FR3068189A1 - RC OSCILLATOR - Google Patents

Abstract

The invention relates to an oscillator (1) comprising: a comparator (3); a generator (7, C, K) of a voltage ramp (VC) whose output is connected to a first input terminal (+) of the comparator and has a first constant current source (7); and one or more second current sources (6, 8) connected to the first input terminal of the comparator and supplying or removing a current proportional or inversely proportional to the temperature.

Description

RC OSCILLATOR

Field

The present description relates generally to electronic circuits and, more particularly, to the production of an RC oscillator.

Presentation of the prior art

The RC oscillators are based on the generation of a ramp which is reset by a level comparator whose reference conditions the period of a digital signal supplied by the comparator. Structurally, these oscillators include current sources as well as resistive and capacitive elements whose values vary with temperature, which affects the stability of the period of the output signal.

summary

One embodiment overcomes all or part of the drawbacks of known RC oscillators.

One embodiment provides an RC oscillator which is temperature stable.

One embodiment proposes a particularly simple solution and requiring no elements of a different nature from those usually present in an RC oscillator.

B16138FR - 17-RO-0254

Thus, one embodiment provides an oscillator comprising:

a comparator;

a generator of a voltage ramp, one output of which is connected to a first comparator input terminal and comprising a first constant current source; and one or more second current sources, connected to the first input terminal of the comparator and supplying or withdrawing a current proportional or inversely proportional to the temperature.

According to one embodiment, the ramp generator comprises:

a capacitive element in series with the first current source; and a switch in parallel on the capacitive element, the switch being controlled by an output of the comparator.

According to one embodiment, the oscillator further comprises a generator of a threshold voltage, one output of which is connected to a second input terminal of the comparator.

According to one embodiment, the threshold voltage generator comprises a third current source in series with a resistor, the midpoint of the series association being connected to said second input terminal of the comparator.

According to one embodiment, the oscillator further comprises a circuit for shaping an output signal from the comparator.

According to one embodiment, the second current source or sources each comprise several switchable current sources.

According to one embodiment, the second current source (s) are dimensioned to correct possible frequency drifts following temperature variations.

One embodiment provides an electronic circuit comprising at least one oscillator.

B16138ER - 17-RO-0254

Brief description of the drawings

These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments made without implied limitation in relation to the attached figures, among which:

FIG. 1 very schematically shows an example of a conventional RC oscillator;

Figures 2A, 2B and 2C illustrate, by timing diagrams, the operation of an RC oscillator;

Figure 3 shows, schematically and partially in the form of blocks, an embodiment of an RC oscillator;

FIG. 4 illustrates the temperature behavior of an RC oscillator; and FIG. 5 is an exemplary embodiment of a correction current source for the oscillator of FIG. 3. Detailed description

The same elements have been designated by the same references to the various figures which have been traced, in particular for the chronograms, without respect for scale. In particular, the structural and / or functional elements common to the various embodiments may have the same references and may have identical structural, dimensional and material properties.

For the sake of clarity, only the steps and elements useful for understanding the embodiments which will be described have been shown and will be detailed. In particular, the final application of the oscillator has not been detailed, the embodiments described being compatible with the usual applications of RC oscillators.

Unless otherwise specified, when reference is made to two elements connected to each other, this means directly connected without any intermediate element other than conductors, and when reference is made to two elements connected to each other, it means that these two elements can be directly

B16138ER - 17-RO-0254 connected (connected) or connected via one or more other elements.

In the description which follows, unless stated otherwise, the expressions approximately, substantially, and of the order of mean to the nearest 10%, preferably to the nearest 5%.

FIG. 1 very schematically shows an example of a conventional RC oscillator.

