A circuit for diagnosing the state of an electric load

Abstract: A circuit for diagnosing the state of a load (LD) comprises a DMOS transistor (DMOS1) interposed between a terminal (OUT) of the load (LD) and voltage comparator circuits (CDT) in order to limit the maximum voltage input to the comparators (CDT) to reduce the circuit area occupied by the comparators (CDT) and consequently the cost of the diagnosis circuit.

Description: The present invention relates, in general, to diagnosis circuits and, more specifically, to circuits for diagnosing the state of an electrical load as defined in the preamble of Claim 1. The invention addresses the problem of the diagnosis of the state of a load when a circuit driving the load is de-activated, that is with its output at high impedance. The load may be in the short-circuited state (to earth, to the supply, or between its terminals), in the open-circuit state (for example, because of a broken contact) or in a normal state, that is, connected and operating correctly. The diagnostic data may be supplied, for example, to a microcontroller which can implement recovery operations to ensure functioning in the event of a breakdown. The invention is particularly advantageous for applications with supply voltages and ambient temperatures which are variable over a wide range, for example, in automotive applications. In this case, the diagnosis circuit has to be able to withstand the maximum output voltage applicable to the load, for example, 50V. Clearly, if the diagnosis circuit is produced in the form of an integrated circuit, components capable of withstanding high voltages occupy a larger area and therefore involve a higher cost. A reduction in the number of high-voltage components is therefore crucial for minimizing the area occupied by the circuit. The circuit also has to withstand the variations which take place in the characteristics of the electronic devices when the ambient temperature varies over a wide range, for example, from -40°C to 125°C. Figure 1 is a circuit diagram showing a typical configuration in which the use of a diagnosis circuit may be necessary. One terminal of an electrical load LD is connected to an output OUT of a driver circuit DR. The driver circuit DR may be, for example, an amplifier, an output stage, or the like. It can be enabled or disabled (with the output OUT at high impedance) according to the value of an enabling logic input EN. When the driver circuit DR is enabled, a logic input IN selects the supply (positive or earth) to which the output OUT is connected. The other terminal TR of the load LD is connected to a supply, also an earth, or to another driver circuit (not shown). This diagram includes the possibilities of so-called "low side", "high side" and "bridge" driver circuits. The basic layout used in diagnosis circuits according to the prior art is shown in Figure 2. In Figure 2 and the following drawings, parts and elements already described with reference to Figure 1 have been attributed the same reference numerals and/or letters. The output of the driver circuit DR, that is, the terminal OUT is connected to two high-voltage polarizing resistors Rpu and Rpd and high-voltage comparators CDT. When the enabling input EN of the driver circuit DR switches, disabling the circuit DR, the power transistors of the circuit DR are cut off and the voltage at the output OUT is brought to a value which depends upon the state of the output OUT, as described below: if the load LD is correctly connected, the voltage at the output OUT is brought to a value close to that of the other terminal TR of the load LD, if the output OUT is short-circuited to earth, the voltage is brought to a value close to 0V, if the output OUT is short-circuited to the supply Vdd, the voltage is brought to the value of the supply Vdd, if the load LD is disconnected, that is, in open circuit, the voltage at the output OUT is brought to a value determined by the voltage divider formed by the resistors Rpu and Rpd. Discrimination between these conditions is carried out by means of the high-voltage comparators CDT. The situation in which the terminals of the load LD are short-circuited is distinguished from that in which the load LD is connected correctly by means of circuits, within the driver circuit DR, which can detect the presence of an overcurrent. This solution has the disadvantage, however, that the resistors Rpu and Rpd and the comparators CDT have to withstand a high voltage, necessitating the use of large components. The solutions according to the prior art also have the disadvantage of being very complex and consequently expensive. US-A-4 962 350 discloses a circuit for detecting the condition of an electrical load connected to the output of a driver circuit, wherein the diagnosis is carried out while the driver circuit is enabled. Upon detection of a short circuit or open circuit load condition, the detecting circuit blocks the driving signal from being applied to the driving circuitry preventing any possible damage to the load. EP-A-0 159 257 discloses an electronic circuit for measuring signals with very small amplitudes and having a potential much different from ground potential. Said signals are amplified, then transformed into current signals which are brought to ground potential before being actually measured. The object of the present invention is to provide a circuit for diagnosing the state of an electrical load which overcomes all of the problems indicated above in a satisfactory manner. According to the present invention, this object is achieved by means of a circuit for diagnosing the state of an electrical load having the features indicated in claim 1. Further advantages and characteristics of the present invention will become clear from the following detailed