth 17. Accordingly, when the transistor 13 is controlled by the driving stage 11 so as to supply a positive current to the load 16, the latter remains at positive voltage, while when the transistor 13 is switched off, a negative overvoltage forms across the inductor 16, as illustrated in figure 3. During this phase, one of the epitaxial wells constituting the device may reach a voltage which is lower than that of the substrate, which condition, as mentioned, is to be avoided. To solve this problem the invention teaches to bias the substrate at the output potential when the latter becomes negative, for example in the manner illustrated in figure 4. Reference should be thus be made to figure 4, wherein the general circuit diagram of the integrated circuit according to the invention is illustrated, in which the parts in common with the known embodiment illustrated in figure 2 have been given the same reference numerals. According to the invention, the driving integrated circuit 20 again comprises a driving stage 11, a power supply 12, a power transistor 13 as well as output 14 and reference 15 terminals for connection to two different terminals of the inductive load 16 referred to the earth 17. Differently from the known embodiment, according to the invention the substrate, indicated at 30, is not clamped to the ground and then to the external earth of the system, but three diodes 21, 22 and 23 are provided to electrically separate or connect the substrate from or to the output and from or to the earth of the system, according to the operating state. In detail, the circuit 20 has a first diode 21 connected with its anode to the substrate 30 and with its cathode to the output terminal 14, a second diode 22 connected with its anode to the substrate 30 and with its cathode to the reference terminal 15 and a third diode 23 connected with its anode to the internal ground 24 of the system and with its cathode to the reference terminal 15. Accordingly, during the conduction of the power transistor 13, when a positive voltage is present on the load, the diode 21 is inversely biased, while the diodes 22 and 23 are forward biased. Therefore the current of the substrate flows through the diode 22 towards the earth. In this case (the drop on the diodes 22 and 23 being approximately equal) the potential of the ground 24 is equal to the potential of the substrate and all the epitaxial wells are mutually isolated. When, upon control of the driving stage 11, the power transistor is switched off and the output voltage becomes negative, the diode 21 is forward biased (thus connecting the substrate to the output terminal) while the diode 22 is inversely biased, separating the substrate from the earth 17. Then the substrate current flows through the diode 21, biasing the substrate 30 at a voltage equal to the (negative) output voltage minus the drop on the diode 21. An embodiment of the circuit of figure 4 is illustrated in figure 5, showing the substrate 30 with P-type polarity and the different epitaxial wells 31-35 with N⁻-type conductivity mutually separated by the P-type isolation regions 36. In detail, the epitaxial wells 31 and 35 are provided for the accommodation of the different layers which form the components of the driving stage 11, while the epitaxial wells 32, 33 and 34 are intended for accommodating the diodes 21, 22 and 23. In particular, the epitaxial well 32 accommodates the P-type base region 37 (defining a circuit ground region) inside which the N⁺-type emitter region 38 is arranged, forming with the previous one the diode 23. The base region 37 is connected to the ground of the system, here indicated by the terminal 24, while the region 38 is electrically connected through line 42 to the reference terminal 15 of the circuit. The epitaxial well 33, forming with the substance 30 the diode 22, comprises in its interior the N⁺-type region 39 for connection to the output terminal 15 in turn connected to the external earth potential of the circuit. Finally the epitaxial well 34, forming with the substrate 30 the diode 21, comprises in its interior the N⁺-type region 40 connected to the output terminal 14 at the voltage V out . In practice therefore, as explained above, when the output voltage V OUT is greater then the external reference potential (earth 17) the junction between the well 34 and the well 30 (diode 21) is reverse biased, while the epitaxial well 33/substrate 30 junction (diode 22) is forward biased and allows the discharge of the current of the substrate towards the terminal 15. In this situation, the substrate has a lower potential than all the epitaxial wells referred to the ground 24, so that an effective separation and isolation thereof is achieved. Instead, when the output voltage goes to a value lower than the ground, the epitaxial well 34/substrate 30 junction is forward biased, linking the substrate 30 to said output voltage (apart from the drop on the diode 21 formed by said junction). Accordingly the current in the substrate can be reliably discharged through the output terminal, without affecting the other epitaxial wells of the driving stage. As is apparent from the preceding description, the invention fully achieves the intended aim and objects. In fact, a driving integrated circuit has been provided which operates reliably regardless of the output voltage of the circuit. Furthermore, the presented solution is simple, cheap, and producible by means of current methods, without requiring a substantial increase in the occupied area. 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: An integrated circuit for driving inductive loads, comprising at least one semiconductor substrate (30) of P-type conductivity and a plurality of isolated epitaxial regions (31-35) of N-type conductivity in said substrate; said integrated circuit having at least one output terminal (14) implemented at one of said isolated epitaxial regions (31-35) and connected to an inductive load (16) and a reference terminal (15) connected to a reference potential (17), said output terminal (14) having a variable output potential; said integrated circuit further comprising a first diode means (21) having its anode connected to said substrate (30) and its cathode connected to said output terminal (14) and a second diode means (22) having its anode connected to said substrate (30) and its cathode connected to said reference terminal (15) for connecting said substrate (30) to said reference terminal (15) when said output potential is greater than said reference potential and for connecting said substrate (30) to said output terminal (14) when said output potential becomes smaller than said reference potential. An integrated circuit according to claim 1, characterized in that it comprises a third diode means (23) interposed between said reference terminal (15) and a region of said circuit constituting a circuit ground region (37). An integrated circuit according to claim 2, characterized in that said third diode means (23) has its anode connected to said ground region (37) and its cathode connected to said reference terminal (15). An integrated circuit according to one or more of the preceding claims, characterized in that it comprises a first isolated epitaxial region (34), connected to said output terminal (14), a second isolated epitaxial region (33), connected to said reference terminal (15), and a third isolated epitaxial region (32), accommodating in its interior said ground region (37). An integrated circuit according to claim 4, characterized in that said first isolated epitaxial region (34) forms with said substrate (30) said first diode means (21), in that said second isolated epitaxial region (33) forms with said substrate (30) said second diode means (22), and in that said third isolated epitaxial region (32) forms with said substrate (30) said third diode means (23).

Inventors: MAGGIONI, GIAMPIETRO, MARCHIO, FABIO, MORELLI, MARCO, TRICOLI, FRANCESCO
Applicants: SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1988
Main classification: H01L27
IPC codes: H03K17, H01L27, H01L21