GB2181612A - Bistable driver for microprocessor output - Google Patents

Abstract

A microcomputer (MC) feeds pulses to control a number of driver circuits (TR1-TRn),the outputs of which are connected to respective electrical loads (V),through respective control circuits (AN1-ANn), each having two inputs (1, 2). One input (1) controls the output voltage of the control circuit and the other (2) switches its output impedance. Each non-inverting driver circuit is rendered bistable by a positive feedback resistor. When a driver circuit is switched on by its control circuit the bistable changes state until it is switched off or until it is tripped during the high impedance phase of the control circuit by a short circuit or overload. The circuit is particularly applicable to motor vehicle and provides output protection. <IMAGE>

Description

SPECIFICATION Output circuit for electronic circuits The invention relates two an output circuit for electronic switches or circuits, particularly microcomputers, for switching electrical loads.

Output circuits with electronic driver circuits are frequently used for the control of electrical loads having a relatively high power consumption, particularly incandescent lamps, relay windings or electric motors. In general, however, the driver circuits used are not proof against short-circuits or overloads. A short-circuit or a state of overload can occur as a re sultofan error during the connection of the electrical load to the driver circuit or in the event of a defect in or at the electrical load. The problem is presented, therefore, of protecting the driver circuit against short-circuit and overloading.

An output circuit with a circuit arrangementforthe protection of an electronic switch is known from the DE-AS 23 10 448. There an electronic switch is used as a driver circuit and a control circuit protecting that switch consists of an AND-circuit at one input of which the switching signal switching on the load is fed in and atthe otherinputofwhich theswitching signal is additionally fed in through an electronic semiconductor component which inverts and delays the switching signal. As a result, the effect is achieved that in the event of a short-circuit, the electronic switch is closed only for the short duration of a differential pulse and only when the load is switched on.If there is no short-circuit at the output ofthe electronic switch, the potential at the output keeps the other input ofthe AND-circuit at a potential such that the electronic switch remains closed after the expiration of the differential pulse.

This previously known circuit has the disadvant age that high costs are necessary for additional com- ponents for the protection of the driver circuit. Obtaining the components and assembling them during production is expensive in time and money.

Output circuits which are already in existence, particularly highly integrated circuits, cannot always be subsequently equipped with the protective circuit because driver circuit and control circuit are not clearly separated from the point of view of the circuitry. If the driver and control circuits are spatially separated, additional electrical conductors are necessary es- tablish the electrical connection.

According to the present invention, there is provided an outputcircuitforan electroniccircuitor switch for switching an electrical load, comprising a driver circuit having an output for connection to the electrical load, and a control circuitforthe pulsewise actuation of the driver circuit, the output of the driver circuit being coupled back to its input through afeedback resistor and the control circuit having a first input th rough which its output voltage can be controlled and a second input th rough which its output impedance can be controlled.

By coupling the output of a non-inverting driver circuit back to the input through a feedback resistor, the driver circuit has the characteristics of a bistable trigger stage. The input ofthe driver is switched on or off by the control circuit depending on whether positive potential from the electronic switch is or is not applied to the first input of the control circuit. As a result ofthe bistable character of the driver circuit, its switching state in the high-impedance phase of the output ofthe control circuit depends only on whether there is a short-circuit atthe output of the drivercircuit. If there is no short-circuit the input of the driver circuit remains uninfluenced by its output.If a shortcircuit or an overload occurs at the output ofthe driver circuit, through a load with too little internal resistance, this state is immediately transmitted to the input of the driver circuit and the driver circuit opens the load circuit.

It is a particular advantage if a clock-pulse generator switches the output of the control circuit to a low impedance pulsewise, particularly at a fixed frequency, through the second input. The pulse dutyfactor between a low-impedance and high-impedance output state of the control circuit is selected so that the driver circuit is not overloaded in the event of a short-circuit. Only a simple timing-pulse generating circuit is necessary on which no heavy demands are made with regard to the frequency constancy and the pulse duration. If a short-circuit or overload state is eliminated during the operation of the output circuit, the electrical load is switched on again with the appearance ofthe next pulse of the clock-pulse gene- rator, in the appropriate circumstances.

