EP1566717A1 - Device for the generation of an improved reference voltage and corresponding integrated circuit - Google Patents

Abstract

The device has reduction units for reducing dependency to a value of a resistor (R1) for a current circulating in a branch (31) of a proportional to absolute temperature current generator. The reduction units have a resistor (R4) of non adjustable value and act in a manner to increase or decrease the current circulating in the branch when resistivity of the resistor (R1) is higher or lower than a reference value respectively. An independent claim is also included for an electronic integrated circuit comprising a device for generating reference voltage.

Description

1. Field of the invention

The field of the invention is that of circuit design electronic and microelectronics. More specifically, the invention relates to field of electrical reference voltage generation, used in all applications that need to be able to have a controlled voltage with very small variations as a function of temperature, variations supply voltage, or variations in the technological parameters of the realization of the different components.

Such reference voltages are particularly needed in battery-powered portable equipment (radiotelephones, laptops, etc.), as well as in systems using complex electronic circuits of high performance, and more generally in integrated circuits based on microcontrollers.

2. Solutions of the prior art

• une première source de courant, appelée PTAT (en anglais "Proportional To Absolute Temperature", en français "proportionnel à la température absolue"), dépend positivement des variations de la température :
• une seconde source de courant, appelée CPTAT (en anglais "Conversely Proportional To Absolute Temperature", en français "inversement proportionnel à la température absolue"), dépend négativement des variations de la température.

In order to be able to generate a reference voltage that depends as little as possible on temperature variations, two current sources with temperature-dependent dependencies are generally used:

• a first source of current, called PTAT (in English "Proportional To Absolute Temperature", in French "proportional to the absolute temperature"), positively depends on the variations of the temperature:
• a second current source, called CPTAT (in English "Conversely Proportional To Absolute Temperature", in French "inversely proportional to the absolute temperature"), depends negatively on the variations of the temperature.

Such a reference voltage source based on currents PTAT / CPTAT is also featured in an article in the journal IEEE Journal of Solid-State Circuits, published in May 1999, entitled "A CMOS Bandgap Reference Circuit with Sub-1-V Operation "(in French) A CMOS reference voltage source operating at a voltage below 1V "), by Hiromeri Bomba et al.

Specifically, the positive temperature coefficient of the source of PTAT current is usually obtained from a voltage difference between two diodes, or between two base-emitter junctions of bipolar transistors, polarized live, and the negative temperature coefficient of the source of CPTAT current is obtained from the voltage across a diode or the base-emitter junction of a live-polarized bipolar transistor.

To make the reference voltage generated independent of the variations of the supply voltage, we proceed conventionally by cascoding or by regulation.

We can refer to the French patent application No. FR 2842317 reference voltage source, temperature sensor, temperature threshold, chip and corresponding system "on behalf of the same Depositor that this patent application for a more detailed description of a example of reference voltage generation device of the prior art.

• une source de courant de type PTAT 10 comprenant deux transistors bipolaires Q2 et Q1, dont le rapport des surfaces d'émetteur vaut S2/S1;
• une source de courant de type CPTAT 11 ;
• une source de courant de polarisation 12, non illustrée sur la figure 1;
• une résistance de sommation des courants Rs 13.

FIG. 1 shows, according to the prior art, an exemplary device for generating a reference voltage, of the "bandgap" type, capable of operating at a low supply voltage, with a low quiescent current. Such a device comprises:

• a PTAT type power source 10 comprising two bipolar transistors Q2 and Q1, the ratio of emitter surfaces of which is S2 / S1;
• a power source of the CPTAT 11 type;
• a source of bias current 12, not shown in Figure 1;
• a summing resistor of currents Rs 13.

A first operational amplifier 14 makes it possible to polarize the bipolar components of the circuit, and generate a current proportional to the temperature (PTAT), whose value can be adjusted by adjusting the value of the resistance R1.

A second operational amplifier 15 is used in follower assembly, and is connected to the smallest bipolar transistor Q1: it is used to generate a current inversely proportional to temperature (CPTAT), whose value may be adjusted by playing on resistance R2.

