Voltage generating circuit

The invention relates to a voltage generating circuit and to a voltage stabilized supply device including such a circuit.

At the present time, numerous voltage generating circuits are known. One of the problems that they have to solve is that of outputting a voltage level that is relatively stable or, at the very least, one that does not exceed a given maximum value, while the voltage level input to the circuit can vary, either above or below a nominal supply voltage.

To solve this problem, it has already been suggested to use components such as Zener diodes shunt connected at the input to the circuit in order to limit the voltage to a given level, in the event of the nominal voltage being exceeded. Furthermore, in order to protect the components present downstream, only a fraction of the voltage input to the circuit is used to generate the output voltage.

It will be appreciated, nonetheless that, with such a solution, the means used to achieve the desired output voltage level have to be over-dimensioned in relation to the means that it would have sufficed to use if all of the input voltage, and not just a fraction thereof, had been used. In the case of charge pump circuits, for example, it is necessary to increase the number of charge stages of the pump or pumps.

As a corollary, in circuits having the same characteristics, the response times to obtain the same output voltage level are longer for a circuit using only a fraction of the input voltage and not its totality. This will apply, in particular, to charge pump circuits having the same number of charge stages but using, or not using, all of the circuit input voltage.

The object of the present invention is to provide a voltage generating circuit and a voltage stabilized supply device including such a circuit that overcome the aforementioned drawbacks and that make it possible to avoid having to over-dimension the means used to obtain at the output the desired voltage level, that is to say, for example, that make it possible to reduce the number of charge stages of the charge pumps, in the event of such components being used.

A further object of the present invention is to provide a voltage generating circuit and a voltage stabilized supply device using such a circuit that make it possible to reduce the start-up times.

Another object of the present invention is to provide a voltage generating circuit and a stabilized voltage supply device including such a circuit that afford protection in the event of a voltage level exceeding the nominal voltage being present at the input.

Further objects and advantages of the invention will emerge in the course of the following description, which is given only by way of illustration and is not intended to limit same.

The invention relates firstly to a voltage generating circuit having:

a voltage input;

voltage boosting means, having an input and an output;

means for varying the voltage supply level of the input to the said boosting means from the said voltage input of the circuit, capable, on one hand, in a first state, of permitting the application, at the input to the said boosting means, of a fraction, that is possibly limited, of the voltage input to the circuit and, in a second state, of increasing the level of voltage applied to the said input to the boosting means and, on the other hand, of permitting the change-over from the said first to the said second state according to a triggering signal, obtained by feedback of the output voltage from the said boosting means to the said means for varying the voltage level, so as to optimize the production at the output from the said voltage boosting means of the desired voltage level.

The invention also relates to a voltage stabilized supply device including such a circuit.

The invention will be more readily understood in the light of the following description of an embodiment taken in conjunction with the accompanying drawing wherein: the FIGURE is a diagram showing an exemplary form of embodiment of the voltage generating circuit according to the invention, including ;an exemplary embodiment of the voltage stabilized supply device.

As illustrated in the figure, the voltage generating device according to the invention has a voltage input, 1 , and voltage boosting means, 2 , having an input, 3 , and an output, 4 , which could be the output from the circuit.

In order to optimize the production, at the output from the said voltage boosting means 2 , of a desired voltage level, the circuit according to the invention further has means, 6 , for varying the level of voltage supply to input 3 of the said boosting means 2 , from the said voltage input 1 of the circuit.

The said means 6 for varying the supply level are designed to be capable, in a first state, of permitting the application, at the input to the said boosting means 2 , of a fraction, that is possibly limited, of the voltage input to the circuit.

<<Possibly limited>> is to be taken as meaning, as explained in more detail hereinafter, that, in the event of a voltage exceeding the nominal voltage being present at the input to the circuit, the voltage applied downstream is then fixed at a chosen maximum level.

The said means 6 for varying the voltage supply level are further designed to be capable, in a second state, of increasing the level of voltage applied to the said input of boosting means 2 .

In addition, they permit the change-over from the said first to the said second state, this being in accordance with a triggering signal, obtained by feedback of the output voltage from the said boosting means 2 to the said means 6 for varying the voltage level.

There is thus provided a circuit that will, firstly, afford protection for the said voltage boosting means 2 and/or the downstream components. Furthermore, the said circuit makes it possible, thanks to the change-over from the said first to the said second state, no longer to use just a given fraction of the said input voltage but a higher voltage, possibly approaching the said input voltage. For the same dimensioning of boosting means 2 , it is then possible to obtain shorter response times.

It should also be noted that, thanks to the feedback, it is the output signal of boosting means 2 themselves that is used to improve the level of their input signal, which thus makes it possible to benefit from positive feedback.