description, given with the aid of the appended drawings, provided by way of non-limiting example, in which: Figures 1 and 2 are diagrams of circuits of the prior art and have already been described, Figure 3 is a diagram of an embodiment of a diagnosis circuit according to the invention, Figure 4 is a cartesian graph showing the operation of the circuit of Figure 3, Figures 5 to 7 are diagrams of alternative embodiments of the diagnosis circuit according to the invention, Figure 8 is a graph illustrating an operating principle of the circuit of Figure 7, and Figure 9 is a diagram of an alternative embodiment of the diagnosis circuit according to the invention. An embodiment of the diagnosis circuit according to the invention will now be described with reference to Figure 3. This drawing shows a driver circuit DR, to the output OUT of which a load LD is connected exactly as described above with reference to the prior art. The invention is based upon the principle that it is not necessary to have a direct connection to the output OUT in order to carry out the diagnosis but that the desired measurements can be taken at a node which has a voltage equal to the voltage, indicated Vout, of the output OUT only when the latter is below a value which can be withstood by low-voltage devices. A DMOS transistor, indicated DMOS1, connected to the output OUT of the driver circuit DR, can withstand a high voltage, for example 50v. The circuits connected downstream of the transistor DMOS1 are at low voltage and the transistor DMOS1 is therefore the only high-voltage component used for diagnosing the state of the load LD. The gate terminal of the transistor DMOS1 is connected to a low-voltage supply Vdd, for example, a 5V supply. The voltage of the source terminal of the transistor DMOS1, indicated as a node S, does not exceed a voltage value Vs max given by: Vs_max = Vdd-Vth where Vth is the threshold voltage of the transistor DMOS1. In fact, if the voltage, indicated Vs, of the node S were greater than the voltage Vs_max, the voltage Vgs, that is, the voltage between the gate and source terminals of the transistor DMOS1, would be less than the threshold voltage Vth and the transistor DMOS1 would therefore be cut off. In this case, the voltage Vs of the node S would be that determined by an amplifier A, the output of which is connected to the node S, and would be less than the voltage Vs_max. The amplifier A is a transconductance differential amplifier and has feedback from its output which is connected to its inverting input. The non-inverting input, on the other hand, is connected to a reference voltage Vrif which is less than the voltage Vs_max and can be produced by, for example, a resistive divider. Figure 4 shows an output current Iouta of the amplifier A, dependent upon a differential input voltage Vdiff equal to the difference between the inverting input voltage, that is, the voltage Vs at the node S, and the voltage at the non-inverting input, that is, the reference voltage Vrif. The transistor DMOS1 is used for conducting current from the source terminal to the drain terminal, that is, from the node S to the output OUT, as well as for conducting current from the drain terminal to the source terminal, that is, from the output OUT to the node S. The maximum value of the current flowing from the source terminal to the drain terminal of the transistor DMOS1 is selected so as to be small enough not to activate the intrinsic diode between the source terminal and the drain terminal of the transistor DMOS1. When the driver circuit DR is disabled, the voltage Vs reached at the node S depends upon the state of the output OUT as described below: if the load LD is correctly connected, it is brought to a value close to that of the other terminal TR of the load LD, if this is below the voltage Vdd-Vth, or to the voltage Vdd-Vth, if the output OUT is short-circuited to earth, it is brought to a value close to 0V, if the output OUT is short-circuited to the positive supply Vdd, it is brought to the voltage Vdd-Vth, if the output OUT is in the open-circuit state, the voltage Vs of the node S is brought by the amplifier A with feedback to a voltage equal to the reference voltage Vrif; in fact, during the transient state, if Vs<Vrif, the current Iouta flows out of the output of the amplifier A and increases the voltage Vs of the node S; if Vs>Vrif, the current Iouta flows into the output of the amplifier A and reduces the voltage Vs of the node S. The voltage Vout of the output OUT is brought to the value of the reference voltage Vrif even in the presence of a leakage current (within or outside the circuit), into or out of the output OUT, of a value up to that of the current Iouta output by the amplifier A. The low-voltage comparators CDT can thus recognize the voltage range in which the node S is situated. In situations in which the output OUT is connected to a positive voltage by means of the load LD or by means of a short-circuit, the amplifier A absorbs a current and the transistor DMOS 1 conducts a current from the drain terminal to the source terminal. In situations in which the output OUT is connected to earth by means of the load LD or by means of a short-circuit, the amplifier A supplies a current and the transistor DMOS1 conducts a current from the source terminal to the drain terminal without activating the intrinsic diode between source and drain. Figure 5 shows an alternative embodiment of the circuit according to the invention in which the amplifier A is replaced by a resistive divider formed by two resistors Rpu and Rpd which, if the output OUT is in the open-circuit state with the driver circuit DR disabled, determines the value of the voltage Vs of the node S. When the transistor DMOS1 conducts a current, with reference to Figure 3, its