These two currents respectively proportional (PTAT) and vice versa proportional (CPTAT) at the temperature are added in a third resistance Rs 13, to generate an adjustable voltage, which can be made independent of the temperature by setting the PTAT and CPTAT currents.

Such a circuit also includes a current source not shown in FIG. 1, which comprises a starting circuit active at power up, and provides the bias current of the two operational amplifiers 14 and 15.

The device of FIG. 1 delivers a reference voltage VREF, the expression of which is given by VREF = Rs (I 1 + I 2) .

For each of the bipolar transistors Q1 and Q2, there is a base-emitter voltage V _BE = kT / q I n I _E / I _S (ie for Q1, V _BE1 = kT / q I n I _{E 1} / I _{S 1} and for Q2, V _{BE 2} = kT / q I n I _{E 2} / I _{S 2} ), where I _E and I _S respectively denote the emitter and saturation currents of transistors Q1 and Q2, and where T is the absolute temperature.

When the voltages at input points A and B in the operational amplifier 14 are identical or v (A) = v (B), one can express Δ V _BE = V _BE1 - V _BE2 as follows: .DELTA.V _BE = kT / q I n I _{S 2} / I _{S 1} , where the currents I _S2 and I _S1 are proportional to the size of the emitters of the bipolar transistors Q2 and Q1.

We then deduce the following expressions: I 1 = DV BE R 1 = kT qR 1 ln S 2 S 1 , which is proportional to the absolute temperature T, with k and q constant, and where S ₂ / S ₁ denotes the ratio of the emitter surfaces of the two bipolar transistors Q ₂ and Q ₁ ,
and I 2 = V _{BE 1} / R ₂ , which is inversely proportional to the temperature T.

VREF reference voltage is then expressed: VREF = Rs (kT / qR ₁ ln S ₂ / S ₁ + V _{BE 1} / R ₂₎ = KTRS / qR ₁ ln S ₂ / S ₁ + RSV _{BE 1} / R ₂ . The first term kTRs / qR ₁ ln S ₂ / S ₁ of this equation is proportional to the absolute temperature T, and the second term RsV _{BE 1} / R ₂ is inversely proportional to T. Thus, if it is possible to equal, in absolute value, the temperature coefficients of each of these two terms, the voltage VREF delivered at the output of the device of FIG. 1 can be, theoretically, made independent of the variations of the temperature T.

Figure 2, which will not be described here in more detail, presents an example embodiment of the device shown schematically in FIG. Figures 1 and 2, the same functional elements are designated by the same numerical references.

The power source (which includes an active start circuit at the start energized, and provides the bias current of both amplifiers 14 and 15) which was not shown in FIG. 1, is illustrated in FIG. Figure 2 under the reference numeral 12.

It has also been proposed to add to generation devices existing reference voltages additional adjustable resistance, in series with the PTAT generator, to adjust the proportional current value at the temperature delivered by the generator. This is called "trimming".

3. Disadvantages of prior art

Reference voltage generation devices of the prior art, such as that those illustrated in Figures 1 and 2 for example, include components integrated, such as polysilicon resistors.

A disadvantage of these components is that their value may vary more approximately 20%, depending on the parameters of the technology in which they are realized (typically, depending on the wafer (or slice) of silicon on which they are realized). These components therefore have absolute precision mediocre, which has the effect of inducing a dispersion of the reference voltage output, depending on the temperature and the parameters technological changes ("process" variations).

A disadvantage of the reference voltage generation techniques of "Bandgap" type of the prior art is therefore the inaccuracy of the voltage generated, in depending on temperature variations and technological parameters.

The addition of an additional resistance adjustable in series with the PTAT generator (trimming resistor) allows you to adjust the value of the current proportional to the temperature delivered by the generator, but requires resistance adjustment as soon as process variations occur.

It is therefore necessary to intervene on each device to adjust the value of the "trimming" resistance according to the variations of the process, this which is particularly tedious.

4. Objectives of the invention

The object of the invention is in particular to overcome these drawbacks of the art prior.

More specifically, an object of the invention is to provide a technique of generating a reference voltage that has increased accuracy compared to the reference voltages generated according to the techniques of the art prior. In particular, the object of the invention is to improve the accuracy of the reference voltage generated with respect to temperature variations and / or technological parameters for the manufacture of components (particularly in part of the use of polysilicon resistors).