The oscillator comprises a comparator 3 (for example, an operational amplifier) of which a first input terminal, for example non-inverting or positive (+), receives a voltage ramp VC and of which a second input terminal, for example inverting or negative (-), receives a threshold voltage VTH. A terminal output from comparator 3 provides the result of the comparison. The output of comparator 3 switches every time the level of the voltage ramp crosses the VTH threshold. The oscillation function is obtained by resetting the voltage ramp. More precisely, the output signal V3 of comparator 3 is used to reset the voltage ramp each time it reaches the threshold VTH, that is to say each time the output switches from a first state ( for example low) to a second state (for example high), which switches the output of the comparator in the opposite direction. Thus, the output signal from comparator 3 has the appearance of a digital pulse signal. This pulse signal is periodic. In the example shown, the output signal from comparator 3 is shaped by a circuit 5 (SHAPE), an output terminal of which supplies, for example, a square signal which can constitute a clock signal CK.

The voltage ramp VC is obtained from a capacitive element, for example a capacitor C, charged by a current source 7 of constant value, and discharged at each output pulse from comparator 3. The current source 7 and the capacitor C are connected in series between two terminals 12 and 14 for applying a DC supply voltage Vdd,

B16138ER - 17-RO-0254 terminal 14 being at a reference potential, for example GND ground. The midpoint 16 of the series association of the current source 7 and of the capacitor C is connected to the positive input terminal of the comparator and, by a switch K, to the terminal 14 (more generally to the other electrode of capacitor C). A control terminal of switch K is connected to the output of comparator 3.

The threshold voltage VTH is obtained from a voltage generator, of the voltage divider assembly type or of a reference voltage generation assembly, consisting for example of a current source 9 in series with a resistive element R between terminals 12 and 14. The midpoint 18 of this series association is connected to the negative input terminal of comparator 3.

In a simplified manner, the current source 9 is a variable resistor as a function of the supply voltage Vdd. The comparator 3 and the shaping circuit 5 are, for example, supplied by the voltage Vdd.

Figures 2A, 2B and 2C illustrate, by timing diagrams, the operation of an RC oscillator. FIG. 2A illustrates an example of the shape of the voltage VC across the terminals of the capacitor C. FIG. 2B illustrates an example of the corresponding shape of the output signal V3 of the comparator 3. FIG. 3C illustrates an example of the corresponding shape of the signal Clock CK supplied by circuit 5.

For simplicity, reference will be made thereafter to respectively high (H) and low (L) states of the digital output signals of comparator 3 and of the shaping circuit 5. In practice, these levels are respectively close to the potentials ( in the example of FIG. 1, Vdd and GND) for supplying comparator 3 and circuit 5, except for voltage drops in the transistors which constitute them.

At each cycle, the voltage VC increases approximately linearly (load under constant current) from (instant tO) of a level VL relatively low compared to

B16138FR - 17-RO-0254 at the VTH loaded level. We will see later that this low level can correspond to an approximately zero level (to the nearest voltage drop in switch K when it is closed). When the capacitor C is discharged, the output of comparator 3 is at low level L.

When the voltage VC reaches the threshold VTH (instant tl), this causes a tilting towards the high state H of the output (voltage V3) of the comparator 3. This tilting (instant t2) triggers on the one hand the closing of the switch K and, in the example shown, the switching of the output of circuit 5 (instant t3). Closing the switch K causes the capacitor C to discharge, the voltage VC of which drops (with a time constant depending on the series resistance of the switch K in the on state). When the voltage VC drops below the threshold VTH (instant t4), this causes a switchover (instant t5) towards the low state of the output V3 of the comparator 3. This switchover triggers the opening of the switch K (instant tO of the next cycle) so a new charge ramp for capacitor C.

FIGS. 2A and 2B show the propagation time (t2-tl) in the comparator 3 for switching from the low state L to the high state H, the closing time of the switch K cumulated with the duration of discharge of the capacitor C (time t4-t2), the time (t5-t4) of propagation in the comparator 3 for a switching from the high state H to the low state L and the time (t0-t5, time interval between time t5 of a cycle and time tO of the following cycle) of opening of switch K.

All these delays are cumulative and the duration of each cycle (time interval between two successive instants tO) therefore corresponds to the charging time of the capacitor C from the level VL to the level VTH plus the total delay D (time interval between the instant tl of a cycle and the instant tO of the following cycle).

In the example shown, it is assumed that the combination of the low series resistance of the switch K to

B16138EN - 17-RO-0254 the on state, the propagation time in comparator 3 and the switching speed of switch K, generates a sufficient closing time of switch K to completely discharge the capacitor C at each cycle.