voltage Vds, that is, the voltage between the drain and source terminals, introduces a small potential difference between the output OUT and the input of the comparators CDT, or node S, even when Vout<Vdd-Vth. This constitutes a source of inaccuracy if the switching thresholds of the comparators CDT are required to relate precisely to the voltage Vout of the output OUT, since the voltage read by the comparators CDT is that of the node S. If it is desired to increase the degree of accuracy, one of the following solutions may be adopted: increasing the area (the number of cells for vertical DMOS transistors) of the transistor DMOS1, reducing its voltage Vds for a given current, duplicating the transistor DMOS1; this solution, which is shown in Figure 6, is described below. The transistor DMOS1 has the task of conducting a current absorbed or supplied by the amplifier A and of limiting the voltage Vs of the node S1. The transistor DMOS2 has the task of making available a voltage equal to the voltage Vout of the output OUT, limited to a maximum value which is not dangerous for the amplifier A and the comparators CDT. The voltage at the node S1 cannot rise above a voltage value Vs1_max given by: Vs1_max = Vdd-Vth1 in which Vth1 is the threshold voltage of the transistor DMOS1. The voltage at the node S2 cannot rise above a voltage value Vs2_max given by: Vs2_max = Vdd+Vd1 where Vd1 is the voltage of a diode D1; this diode D1 may be formed by the base-collector junction of an NPN bipolar transistor. The voltage Vds of the transistor DMOS2 for voltages Vout of less than Vdd at the output OUT, is almost zero since the current coming from or absorbed by the amplifier A passes through the transistor DMOS1 and because the amplifier A and the comparators CDT have high-impedance inputs; they therefore receive a voltage Vs2 of the node S2 which, if Vout<Vdd, is almost identical to the voltage Vout of the output OUT. The diode D1 prevents the source terminal of the transistor DMOS2 from assuming high voltage values which could damage the gate oxide of the DMOS2 transistor as a result of stray capacitances during rapid transitions of the output OUT. Figure 7 shows a circuit illustrating a possible embodiment of the comparators CDT which comprise two comparator circuits CMP1 and CMP2. Two thresholds Vr1 and Vr2 of the comparator circuits CMP1 and CMP2 are shown in Figure 8. The reference voltage Vrif of the amplifier A is between the thresholds Vr1 and Vr2. The maximum value which the voltage of the node S2 can assume is given by Vdd+Vd1. The two comparator circuits CMP1 and CMP2 enable the range in which the output OUT is situated to be recognized for the voltage values which are of interest (for 0V<Vout<Vdd, the voltages Vout and Vs2 coincide). The indications of the comparators CDT and of the overcurrent detection circuits included in the driver circuit DR thus enable the condition of the load LD to be known. Another particular embodiment of the invention is shown in Figure 9. Figure 9 shows a particular embodiment of the circuit in which the low-voltage comparators CDT are protected by a DMOS transistor DMOS2 and by a diode D2. The functions of the transistor DMOS2 and of the diode D2 are similar to those of the transistor DMOS2 and of the diode D1 shown in the circuit of Figure 6. The invention has been developed for a specific application for diagnosing the state of a load LD in a "low side" driver circuit. For this purpose, the diagram of Figure 7 was used. The values of the thresholds vr1, Vr2 and of the reference voltage Vrif are: Vr1 = 2V Vr2 = 1V Vrif = 1.5V. The diagnosis circuit according to the invention is therefore particularly advantageous in applications in which the supply voltage and the ambient temperature may vary over a wide range. In particular, the circuit according to the invention enables the number of high-voltage components to be reduced, reducing the area occupied by the circuit.

Claims: A circuit for diagnosing the state of an electrical load (LD) connected to an output (OUT) of a driver circuit (DR) wherein the diagnosis is carried out while the driver circuit (DR) is disabled, comprising voltage-comparator means (CDT) connected to the output (OUT) of the driver circuit (DR) ; characterised in that it comprises a DMOS-FET transistor (DMOS1) interposed between the output (OUT) and the voltage comparator means (CDT) and having the source (S) connected to the input terminal of said comparator means (CDT) and the gate coupled to a reference voltage (Vdd) in order to reduce the maximum voltage at the input (S) to the comparator means (CDT), and in that the comparator means (CDT) are of the low-voltage type. A circuit according to Claim 1, characterised in that it comprises amplifier means (A) connected to the input node (S) of the comparator means (CDT) and configured for generating a current (lout) flowing into or out of the input node (S) in dependence on a voltage difference (Vdiff) between a voltage (Vs) of the input node (S) and a reference voltage (Vrif). A circuit according to Claim 2, characterised in that the amplifier means comprise a transconductance differential amplifier (A) having an output connected to the input node (S), a non-inverting input connected to the reference voltage (Vrif), and an inverting input connected to the output of the transconductance amplifier (A). A circuit according to Claim 1, characterised in that a reference voltage (Vrif) is connected to the input node (S) .

Inventors: CHICCA STEFANIA, CRESPI ANGELO, MASTELLA PAOLO, POLETTO VANNI, CHICCA, STEFANIA, CRESPI, ANGELO, MASTELLA, PAOLO, POLETTO, VANNI
Applicants: MAGNETI MARELLI SPA, ST MICROELECTRONICS SRL, MAGNETI MARELLI S.p.A., STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G01R31
IPC codes: G01R15, G01R31, G01R19