Byway of example, the invention will be described in more detail below with reference to the ac companyingdrawing in which the single figure shows one embodiment of the subject of the invention with a microcomputer and control and driver circuits as integrated circuits in a motorvehicle.

In the figure, the positive pole of a current source B, which is realised in the form of a starter battery of a motor vehicle, is connected to the first connection of a switch S, which is realised in the form of the ignition switch of the motorvehicle. The second connection of the switch S is electrically connected to the positive wire + UB to which the positive potential of the positive pole of the current source B is applied when the switch S is closed. The switch S is shown in the open switching position in the figure. Electrically connected to the negative pole of the current source B is the negative wire - U B to which the negative supply voltage is therefore applied. The negative wire -UB may also be additionally connected to the earth ofthe motorvehiclethrough an earth wire. A microcomputer MC, an integrated control module AN and an integrated drivermoduleTRare each elec- trically connected to the positive wire + UB and the negative wire - UB for the current supply, possibly through voltage dividers or voltage controls.

A plurality of control circuitsANI toANn are integrated within the module AN. The control circuits ANI to ANn are identical in construction to one another. This means that, inside the control module AN, the circuits ANI to ANn are connected via current supply lines to the corresponding current supply lines of the module AN. Each of the circuits ANI to ANn has two inputs, the first inputs being directly connected, through first connecting lines 1, to cor- responding outputs ofthe microcomputer MC. The second inputs of the circuits ANI to ANn are electrically connected to one another inside the module AN so that, outside the module, they are electrically connected through a common connecting line 2 to the clock-pulse generatorTas part of the microcomputer MC.Each of the control circuits ANI to ANn is connected, through second connecting lines 3, to corresponding driver circuits or output-stage circuits TRI to TRn which are constructed in the form of parts of the driver moduleTR.

The driver circuits TRI to TRn are identical in construction as part of the integrated driver module TR.

This means that each of the driver circuits TRI to TRn is electrically connected, through current supply lines, to the corresponding current supply lines of the integrated driver module TR. Each ofthe driver mod ules TRI to TRn has an output which is electrically connected, through third connecting lines 4, to the particular electrical load. In the figure, by way of example, the output ofthe driver circuit TRI is electrically connected to one terminal of an incandescent lamp V as an electrical load. The other terminal ofthe lamp is electrically connected to the positive supply voltage ofthe wire +UB. The output of the drivercircuits TRI to TRn is coupled back, through feedback resistors RI to Rn, to the input ofthe particular driver circuit or output-stage circuit TRI to TRn.

For reasons of clarity, onlythe actuating circuits ANI and ANn and the driver circuits TRI and TRn are illustrated in the figure.

An electrical potential which corresponds to the switching state ofthe driver circuits TRI to TRn to be controlled by the microcomputer MC is applied, through thefirst connecting lines 1 ,tothe first inputs ofthe control circuits ANI to ANn. The potential atthe first input of each control circuit determinesthe output voltage of the respective control circuitwhich can befed,throughthe second connecting line3,to the input of the associated driver circuit. An electrical potential which determines the output resistance or the output impedance of the control circuits ANI to ANn and is produced by the clock-pulse generatorT, which is constructed in the form of part ofthe microcomputer MC, is applied, through the common connecting line 2, to the second inputs of the control circuits.This clock-pulse generator alternately produces positive and negative or earth potential and has a fixed frequency. The ratio ofthetimes of positive potential and the times of negative potential is selected so that the clock-pulse generatorTonly has high potential substantially pulsewise. If positive potential is applied to the first inputs of the control cir cuitsANI to ANn, these circuits have a positive output voltage. If negative or earth potential is applied to the first inputs, the circuits have a negative output voltage or earth potential. If positive potential is applied to the second inputs of the control circuits ANI to ANn, the outputs of these circuits have a low impedance, that is to say the outputs have a low output resistance.If negative or earth potential is applied to the second inputs ofthe control circuits ANI to ANn,the outputs of these circuits have a high impedance, that is to say the outputs have a great output resistance.