In other words, the invention aims to provide a technique of generation of a reference electrical voltage which makes it possible to reduce the dispersion of the output voltage of a "bandgap" type device.

Another object of the invention is to propose such a technique which is simple and inexpensive to implement, and which does not require the setting of specific components.

In particular, the invention aims to provide such a technique that limits the adjustment of the value of the components, after their assembly, when their operating conditions change.

Another object of the invention is to propose such a technique which does not significantly increase the complexity of the generation devices reference voltage, compared to the prior art.

The invention also aims to provide such a technique that is well adapted to devices for generating reference electrical voltages low voltage operating by summing currents.

5. Essential characteristics of the invention

These objectives, as well as others that will emerge later, are achieved at using a device for generating a reference voltage comprising a first and a second current generator delivering respectively a proportional current and an inversely proportional current temperature, and means for summing said currents, so as to obtain a voltage independent of said temperature, said first generator of current comprising at least one operational amplifier and two branches parallel, a first branch comprising a first current source, controlled by the operational amplifier, and a first bipolar transistor, and a second branch comprising a second current source controlled by the operational amplifier, a first resistor and a second transistor bipolar.

According to the invention, such a device for generating an electrical voltage of reference includes means of reducing dependence on the value of said first resistance of the current flowing in said first branch, said reduction means comprising at least a second resistance of non-adjustable value.

Thus, the invention is based on an entirely new and inventive approach of the generation of a reference voltage, independent of the temperature and variations in manufacturing processes of the components constituting such a device. Indeed, the invention proposes a technique for generating a voltage of reference that has improved accuracy compared to the techniques of art previous, thanks to a reduction of the sensitivity to the values of the resistances used.

This technique is based on a "bandgap" type device based on operational amplifiers.

This type of bandgap makes it possible in particular to supply an output voltage adjustable and between 0 V and the supply voltage. It can also operate at voltages below 1V.

The new introduction of ways to reduce dependence on value of the resistances makes it possible to overcome the strong dispersion of the voltage of reference generated at the output, induced by the variations of plus or minus 20% of resistance values (in polysilicon, for example) according to the parameters technological aspects of their manufacture.

If this second resistance is carried out according to the same process that the first resistance, the evolution of its value will thus be similar to that of the first resistor, which allows a fine compensation of the dependence on the value of the first resistance of the current flowing in the first branch.

The use of such a non-adjustable resistance allows in particular to overcome the problems of adjustment of the components, since the resistance value is set as soon as it is integrated into the reference voltage generation.

The invention thus makes it possible to eliminate a step of adjusting the components, which was according to the prior art necessary as soon as a variation of the resistivity occurred.

Advantageously, said reduction means act in such a way as to increase, respectively reduce, the current flowing in said first branches when the resistivity of said first resistance is greater, respectively less than a reference value.

This maintains a relative equilibrium between the currents generated by each of the first and second current generators of the device, when the technological parameters are changing, which makes it possible to reduce the dispersion of referenced voltage generated at the output.

Advantageously, said second resistor is placed on said second branch, on a link established between said first and second sources current.

This second resistor is thus placed in series with the transistor bipolar of the second limb.

The second resistor can in particular be connected in series between the second current source and a power supply of the generation device voltage.

Preferably, said second resistance is chosen so as to what the ratio of said currents proportional and inversely proportional to the temperature remains within a predetermined range of values when the value of said first resistance varies.

This range of values is as narrow as possible, so as to ensure that the ratio of currents generated by each of the first and second generators is as constant as possible, depending on the evolution of the technological parameters.

Advantageously, the first and second resistors are produced according to the same technology, so as to present the same behavior depending variations in operating conditions of said device.

In particular, the first and second resistors may be polysilicon resistors made on the same wafer.

The invention also relates to an electronic integrated circuit comprising a device for generating a reference voltage comprising a first and second current generators respectively delivering a current proportional and a current inversely proportional to the temperature, and means for summing said currents so as to obtain a voltage independent of said temperature. The first power generator includes at least one operational amplifier and two branches in parallel, namely a first branch comprising a first current source controlled by the operational amplifier, and a first bipolar transistor, and a second branch comprising a second current source controlled by the amplifier operational, a first resistor and a second bipolar transistor.

