Abstract:
A regulating device for receiving a variable voltage and delivering a constant voltage includes a regulating element that includes a circuit for comparing the variable voltage with a reference voltage, a circuit for dividing the variable voltage by a factor, and a switching circuit for supplying the regulating element with a voltage equal either to the variable voltage or to the divided variable voltage. The switching circuit may be controlled by the comparison circuit in such a way that the regulating element is supplied with the variable voltage if a voltage condition is not satisfied and with the divided variable voltage if the voltage condition is satisfied.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of voltage regulators, and, more particularly, to voltage regulators of a type delivering a constant DC voltage while being supplied with a DC voltage prone to variation. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a regulator which is required to output a voltage of 3.3 volts will be supplied with a DC voltage of about 5.1 to 9.5 volts. The upper limit of supply of a regulator depends essentially on the technology of the active components with which it is provided. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a voltage regulator which is not damaged when the voltage to which it is subjected exceeds the breakdown voltage of its active components. 
     A regulating device according to present the invention is supplied with a variable voltage V v  and delivers a constant voltage for supplying consumer elements. The device may include a regulating element, a means or circuit for comparing the variable voltage V v  with a reference voltage V ref , a means or circuit for dividing the variable voltage V v  by a factor k 1 , and a switching means or circuit able to supply the regulating element with a voltage V r  equal either to the variable voltage V v , or to the divided variable voltage V v /k 1 . The switching means may be controlled by the comparison means in such a way that the regulating element is supplied with the variable voltage V v  if a voltage condition is satisfied and with the divided variable voltage V v /k 1  if the voltage condition is not satisfied. This is because the variable voltage V v  is prone to take values greater than those which the active components of the device are capable of supporting. 
     Advantageously, the comparison means may include a means or circuit for dividing the variable voltage V v  by a factor k 2  to obtain a comparison voltage V comp =V v /k 2 , and an amplifier arranged as a comparator receiving on one input the comparison voltage V comp  and on another input the reference voltage V ref  to output a control signal V + . The control signal V +  may be of a conventional value  1  if the comparison voltage V comp  is greater than the reference voltage V ref  and of a conventional value  0  if the comparison voltage V comp  is less than the reference voltage V ref . 
     The device may include an inverter at the output of the amplifier to obtain an inverse control signal V − . The means for dividing by the factor k 2  may include at least two resistors arranged in series between the variable voltage V v  and ground. The means for dividing by the factor k 1  may include at least two resistors arranged in series between the variable voltage V v  and ground. The means for dividing by the factor k 1  and the means for dividing by the factor k 2  may include at least one common resistor. 
     Advantageously, the switching means may include a first transistor, one terminal of which is connected to the input of the switching means and sees the variable voltage V v . Another terminal of the first transistor is connected to the output of the switching means and sees the voltage V r . A control terminal of the first transistor is linked to a control means or circuit generating a voltage able to turn on the first transistor if the voltage condition is not satisfied or to turn off the transistor if the voltage condition is satisfied. The first transistor may be a MOS transistor. 
     The switching means may also include at least one second transistor, one terminal of which is connected to the input of the switching means and sees the variable voltage V v . Another terminal of the second transistor is connected to the output of the switching means and sees the voltage V r . Furthermore, a control terminal of the second transistor sees a control voltage equal to the divided variable voltage V v /k 1  that is able to turn off the second transistor if the voltage condition is not satisfied and to turn on the second transistor if the voltage condition is satisfied. Thus, the voltage V r  is equal to the divided variable voltage V v /k 1 . The second transistor may be replaced by a cascode arrangement of several transistors, for example bipolar transistors, to deliver more current at the output of the switching means. 
     Advantageously, the means for controlling the first transistor may include a third transistor controlled by an output voltage from the comparison means and a fourth transistor controlled by the inverse of the output voltage from the comparison means. The third transistor may be connected at one terminal to ground and at another terminal to the output of the switching means seeing the voltage V r  by way of two resistors R 21  and R 22  in series. A common point shared by the two resistors sees the voltage V r  if the voltage condition is not satisfied and a voltage equal to V r *R 21 /(R 21 +R 22 ) if the voltage condition is satisfied. 