It is assumed that the shaping circuit 5 performs a function of divider (frequency) by two, that is to say that it causes a switching of the state of its output at each pulse of the output signal V3 of comparator 3. Depending on the embodiment of circuit 5, it is triggered on a rising edge (case shown) or on a falling edge. In the example of FIG. 2C, the time interval between two successive instants t0 therefore represents the half-period of the clock CK. The frequency of the clock CK is therefore equal to 1 / (2 (RC + D)). Other configurations of shaping circuits can be used (for example a divider by 3, 4, etc.)

The frequency stability of an RC oscillator therefore depends on the stability of the values R, C and D. However, all these values depend on the temperature. Consequently, an oscillator as shown in FIG. 1 is highly dependent on the temperature. This limits the applications of an RC oscillator, in particular when the application requires a clock whose frequency is stable in temperature, for which we are today generally forced to use a quartz oscillator.

FIG. 3 represents, schematically and partially in the form of blocks, an embodiment of an RC oscillator 1.

We find all the elements of the oscillator of figure 1, namely:

a comparator 3, a first input terminal of which, for example non-inverting or positive (+), receives a voltage ramp VC and a second input terminal of which, for example inverting or negative (-), receives a threshold voltage HTV;

a current source 7 of constant value in series with a capacitive element, for example a capacitor C, between

B16138EN - 17-RO-0254 two terminals 12 and 14 for applying a direct voltage Vdd referenced for example to ground GND, the midpoint 16 of the series association being connected to the positive input of comparator 3;

a current source 9 in series with a resistive element, for example a resistor R, between the terminals 12 and 14, the midpoint 18 of the association in series being connected to the negative input of the comparator 3;

a switch K shorting the electrodes of the capacitor C, controlled by the output of comparator 3; and preferably a circuit 5 for shaping the output pulses of comparator 3.

According to one embodiment, the oscillator 1 also comprises at least one source of correction current 6, 8, injecting or taking a current from the node 16. In the example of FIG. 3, two current sources are provided 6 and 8, respectively in parallel on the current source 7 to inject a correction current on the node 16 and in parallel on the capacitor C to draw a part of the current injected by the source 7. The correction current source (s) are with a negative or positive temperature coefficient, that is to say that the current which they supply or take varies substantially linearly with temperature, with a positive or negative coefficient.

Associating a current source with a positive or negative temperature coefficient with the constant current source 7 makes it possible to compensate for the temperature drifts of the quantities R, C and D.

Depending on the nature of the components, the quantities R, C and D decrease or increase when the temperature increases.

According to one embodiment, the current source 6 supplies a current, which is added to the current supplied by the current source 7, to charge the capacitor C. The current source 6 is, for example, with a negative temperature coefficient (inversely proportional to temperature or "complementary

B16138EN - 17-RO-0254 to absolute temperature ", CTAT), that is to say that the current which it supplies decreases when the temperature increases. In a case where the value of the resistance R decreases with temperature, as does the delay D, such a source 6 makes it possible to compensate for this temperature drift.

According to another embodiment, the current source 8 draws a current which slows down the charge of the capacitor C.

A similar reasoning can be carried out with current sources 6 and 8 with positive temperature coefficients (proportional to temperature or "proportional to absolute temperature", PTAT), that is to say which provide a current whose value increases with the temperature. Similarly, it is possible, depending on the characteristics of the current sources, to mix the different solutions.

The fact of using one or more sources of correction current makes it possible to correct temperature drifts expected from a circuit making an RC oscillator.

Figure 4 illustrates the effect of correction current sources in the temperature response of an RC oscillator.

This figure represents, very schematically, the evolution of the frequency f of the output signal CK as a function of the temperature T. provide a signal CK

The oscillator 1 is dimensioned for at a nominal frequency fO under nominal temperature conditions TO (typically, the nominal operating temperature is a few tens of degrees). According to the constitution of the oscillator (according to the nature of the resistance, capacitor and comparator and in particular of their positive or negative temperature coefficients), a variation of temperature results in an increase or a decrease in frequency according to whether the temperature increases or decreases , Or vice versa. FIG. 4 illustrates, by dotted lines, the range of possible drift as a function of the temperature. The inventors have found that the frequency is,

B16138ER - 17-RO-0254 in a conventional oscillator (Figure 1) capable of varying by several percent (for example of the order of 3 to 5%).