If positive potential is applied to the inputs of the driver circuits TRI to TRn, the loads V are switched off. If negative or earth potential is applied to the inputs ofthe driver circuits TRI to TRn, the loads V are switched on. The driver circuits TRI to TRn do not switch in an inverting manner. If the loads are switched on, each load circuit is formed from the cuit rent source B, the switch S, the positive line +UB,the load V, the associated driver circuit TRI to TRn and the negative or earth line -UB. Thus, when the incandescent lamp controlled by circuitANI is switched on, the first connection ofthe lamp is electrically connected to the negative supply voltage of the negative or earth line -UB through the drivercircuitTRI.A positive potential at the first input of the control circuit ANI therefore means the load V is switched off, and a negative or earth potential at that first input means the load is switched on. It is also possible, however, to reverse the switching logic if the load V is connectedtothe earth line orthe negative line -UB. Forthis purpose, the switching logic in the microcomputer MC can be reversed, for example, orthe control circuits must switch in an inverting manner.

The driver circuits then switch the load on with a positive input potential.

The operation of the device shown in the figure as an example ofthe invention will now be described.

Since all the control circuits ANI to ANn and all the driver circuits TRI to TRn are alike in construction and are connected alike, it is sufficient in the following to refer only to the operation of the first control circuit ANI together with the first driver circuit TRI.

If the ignition switch S is closed, the microcomputer MC, the control module AN and the driver module TR are supplied with currentfrom the battery B through the positive line + UB and the negative or earth line - UB. At the same time, positive potential is applied to the second connection ofthe lamp V through the positive line + UB. The clock-pulse generatorT is supplied with currentthrough the microcomputer MC. Through the control moduleAN, among other things, the first control circuitANl is supplied with current. Through the driver module TR, among other things, the first driver circuitTRI is supplied with current.

If positive potential is now applied to the first input of the first control circuit AN I, positive potential also appears atthe output of that circuit and hence also at the input of the first driver circuitTR1. Thus the driver circuit TRI is blocked and the lamp V is switched off.

This positive potential at the output ofthe first control circuitANI is only effective at the input of the first driver circuit TRI, however, when positive potential from the clock-pulse generatorT, as part ofthe microcomputer MC, is applied to the second input ofthe control circuitANI through the common connecting line 2, for only then is the output of the first actuating circuitANl art a low impedance. This positive potential isonlyapplied pulsewise, however, to the second input of the control circuit and is repeated at a fixed frequency.

If negative or earth potential is applied to the first input ofthe first control circuitANI, the output of that circuit has a negative output voltage or earth potential. Again, this negative output voltage is only effective when positive potential from the clock-pulse generatorT is applied to the second input ofthe control circuit, whereby the output of the circuit has a low impedance. Thereby the negative potential ofthe output of the first control circuit AN I is impressed on the input of the first drivercircuitTRI, and thefirst driver circuit connects the firstterminal ofthe lamp to the earth line -UB so that the load circuit is closed and the lamp V is switched on.After the expiration of the pulse-like short phase of positive potential at the second input ofthe control circuit ANI, the output of that circuit becomes highly resistive. As a result, the output ofthe circuit is decoupled from the input of the first driver circuit TRI. Nevertheless, the first driver circuit remains conductive and so the load circuit continues to remain closed because the output ofthe first driver circuit is coupled back to its input through thefirstfeedback resistor Ri.

This feedback works as follows. If the load circuit is closed and if the load does not have too low an internal resistance or if the output of the first driver circuit TRI is not short-circuited, the voltage between the positive line+UB and the negative or earth line -UB falls substantially completely across the internal resistance of the load V. Thus substantially negative or earth potential appears at the output of the first driver circuit TRI. This negative or earth potential is impressed on the input ofthe first drivercircuit TRI through the first feed back resistor RI. This applies so long as the output of the first actuating circuit AN I is in its high-impedance state. If the output of the first actuating circuit ANI is again switched into its low-impedance state, its output voltage again determines the switching state ofthe first driver circuit TRI.Therefore, if negative or earth potential continuesto be applied to thefirst input ofthe first control circuitANI, thefirst driver circuitTRI remains switched on and so the load circuit remains closed.

If positive potential is now applied to the first input ofthe first control circuitANI, however, the output of the control circuit ANI and hence the input ofthefirst driver circuitTRI has positive potential. The first drivercircuitTRI breaksthe load circuitand the lamp V is switched off.