     The fourth transistor may be connected by one terminal to ground and by another terminal to the output of the switching means by a fifth transistor whose control terminal is connected to the common point shared by the two resistors. The fifth transistor may be on if the voltage condition is satisfied and off if the voltage condition is not satisfied so that the common point shared by the fourth and fifth transistors sees a voltage which is substantially  0  if the voltage condition is not satisfied and substantially equal to the voltage V r  if the voltage condition is satisfied. The third and fourth transistors may be MOS transistors with their sources connected to ground. The fifth transistor may also be a MOS transistor with a source connected to the voltage V r . 
     Advantageously, the means for controlling the first transistor may furthermore include a sixth transistor having a terminal connected to the common point shared by the fourth and fifth transistors. The other terminal and the control terminal of the sixth transistor may be short circuited and linked to the output of the switching means seeing the voltage V r  by two resistors R 27  and R 28  in series. The sixth transistor may be on if the voltage condition is not satisfied and off if the voltage condition is satisfied. The common point shared by the two resistors R 27  and R 28  sees the voltage V r *R 27 /(R 27 +R 28 ) if the voltage condition is not satisfied and the voltage V r  if the voltage condition is satisfied. 
     A seventh transistor is provided with a control terminal connected to the common point shared by the two resistors R 27  and R 28 . A terminal thereof may be connected to the common point shared by the fourth and fifth transistors. Another terminal of the seventh transistor may be connected to the common point shared by the fourth and fifth transistors by a resistor R 33 . Also, the remaining terminal of the seventh transistor may be connected to the control terminal of the first transistor of the switching means by a resistor R 32 . A resistor R 31  links the control terminal of the first transistor and the input of the switching means which sees the variable voltage V v . 
     This is done in such a way that the seventh transistor is on if the voltage condition is not satisfied (the control terminal of the first transistor being subjected to a voltage substantially equal to V v *R 32 /(R 31 +R 32 ) able to turn it on) and that the seventh transistor is off if the voltage condition is satisfied (the control terminal of the first transistor being subjected to a voltage substantially equal to V v  able to turn it off). The sixth transistor may be a bipolar transistor with short-circuited collector and base. The seventh transistor may be a bipolar transistor with the collector linked to the common point shared by the resistors R 32  and R 33 . 
     By way of example, a regulator may be embodied in HF 5  CMOS technology for which the breakdown voltage is about 15 volts. The general principle is to detect the voltage applied with respect to a threshold of 12.5 volts by a resistive bridge and a comparator, and to switch the regulating structure while maintaining normal operation if the voltage is below 12 volts and by dividing the voltage applied if it is greater than 12 volts. Thus, not only is the regulator protected against destruction in the event of an excessive supply voltage, but additionally the regulator continues to operate satisfactorily at a voltage above the breakdown voltage. The regulator is designed in such a way that none of its components prone to break down at about 15 volts are subjected to such a voltage. 
     A regulating process according to the invention provides a constant voltage for supplying consumer elements from a variable voltage V v , in which process the variable voltage V v  is compared with a reference voltage V ref , the variable voltage V v  is divided by a factor k 1 , and the regulating element is supplied with an equal voltage V r  either via the variable voltage V v  or via the divided variable voltage V v /k 1  by switching between the two voltages. The switching may be controlled as a function of the comparison in such a way that the regulating element is supplied with the variable voltage V v  if a voltage condition is not satisfied and with the divided variable voltage V v /k 1  if the voltage condition is satisfied. The variable voltage V v  is prone to take values greater than those which the active components of the device are capable of supporting. 
     The invention also applies to the automobile field, and in particular to inflatable safety bags. Thus, it is possible to construct a regulator which supports a supply voltage greater than that normally permitted by the technology used. This exhibits numerous advantages in terms of choice of technology, reduction of the silicon area used and optimization. Indeed, in the case of an automobile, the regulator is normally supplied from a battery or an alternator operating at 12 volts. However, should the battery be unplugged, the output voltage from the alternator could reach much higher values. The electrical network of the vehicle is also subjected to radiation due to the high voltage used by the ignition spark plugs of the engine. As a result, a regulator mounted in an automobile must be able to support voltages of up to 25 volts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood upon studying the detailed description of the embodiment given by way of non-limiting example and illustrated by the appended drawings, in which: 
     FIG. 1 is a schematic block diagram of a regulating device according to the present invention; 
     FIG. 2 is a schematic block diagram of the comparison means according to the present invention; and 
     FIG. 3 is a schematic diagram of the switching means according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As may be seen in FIG. 1, a regulating device according to the present invention includes a regulating element  1  of a conventional type that is supplied with a DC voltage prone to vary between 5.1 and 12.5 volts and outputting a regulated voltage of 3.3 volts, for example. The regulating device also includes a reference module  2  which delivers a reference voltage to the regulating element, enabling it to formulate the regulated voltage. 