By correctly dimensioning the correction current source or sources, it is possible to considerably reduce the temperature sensitivity of the oscillator. This sizing

is done at the circuit design, for example by simulating comprising it of the oscillator without source of current of correction and in adjusting values from where of the currents of correction. The mixing sources of current at

positive and negative temperature coefficients and / or injecting and drawing a current with current sources having different temperature responses makes it possible to optimize the correction of the temperature behavior of the oscillator.

been significantly enlarged

An advantage of the embodiments which have been described is that by making the RC oscillator independent of temperature variations, considerably the range of possible applications for such an oscillator. In particular, this makes it possible to use an RC oscillator where the stability required for the generated frequency has hitherto required the use of a quartz or ceramic resonator oscillator. We can then take advantage of the advantages of the RC oscillator compared to a quartz oscillator or ceramic resonator in terms of low cost and integrable character.

According to a preferred embodiment, the correction carried out with source 6 or 8 or with sources 6 and 8 is configurable, that is to say that each current source consists of several (for example a number between 2 and 20) elementary sources connected in parallel.

FIG. 5 is an exemplary embodiment of a configurable correction current source, that is to say an embodiment of the current source, for example 6, of FIG.

3.

According to this example, the current source 6 consists of n current sources 61, 62, 6n, switchable

B16138ER - 17-RO-0254 individually (switches Kl, K2, Kn), connected in parallel. The n switches are, for example, controlled by a digital signal from the circuit in which the oscillator is integrated. For example, a non-volatile memory 20 or registers are used to store a configuration word from the source 6.

Where appropriate, it is possible, within the same current source 6, to use individual current sources having positive temperature coefficients for some and negative for others.

Particular embodiments have been described. Various variants and modifications will appear to those skilled in the art. In particular, we considered the case of a positive increasing ramp generation. However, a similar oscillator can be produced from a generation of negative slope. Likewise, the potentials and voltages mentioned are only examples and can be adapted according to the application. In addition, the capacitive current ramp generator may take other forms than that described, for example, a capacitive element in the feedback loop of an operational amplifier. In addition, the voltage threshold VTH can be generated by another circuit, for example, a circuit for generating a reference voltage of the bandgap type. Finally, the capacitive element may consist of several capacitors in parallel. The resistive element can also consist of several resistors in parallel and / or in series.

Note that those skilled in the art can combine various elements of these various embodiments and variants without showing inventive step. Finally, the implementation of the embodiments which have been described is within the reach of those skilled in the art on the basis of the functional indications given above and by using circuits in themselves usual.

Claims (7)

1. Oscillator (1) comprising: a comparator (3); a generator (7, C, K) of a voltage ramp (VC) whose output is connected to a first input terminal (+) of the comparator and comprising a first constant current source (7); and one or more second current sources (6, 8), connected to the first input terminal of the comparator and supplying or withdrawing a current proportional or inversely proportional to the temperature. 2. Oscillator according to claim 1, in which the ramp generator comprises: a capacitive element (C) in series with the first current source (7); and a switch in parallel on the capacitive element, said switch being controlled by an output of the comparator (3). 3. An oscillator according to claim 1 or 2, further comprising a generator (9, R) of a threshold voltage (VTH), one output of which is connected to a second input terminal (-) of the comparator (3). 4. An oscillator according to claim 3, in which said threshold voltage generator (VTH) comprises a third current source (9) in series with a resistor (R), the midpoint of the association in series being connected to said second input terminal (-) of the comparator (3). 5. An oscillator according to any one of claims 1 to 4, further comprising a circuit (5) for shaping an output signal from the comparator (3). 6. An oscillator according to any one of claims 1 to 5, in which the second current source or sources (6, 8) each comprise several switchable current sources. B16138FR - 17-RO-0254 7. Electronic circuit comprising at least one oscillator according to any one of claims 1 to 6.