The circuit according to the invention thus makes it possible, without prejudice to performance, to be able to contemplate voltage generation protected from over-voltages at the input. It will include more precisely for this purpose voltage limiting means, at this point R, 5 .

As illustrated, the said means 6 for varying the voltage supply level of the input to the said boosting means 2 are constituted, for example, by two transistors, T 1 , T 2 , whereof the drain and source are connected, respectively, to the input 1 of the circuit and to the input 3 of boosting means 2 , and the gate of which, in the case of the first thereof, T 1 , is connected to the said means for limiting the voltage at the input to the generator and, in the case of the second thereof, T 2 , to output 4 of the said boosting means 2 .

The said transistors T 1 T 2 , are, in particular, high voltage active devices.

The operation of such a circuit is described hereafter. For a nominal voltage input to the circuit, the input voltage of voltage boosting means 2 is initially equal to the input voltage of the circuit less the voltage between the gate and the source of transistor T 1 .

Voltage boosting means 2 then perform their function, and the voltage at the gate of second transistor T 2 rises above the voltage at the gate of first transistor T 1 . The said second transistor T 2 consequently becomes more conductive and the latter takes over the said transistor T 1 , while permitting the application at the input to the said voltage boosting means 2 of practically the entire input voltage of the circuit.

When the input voltage of the circuit far exceeds the nominal voltage, the voltage input to boosting means 2 is, initially, equal to the voltage supplied by limiting means R, 5 less the voltage between the gate and the source of first transistor T 1 .

As in the preceding case, boosting means 2 then perform their function. The input voltage of voltage boosting means 2 becomes equal to that of built-in limiting means 8 , 9 , 10 and 2 minus the voltage between the gate and the source of second transistor T 2 if this voltage is higher than the initial input voltage.

Thus, in the event of a nominal voltage being present at the input to the device, the said first and second transistors T 1 , T 2 operate in linear mode, while, in the event of a voltage far higher than nominal voltage, they operate in saturated mode.

The said limiting means are constituted, in particular, by a component with three connections, the first connection being designed to be connected to the voltage to be limited, the second connection being designed to be connected to a lower supply voltage, e.g. the earth, and the third being designed to supply a given limited voltage, through the choice of the said component.

These can be, for example, a resistor R and a Zener diode 5 connected in series. The resistance is connected between the voltage input and the gate of the first transistor, T 1 . The Zener diode is connected between the gate of the first transistor T 1 and the lowest supply voltage, e.g. the earthed point 7 . The said diode is connected so as to block an input voltage of the circuit corresponding to the input voltage of the device, which is positive. The means could also consist of a depletion mode MOS transistor.

The said boosting means 2 are constituted, for example, by a charge pump, that is to say a component constituted, among others, by capacitors and/or diodes or other switching means, disposed in successive stages.

They thus have a level of amplification of the given voltage, possibly variable as a function of the number of stages used.

The invention also relates to a voltage stabilized supply device including a circuit as described above.

There can be a third transistor, T 3 , the drain of which is connected to the said voltage input 1 of the circuit and the gate of which is connected to the output 4 of the said voltage boosting means 2 .

Thanks to the action of the voltage boosting means 2 , the said third transistor T 3 can then be considered as completely conductive, as from the moment at which the voltage at its gate is sufficiently high.

There is thus supplied at the source of the said third transistor T 3 an output voltage of device 8 that is substantially equal to the said voltage input to the circuit or to the said maximum voltage, hence regulated.

Furthermore, as already mentioned, the said boosting means 2 can have a variable voltage amplifying level, and means 9 for adjusting the said level according to the output voltage of the said device can be provided if necessary.

These can be, for example, a component making it possible to compare the output voltage of the said device with a reference voltage 10 and, in the event of the latter being exceeded, to control the operation of the charge pump, for example its sequencing.

The output voltage of the latter then drops, thus permitting a reduction in the level of the output voltage of the device to the extent of dropping back beneath the said reference voltage.

It should be noted that the terminology used (<<source>>, <<drain>>, <<gate>>) in designating the connections of the first, second and third transistors, T 1 , T 2 , T 3 , makes no presupposition regarding the type concerned. These can, in fact, be, for example, transistors of the MOS type, in particular DMOS type transistors in the event of a risk, in particular, of a high input voltage. They can also, however, be, among others, bipolar or IGBT transistors.

Other forms of embodiment of the invention, within the grasp of a man of the art, could, of course, have been contemplated without thereby departing from the scope of the invention, in particular as regards the mode of regulation of the output voltage about the reference voltage.