     The regulating device includes a means  3  for dividing the supply voltage V v , which is between 5.1 and 25 volts, for example. The dividing means  3  may divide the supply voltage by two, for example. The regulating device also includes a switching means  4 , of which one input is linked directly to the supply voltage V v . Another input is linked to the output of the dividing means  3  and sees a voltage V r  lying between 5.1 and 12.5 volts. The output supplies the regulating element  1  and a control input receives a setting originating from a comparison means  5 . The comparison means  5  includes an amplifier  6  arranged as a comparator whose negative terminal receives a reference voltage V ref  originating from the reference module  2 , and whose positive terminal receives a voltage V comp  proportional to the supply voltage V v  by way of two resistors  7  and  8  arranged in series between the voltage supply lying between 5.1 and 25 volts and ground. The positive terminal of the amplifier  6  is linked to the common point between the two resistors  7  and  8 . The output of the amplifier  6  is linked to the control input of the switching means  4 . 
     In the case of FIG. 1, it may be seen that the switching means  4  supplies the regulating element  1  directly with the supply voltage V v . This stems from the fact that the supply voltage V v  is less than 12.5 volts. If the supply voltage is greater than 12.5 volts, the amplifier  6  outputs an opposite setting. The switching means  4  then supplies the regulating element  1  from the divided voltage V r  delivered by the dividing means  3 . Thus, the regulating element  1  still sees at its input a voltage of less than or equal to 12.5 volts, while the regulating device as a whole sees a supply voltage of less than or equal to 25 volts. 
     In FIG. 2, it may be seen that the module  9 , which groups together the dividing means  3  and the switching means  4 , is linked to the output of the amplifier  6  through which it receives a control signal V +  from the conductor  10 . The module  9  also receives an inverse control signal V −  obtained by an inverter  11  receiving as an input the control signal V + . The comparison voltage V comp  is delivered to the negative terminal of the amplifier  6  by the module  9 . The resistors making it possible to obtain the comparison voltage V comp  may also be integrated into the module  9 . The module  9  is illustrated in detail in FIG.  3 . 
     To aid understanding, the module  9  may be regarded as being divided into three parts possessing different functionalities, which are separated by dotted lines in FIG. 3 for convenience. The dividing means  3  is formed by three resistors R 12 , R 13  and R 14  connected in series between a line delivering the variable supply voltage V v  (varying between 5.1 and 25 volts) and the ground line. It will be understood that the ground may be floating, i.e., the voltage V v  is understood as taken with respect to ground. 
     The comparison voltage V comp  is tapped off at the common point between the resistors R 13  and R 14  and a divided voltage V d  is tapped off at the common point between the resistors R 12  and R 13 . Therefore, V comp =V v *R 14 /(R 12 +R 13 +R 14 ) and V d =V v *(R 13 +R 14 )(R 12 +R 13 +R 14 ). Stated otherwise, k 1 =(R 13 +R 14 )/(R 12 +R 13 +R 14 ) and k 2 =R 14 /(R 12 +R 13 +R 14 ). By way of example, R 12  may be equal to 55 kOhms, R 13  may be equal to 45 kOhms and R 14  may be equal to 25 kOhms, from which it follows that V comp  is less than or equal to 5 volts and that V d  is less than or equal to 14 volts. 
     The remainder of the module  9  forms the switching means  4  and can be divided into a switching part  15  and a switching control part  16 . The switching part  15  includes a first MOS-type transistor T 17 . The sourse of the first transistor T 17  is linked to the supply voltage V v . The drain of the first transistor T 17  is linked to the output of the module  9  which delivers a supply voltage V r  of less than or equal to 12.5 volts to the regulating element  1  of FIG.  1 . Furthermore, the gate of the first transistor T 17  receives a control signal originating from the switching control part  16 , which will be described below. 