If the driver circuit TRI is conducting and the load ci rcuit with the lamp V is therefore closed, and if the input of the driver circuit TRI is decoupled from the output of the control circu it AN I by a high output im pedance of the output ofthe control circuit ANI, then, in the event a short-circuit atthe lampVorinthe event of too low an internal resistance of the lamp, the voltage between the positive line + UB and the negative or earth line -UBdropssubstantiallyac- ross the internal resistance ofthe first driver circuit TRI. This means that the outputofthe driver circuit TRI has substantially the positive potential of the line +UB.Thus,for a shorttime, input and output of the first driver circuit TRI have different potentials. Then, however, the potential of the output of the circuitTRI is impressed on its input through the feedback resistor RI. In this case, input and output of the drivercircuit TRI again have the same potential, namely positive potential. The positive potential at the input of the circuit TRI has the effectthat the circuit breaks the loadcircuitwiththelamp V.

If the output of the first control circuit ANI now briefly has a low output resistance as a result of a positive voltage pulse at its second inputfrom the clock-pulse generator T, the output of the control circuitANl and the inputofthe driver circuit TRI are again coupled for the shorttime of the positive voltage pulse. If negative potential is now applied to the output of the first control circuitANI and hence to the input ofthefirst driver circuit TRI, the circuit TRI becomes conducting for the short time of the positive voltage pulse, even in the event of a short-circuit or overload, and the load circu it with the lamp V is thus closed.The duration ofthe short positive voltage pulse from the clock-pulse generator is chosen to be so short, however, that when the voltage in the load circuit drops across the internal resistance of the first driver circuit TRI in the event of a short-circuit, the driver circuit is not overloaded as a result of the generation of heat. The permissible duration of the positive voltage pulse of thetiming-pulse generatorcircuitTis essentially determined by the internal resistance of the switching path of the driver circuitTRI in the load circuit and by the heat capacity of this switching path.

After the expiration of the duration of the voltage pulse of the clock-pulse generator T, which pulse recurs periodically at a fixed frequency, the output of the control circuitANI again has a high impedance and so the input of the driver circuit TRI is again decoupled from the output of its control circuit.Ifthe state of short-circuit or overload in the load circuit has now been eliminated, for example because the defective orfaulty load has been exchanged for an operative load with the correct electrical connected values, the driver circuit TRI becomes conducting and so the load circuit is again closed if negative or earth potential continues to appear the output ofthe control circuit ANI and as soon as the output of the control circuit is again briefly switched to low impedance by the positive voltage pulse of the clock-pulse generatorTafterexpiration of the period between pulses.

It is also possible for the clock-pulse generator additionally to switch the output to a low impedance through the second input ofthe control circuitANI on each appearance of a change in potential at the first input ofthe control circuit, in orderto avoid a delay in the switching on and off ofthe load V. That is to say, in addition to the predetermined frequency operation ofthe clock-pulse generatorT,with every change of potential at the first input of a control circuit corresponding to the associated load being switched on or off the clock-pulse generator is arranged to switch the output of that control circuit. By this means the command to switch the associated load on or off is realised immediately by the control circuit.

The device of the example shown in the figure has the particular advantage that when a plurality of electrical loads in a plurality of different load circuits are switched by an electronic switch or circuit, then in the event of a state ofshort-circuit or overload in one of the load circuits, the other load circuits can still be actuated by the electronic switch or circuit through the output circuit described.The elimination of states of short-circuit or overload by replacing defective loads or by exchanging loads with too little internal resistance for loads with a sufficiently high internal resistance can be carried outwhile the electronic circuit and the output circuits are in operation so that operations having a critical timing, in which electrical loads are switched, are not disturbed by a state of short-circuit or overload in another load circuit nor by its elimination. As an example of an operation the timing of which is critical, there is the control of ignition and/or petrol injection of a motor-vehicle engine which also depends, inter alia, on the instantaneous crankshaft angle of the engine. This control or regulation of ignition or petrol injection can be effected by the same electronic circuit in the example of the microcomputer MC shown inthefigure.