Such a generation device comprises means for reducing the dependence on the value of said first resistance of the current flowing in said first branch, said reduction means comprising at least one second non-adjustable resistance.

6. List of figures

• la figure 1, déjà commentée précédemment en relation avec l'art antérieur, présente un synoptique d'un dispositif de génération d'une tension de référence de type "bandgap" ;
• la figure 2, également commentée ci-dessus en relation avec l'art antérieur, illustre un exemple de réalisation du dispositif de la figure 1 ;
• la figure 3 illustre les transistors bipolaires et les miroirs de courant utilisés pour générer un courant PTAT dans le dispositif de la figure 2 ;
• la figure 4 présente les courbes des tensions d'entrée de l'amplificateur opérationnel 14 de la figure 2 en fonction du courant I1 ;
• la figure 5 illustre le déplacement de la courbe de tension d'entrée V(IN-M) de la figure 4, sous l'effet du changement de résistivité des composants du dispositif de la figure 2 ;
• la figure 6 présente le schéma général d'un dispositif de génération de tension de référence "bandgap" selon l'invention, dans lequel une résistance R4 supplémentaire a été ajoutée dans le générateur PTAT pour compenser les variations de résistivité des composants ;
• la figure 7 décrit plus en détail le générateur PTAT du dispositif de la figure 6 ;
• la figure 8 présente les courbes représentatives de la tension de référence générée en sortie d'un dispositif "bandgap" de l'art antérieur et d'un dispositif "bandgap" de l'invention, en fonction de la résistivité nominale des composants résistifs utilisés dans de tels dispositifs ;
• la figure 9 présente les courbes représentatives de la tension de référence générée en sortie d'un dispositif "bandgap" de l'art antérieur et d'un dispositif "bandgap" de l'invention, en fonction de la température ;
• la figure 10 présente un histogramme de mesures de tensions de référence VREF en sortie d'un dispositif conforme à l'invention, réalisées à partir de 7 wafers (ou tranches de silicium) distincts.

Other features and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given as a simple illustrative and non-limiting example, and the appended drawings, among which:

• FIG. 1, already commented previously in relation to the prior art, presents a block diagram of a device for generating a reference voltage of the "bandgap"type;
• Figure 2, also commented above in relation to the prior art, illustrates an embodiment of the device of Figure 1;
• Fig. 3 illustrates the bipolar transistors and current mirrors used to generate a PTAT current in the device of Fig. 2;
• FIG. 4 shows the curves of the input voltages of the operational amplifier 14 of FIG. 2 as a function of the current I1;
• FIG. 5 illustrates the displacement of the input voltage curve V (IN-M) of FIG. 4, under the effect of the resistivity change of the components of the device of FIG. 2;
• FIG. 6 presents the general diagram of a "bandgap" reference voltage generating device according to the invention, in which an additional resistor R4 has been added in the PTAT generator to compensate for the resistivity variations of the components;
• FIG. 7 describes in more detail the PTAT generator of the device of FIG. 6;
• FIG. 8 shows the curves representative of the reference voltage generated at the output of a "bandgap" device of the prior art and of a "bandgap" device of the invention, as a function of the nominal resistivity of the resistive components used. in such devices;
• FIG. 9 shows the curves representative of the reference voltage generated at the output of a "bandgap" device of the prior art and of a "bandgap" device of the invention, as a function of the temperature;
• FIG. 10 shows a histogram of reference voltage measurements VREF at the output of a device according to the invention, made from 7 separate wafers (or silicon wafers).

7. Description of an embodiment of the invention

The general principle of the invention lies in the introduction of means to reduce the dependence on the value of the resistors of the current type PTAT in a reference voltage generation device by summation of currents.

With reference to FIGS. 3 to 5, the problem of the art is presented prior art that the invention solves.

• la première branche 31 comprend un premier transistor bipolaire Q1 de type pnp et une source de courant formée par le transistor pmos M1 monté en miroir de courant ;
• la deuxième branche 32 comprend un deuxième transistor bipolaire Q2 de type pnp, une source de courant formée par le transistor pmos M2 monté en miroir de courant et une première résistance R1.