     The switching part  15  also includes a second PNP bipolar type transistor T 18  whose collector is linked to the supply line under the voltage V v . The base of the second transistor T 18  is linked to the common point shared by the resistors R 12  and R 13  and sees the voltage V d . The emitter of the second transistor T 18  is linked to an additional bipolar transistor T 19  forming a cascode arrangement to be able to deliver a high output current under the voltage V r . If the output current demanded is lower, the presence of the additional transistor T 19  is not necessary. The transistor T 19 , of an NPN type, has its collector linked to the supply line under the voltage V v , its base linked to the emitter of the transistor T 18 , and its emitter linked to the output line of the voltage V r . 
     It may therefore be seen that if the transistor T 17  is on V r =V v  and the two bipolar transistors T 18  and T 19  are short circuited and hence off. Conversely, if the transistor T 17  is off, the voltage at the emitter of the transistor T 19  is less than or equal to 12.5 volts. As soon as the difference between the voltages V d  and V r  becomes sufficient to turn on the transistors T 18  and T 19  (i.e., greater than or equal to 1.4 volts in general), the transistors T 18  and T 19  are turned on. One thus obtains a voltage V r  equal to the voltage V d  to within the base-emitter voltages of the transistors T 18  and T 19 . More precisely, V r =V d −V BET18 −V BET19 . For example, if at this moment V v =25 volts, V d =14 volts and V r  is substantially equal to 12.5 volts. To increase the breakdown voltage of the transistors T 17 , T 18  and T 19 , they may be embodied in an insulated-well fashion. However, it should be noted that the maximum voltage which these transistors see is equal to V v −V r  and therefore does not exceed 12.5 volts. 
     The switching control part  16  serves to generate the control signal for the gate of the transistor T 17  in such a way that the transistor T 17  is off if the variable voltage V v  is greater than 12.5 volts and is on in the other cases. The switching control part receives the control signal V +  and the inverse control signal V −  originating from the comparison means  5 , illustrated in FIG.  1 . 
     The switching control part  16  includes a MOS transistor T 20  whose source is connected to ground, whose gate receives the control signal V + , and whose drain is connected to the output line under the voltage V r  by way of two resistors in series R 21  and R 22 . The switching control part  16  may also include a MOS transistor T 23 , of the same type as above, whose source is linked to ground, whose gate receives the inverse control signal V − , and whose drain is linked to a point  24 . A MOS transistor T 25  has its drain linked to the point  24 , its source linked to the output line under the voltage V r , and its gate linked to the common point between the resistors R 21  and R 22 . 
     An NPN-type bipolar transistor T 26  has its emitter linked to the point  24  and its base and its collector short circuited and linked to the output line at the voltage V r  by two resistors R 27  and R 28 . An NPN-type bipolar transistor T 29  has its emitter linked to the point  24 , its base linked to the common point between the resistors R 27  and R 28 , and its collector linked to a point  30 . 
     Three resistors in series, R 31 , R 32  and R 33 , are disposed between the input line at the variable voltage V v  and the point  24 . The point  30  is the common point between the resistors R 32  and R 33 . Stated otherwise, when it is on, the transistor T 29  is able to short circuit the resistor R 33 . The common point between the resistors R 31  and R 32  is linked to the gate of the transistor T 17  of the switching part  15  and therefore delivers the control signal to it. 
     The manner of operation of the switching control part  16  is as follows. If the voltage V v  is greater than 12.5 volts, the transistor T 20  receives a positive control signal V +  which turns it on, while the transistor T 23  receives a zero inverse control signal V −  which turns it off. The gate of the transistor T 25  is subjected to a voltage markedly less than the voltage V r  on its source. To this end, R 21  may be  150  kOhms and R 22  may be 100 kOhms. The voltage at the point  24  is therefore equal to the supply voltage V r  plus the almost zero voltage between the drain and the source of the transistor T 25 . The voltage between the base and the emitter of the transistor T 26  is zero. The transistor T 26  is off. The same holds for the transistor T 29 . 