The outputcircuit described above hasthepart- icular advantage that it is simple and inexpensive to construct. Only one resistor is necessary for each line that is protected. The driver circuit and the control circuit are largelyseparated with regard to their circuitry so that the simple feedbackthrough the resistor is rendered possible. Thus retrofitting to output circuits already in existence is also simple.

The construction of an output circuit according to the invention is particularlysimplewhen, as in relatively large electronic circuits, as for example central electrical or central microprocessor systems, the control circuits and/orthe driver circuits forthe large numberofoutputs necessary, are already present. In central microprocessor systems, for example, the clock-pulse generator can be produced as part of the microcomputer MC by adding appropriate program parts to the computer program.If the drivercircuits are already combined in an integrated driver module TR, it is sufficient, in orderto establish the electrical connections between the driver module and the printed circuit board of the rest ofthe elec- tronic circuit, to provide a socket or other connector as an intermediate member in which the required feedback resistors Rare integrated. In this case, it is not necessary to alterthe layout of the printed circuit for the retrofitting of the output circuit. Thus theretrofitting of an electroniccircuitwith an output circuit according to the invention can be affected part icularlysimplyand inexpensively.

Output circuits according to the invention may be particularly suitable for use in motor vehicles with electronic circuits which are susceptible to the spe cial installation conditions, such as vibrations in the vehicle and environmental influences. This also app lies to the electrical loads, for example, such as in candescent lamps in which the filament can shake loose from the holding stems and may easily cause short-circuits in the base where the stems lead in.

Aso-called shift register may also be used as a con trol circuit AN to which the information correspond ing to the switching states ofthe driver circuits TR1 to TRn is transmitted serially. The shift registerthen likewise comprises a second input 2, the input pot ential of which determines the resistance at the out puts ofthe shift register.

Claims (16)

1. An output circuit for an electronic circuit or switch for switching an electrical load, comprising a driver circuit having an output for connection to the electrical load, and a control circuitforthe pulsewise actuation of the driver circuit, the output of the driver circuit being coupled back to its input through afeedback resistor and the control circuit having a first input through which its output voltage can be controlled and a second input through which its output impedance can be controlled.

2. An output circuit as claimed in Claim 1, wherein a clock-pulse generator switches the output of the control circuit to a low impedance, pulsewise, through the second input.

3. An output circuit as claimed in Claim 2, wherein the clock-pulse generator switches said output at a fixed frequency.

4. An output circuit as claimed in Claim 2 or Claim 3, wherein the clock-pulse generator switches the output of the control circuit to a low impedance, pulsewise, on each change of potential at the first input of the control circuit.

5. An output circuit as claimed in any one of claims 2 to 4, wherein the clock-pulse generator forms part of a microcomputer.

6. An output circuit as claimed in any one ofthe preceding claims, wherein the driver circuit is constructed in the form of an integrated driver module.

7. An output circuit as claimed in Claim 6, wherein the driver module comprise a plurality of driver circuits.

8. An output circuit as claimed in Claim 6 or Claim 7, wherein the or each driver circuit of the integrated driver module has a connectorfor its electrical connection to the rest ofthe circuit, the feed back resistor being partoftheconnector.

9. An output circuit as claimed in any one ofthe preceding claims, wherein the control circuit is constructed in the form of an integrated control module.

10. An output circuit as claimed in Claim 9, wherein the control module comprises a plurality of control circuits.

11. An outputcircuitas claimed in Claim 10, wherein the control module comprises as many first inputs and outputs as there are control circuits included in the module and a single second inputforthe joint switching of the outputs ofthe control circuits to low impedance.

12. An output circuit as claimed in any one of the preceding claims for switching a plurality of electrical loads, there being respective driver circuits for said loads, each with a feedback resistor.

13. An outputcircuitasclaimed in Claim 12, wherein respective control circuits are provided for the individual driver circuits.

14. An output circuit as claimed in Claim 12, wherein a shift register provides a control circuitfor the respective driver circuits.

15. An output circu it for an electronic circuit or switch, constructed and arranged for use and operation substantially as herein described.

16. An output circuit as claimed in any one of the preceding claims adapted for use in a motor vehicle.