To do this, FIG. 3 illustrates in detail the current generator of PTAT type referenced 10 in FIGS. 1 and 2. Such a generator 10 comprises two branches in parallel 31 and 32:

• the first branch 31 comprises a first bipolar transistor Q1 pnp type and a current source formed by the pmos transistor M1 mounted current mirror;
• the second branch 32 comprises a second pnp bipolar transistor Q2, a current source formed by the PMOS transistor M2 mounted in current mirror and a first resistor R1.

An additional PMOS transistor M0 and a current source 10 have been added to supply power to the bipolar transistors Q1 and Q2.

The voltages at the in_p and in_m points, denoted by V (in_p) and V (in_m), represent the two input voltages at the points A and B of the operational amplifier 14 of FIGS. 1 and 2, as a function of the (identical) current. injected on these points A and B. As illustrated in FIG. 4, which represents the evolution of these two voltages V (in_p) and V (in_m) as a function of the current I1 in the branches 31 and 32, we have V (in_p ) = V (in_m) at the control point P ("regulating point"). Note that, in FIG. 4, the abscissa of the two curves corresponds to the (identical) current injected at points A and B (expressed in tens of microamperes μA, ie 1. ^e-5 A). The ordinate of these curves corresponds to the voltage, expressed in volts V, at points A and B.

When the value of the resistor R1 decreases (due to changes in the technological parameters of manufacture, also called "process variations"), the current I 1 = Δ V _BE / R ₁ in the second branch 32 increases, in accordance with equation: I 1 = ΔV _BE / R ₁ = kT / qR ₁ ln S ₂ / S ₁ , according to a linear variation.

The control point P of the bandgap generating device (ie the point where V (in_p) = V (in_m)), then moves from the point P to the point P ', under effect of the displacement of the curve representative of the voltage V (in_m), as illustrated by FIG. 5. Again, the abscissa of the two curves corresponds to the (identical) current injected at the points A and B (expressed in tens microamperes μA, ie 1. ^e-5 A). The ordinate of these curves corresponds to the voltage, expressed in volts V, at points A and B.

The control point P corresponds to an initial value of the resistance R1, and the new regulation point P 'corresponds to a decrease of 20% of the value of R1 with respect to point P.

où IS1 est une constante et où V_R2 désigne la tension aux bornes de la résistance R₂ ;
et I ₂ = V _R2 / R2 = V _BE1 / R2.At the same time, the current flowing through the resistor R2 of the current generator CPTAT 11 of FIGS. 1 and 2 increases, since the voltage V _BE1 which is the base-emitter voltage of the bipolar transistor Q1 also increases. We have indeed:

where IS1 is a constant and where V _R2 denotes the voltage across the resistor R ₂ ;
and I ₂ = V _{R 2} / R 2 = V _{BE 1} / R 2.

Accordingly, when the value of the resistor R1 decreases as a function of process variations (typically in a proportion of about 20%), the currents I1 and I2 both increase, in accordance with the movement of the control point P illustrated by the FIG. 5, and the voltage delivered at the output of the reference voltage generating device (of the "bandgap" type) then increases according to the equation: VREF = Rs (I 1 + I 2).

However, as indicated above, the current I1 increases linearly with R1 according to a law in K / R1, where K is a constant (because I 1 = kT / qR ₁ ln S ₂ / S ₁ ), whereas the current 12 increases, on the one hand, linearly with R2 according to a law in K '/ R2, where K' is a constant, and on the other hand, logarithmically according to a law in ln (1 / R1).

In the expression VREF = kTRs / qR _{1 In} S ₂ / S ₁ + RsV _{BE 1} / R ₂ , the first term of the equation, in Rs / R ₁ , therefore remains constant when the resistivity of the polysilicon components varies, while the second term varies as a function of the absolute value of the resistivity p of these components.

• d'une part, on constate une augmentation de la dispersion de la tension de sortie VREF ;
• d'autre part, le coefficient de température de la tension VREF se dérègle, car le courant 12 (qui dépend négativement de la température, de type CPTAT) augmente plus vite que le courant I1 (qui dépend positivement de la température, de type PTAT).