     It follows that a current flows between the input line at the voltage V v  and the point  24  by the resistors R 31 , R 32  and R 33 . However, a small value is chosen for the resistor R 31  relative to the resistor R 33  (e.g., on the order of 10%) so that the voltage on the gate of the transistor T 17  is high and very close to the variable voltage V v . The transistor T 17  is thus turned off. By way of example, R 31  may be 30 kOhms, R 32  may be 45 kOhms, and R 33  may be 300 kOhms. 
     If the voltage V v  is less than or equal to 12.5 volts, the manner of operation is as follows. The transistor T 20  receives on its gate a zero control signal V +  which turns it off, while the transistor T 23  receives on its gate a positive inverse control signal V −  which turns it on. The voltage at the point  24  is therefore substantially zero. The transistor T 25  has its gate and its source substantially at the same potential and is therefore off. The transistor T 26  is turned on by virtue of the current flowing from the output line at the voltage V r  through the resistors R 28  and R 27 . By reason of its arrangement, the transistor T 26  behaves like a diode. It is therefore on as soon as the voltage V r  becomes greater than 0.7 volts. 
     By way of example, identical values equal to 100 kOhms may be chosen for the resistors R 27  and R 28 . The base of the transistor T 29  is subjected to a voltage substantially equal to 0.7 volts plus half the difference between the voltage V r  and 0.7 volts. Stated otherwise, the transistor T 29  turns on as soon as the voltage V r  exceeds 0.7 volts. The transistor T 29  thus short circuits the resistor R 33 . The voltage at the point  30  is therefore close to zero. The voltage at the gate of the transistor T 17  is substantially equal to V v *R 32 /(R 31 +R 32 )=0.4*V v , this being sufficient to turn on the transistor T 17  even if the voltage V v  is getting close to its threshold of 5.1 volts. The transistor T 17  then short circuits the bipolar transistors T 18  and T 19  which turn off. 
     On startup, even if the variable voltage V v  is less than 12.5 volts, the bipolar transistors T 18  and T 19  of the cascode arrangement, which are not yet short circuited by the transistor T 17 , are naturally on. Thus, in the short interval of time between the application of the voltage V v  and the switching of the transistor T 17 , the regulating device operates by dividing the supply voltage, thereby guaranteeing the safety of the regulating element  1 . 
     By way of example, if, on startup, the regulating device receives a voltage V v  equal to 10 volts, the base of the transistor T 18  is subjected to a voltage on the order of 5.6 volts. It follows that the transistors T 18  and T 19  are conducting, thereby making it possible to have V r  equal to about 4.2 volts on the output line. This causes biasing of the transistors T 20 , T 23 , T 25 , T 26 , T 29  and the outputting of a control signal on the gate of the transistor T 17  able to turn it on and to short circuit the transistors T 18  and T 19 , in such a way that the voltage V r  reaches 10 volts. 
     It may also be noted that the various transistors are not subjected to voltages above 15 volts. This is because the transistor T 20  is between V r  and ground and is subjected to a maximum of 12.5 volts. The transistor T 23  between the point  24  and ground can be subjected to a voltage of slightly greater than 12.5 volts. It is in any event limited by the fact that the resistor R 33  is of a high value and will therefore tend to limit the current passing through the transistor T 25  when it is on. When it is off, the transistor T 25  is subjected to the voltage V r . When it is off, the transistor T 26  is likewise subjected to the voltage V r . When it is off, the transistor T 29  is subjected to the voltage across the terminals of the resistor R 33 , which always remains less than the voltage difference between V v  and V r , and hence less than 12.5 volts. 
     Thus, the switching control means  16  is supplied, in general, with the voltage V r  limited to a maximum of 12.5 volts, while being able to control the gate of the transistor T 17  to a voltage of between 12.5 and 25 volts. By virtue of the invention, it is possible to embody a voltage regulator in integrated technology (e.g., HF 5  CMOS, Bi-CMOS not supporting high voltages), while the regulator will be able to support markedly higher voltages, e.g., double. 
     Thus, it is possible to use economical integration technologies allowing high operating speeds while having a regulator which is compatible with a difficult environment prone to high overvoltages. It should be noted that it is possible to increase the speed of operation of the switching means by reducing the value of the resistors R 31  and R 32 , in such a way that the gate capacitances of the MOS transistor T 17  charge up more rapidly.