The resulting overall effect is therefore twofold:

• on the one hand, there is an increase in the dispersion of the output voltage VREF;
• on the other hand, the temperature coefficient of the voltage VREF is derailed, since the current 12 (which depends negatively on the temperature, of the CPTAT type) increases faster than the current I1 (which depends positively on the temperature, of the PTAT type ).

It is to overcome these problems that the inventors of the present patent application propose a new type of device for generating reference voltage, a particular embodiment of which is illustrated in FIG. 6.

The assembly of FIG. 6 corresponds to the assembly of FIGS. 1 and 2, in FIG. which has been added an additional transistor R4 in series in the second current branch 32 of the current mirror of the current generator PTAT 10. Such additional resistance R4, of non-adjustable value, aims to reduce the sensitivity of the output voltage VREF to variations in the values of the resistive components of the device.

More precisely, the effect of the resistor R4 can be illustrated from the diagram of FIG. 7. The current flowing in the first branch 31 of the PTAT generator is denoted by I _M1 , and by I _M2 the current flowing in the second branch 32 of the PTAT generator.

où V _{gs M1} et V _{gs M2} désignent respectivement la tension entre la grille et la source des transistors M1 et M2, et où V_T est la tension de seuil de ces transistors.The relation between the values of the currents I _M1 and I _M2 can be expressed in the form: I M 1 I M2 = V gs M1 - V T V gs M 2 - V T and

where V _{gs M 1} and V _{gs M 2} respectively denote the voltage between the gate and the source of transistors M1 and M2, and where V _T is the threshold voltage of these transistors.

When the value of R1 decreases, the current I _M2 through the transistor M2 increases as previously described in relation to FIG. 3. At the same time, the value of the resistor R4 also decreases, because the resistors R1 and R4 are produced according to FIG. the same technology: for example, R1 and R4 are two polysilicon resistors made on the same wafer.

It should be noted that resistance R4 has a value of adjustable. Here, it is the process variations that slightly modify the value of this resistance. No intervention to adjust ("trimmer") the value of R4 is not necessary.

diminue donc également, et le rapport I_M1/I_M2 décroít donc aussi.When R4 decreases,

therefore also decreases, and the ratio I _M1 / I _M2 also decreases.

• d'une part, la valeur du courant I_M2 croít, en raison de la diminution de R1 ;
• d'autre part, le rapport I_M1/I_M2 diminue, en raison de la diminution de la valeur de R4.

In summary, we thus obtain two opposite effects:

• on the one hand, the value of the current I _M2 increases, because of the decrease of R1;
• on the other hand, the ratio I _M1 / I _M2 decreases, because of the decrease of the value of R4.

By adjusting the ratio R4 / R1, it is therefore possible to keep the current I _M1 almost constant when the resistivity of the components changes, depending on the variations of the technological parameters. The voltage V _BE1 then remains constant and the current CPTAT I 2 = V _{BE 1} / R ₂ only depends on R2.

The invention thus proposes a technique for generating a voltage of reference with improved accuracy compared to the techniques of the art previous, thanks to a reduction of the sensitivity to the values of the resistances, and not requiring the readjustment of the value of the components in case of variations temperature, diet, ...

To take again the notations previously used in relation with FIG. 3, the current I1 = I _M2 changes according to the resistivity of the components following a linear law in K / R (where R is a resistance value and K is a constant) and the current 12 also changes according to the resistivity of the components following a quasi-linear law. Thus, the reference voltage delivered at the output of the device VREF = Rs (I 1 + I 2) may have a more precise temperature coefficient, since the dispersion of the I1 / I2 ratio is reduced.

• tel qu'illustré en figure 2, i.e. ne présentant pas de résistance R4 supplémentaire (courbe référencée 81) ;
• tel qu'illustré en figure 7, i.e. présentant une résistance R4 supplémentaire, selon l'invention (courbe référencée 82).

This is illustrated in FIG. 8, on which the evolution of the reference voltage VREF is represented as a function of the resistivity variations of the components of a reference voltage generation device:

• as shown in FIG. 2, ie not exhibiting any additional resistance R4 (curve referenced 81);
• as illustrated in FIG. 7, ie having an additional resistance R4 according to the invention (curve referenced 82).

The abscissa of the curves of FIG. 8 represents the resistivity of the polysilicon with respect to the nominal resistivity (thus, an abscissa of 1.2 corresponds for example to a 20% increase in the resistivity), and the ordinate VREF corresponds to the output voltage of the "bandgap", expressed in Volts.

As can be seen, the reference voltage VREF delivered in the output of the "bandgap" device of the invention hardly depends on the process variations: indeed, when the resistivity of the components of the device evolves, the VREF voltage remains almost constant (referenced curve 82). According to the prior art however (curve referenced 81), the voltage VREF decreased sharply as the resistivity of the components increased.

Figure 9 shows the evolution of the reference voltage VREF as a function of temperature, for each of these two cases (with (curve referenced 91) or without (curve referenced 92) additional resistance R4), for a resistivity of the polysilicon components equal to 1.2 times their resistivity nominal.

In FIG. 9, the abscissa of the curves represents the temperature, expressed in degrees Celcius (° C), and their ordinate represents the output voltage VREF of "bandgap", expressed in Volts (V). In both cases, for a resistivity of polysilicon equal to 1, the variation of VREF with the temperature is almost zero.

As can be seen, the stability, as a function of temperature, of the voltage VREF generated at the output of the device "bandgap" is better in the where, in accordance with the invention, a resistance R4 has been added in series in the branch 32 of the current mirror of the generator PTAT 10.

Figure 10 shows a histogram of different voltage measurements VREF bandgap reference numbers obtained from 7 different wafers. More precisely, this histogram corresponds to measurements of the output voltage of the "bandgap", for a solution where an R4 resistance has been added. These measures were made at 25 ° C. The abscissa of the histogram corresponds to the different values of voltage VREF measured (in volts), and the ordinate of each bar of the histogram represents the frequency (i.e. the number of pieces) for each value of the voltage VREF on the abscissa (no unit of measurement is therefore associated with the values obtained on the ordinate).

Other embodiments of the invention could be envisaged. In effect, in the example presented above in relation to FIG. 6, the means of reduction of the dependence on the value of the resistance R1 of the circulating current in the first branch 31 of the PTAT current generator consist of a resistance R4 placed in series in this branch.

However, these means could also consist of an additional current injected into the first branch 31 of the current generator PTAT, which would compensate for the variations of the current I _M1 due to the resistivity change of R1. In particular, these means could consist of an additional current source proportional to the current I1 placed in shunt on the bipolar transistor Q1.

These means could also consist of one or more resistors additional, external to the current generator circuit PTAT 10.

Note also that the use of external resistors R1, R2 and Rs to the circuit, and accurate, would also improve the stability of the resistance, but would increase both the number of inputs / outputs and the number of components used, and would therefore lead to an overall increase in the cost of "bandgap" type device of the invention.

Claims (8)

A device for generating a reference voltage comprising first and second current generators respectively delivering a proportional current and a current inversely proportional to the temperature, and means for summing said currents, so as to obtain a voltage independent of said temperature,
said first current generator comprising at least one operational amplifier (14) and two branches in parallel, a first branch (31) comprising a first current source and a first bipolar transistor, and a second branch (32) comprising a second source of current, a first resistor (R1) and a second bipolar transistor,
characterized in that it comprises means for reducing the dependence on the value of said first resistance (R1) of the current flowing in said first branch (31), said reduction means comprising at least one second non-adjustable resistance ( R4). Generation device according to claim 1, characterized in that said reduction means act to increase, respectively reduce, the current flowing in said first branch (31) when the resistivity of said first resistor (R1) is greater, respectively lower at a reference value. Generation device according to any one of claims 1 and 2, characterized in that said second resistor (R4) is placed on said second branch (32) on a link established between said first and second current sources. Generation device according to claim 3, characterized in that said second resistor (R4) is connected in series between said second current source and a power supply of said device. Generation device according to any one of claims 1 to 4, characterized in that said second resistor (R4) is chosen so that the ratio of said currents proportional and inversely proportional to the temperature remains within a predetermined range of values when the value of said first resistance (R1) varies. Generation device according to any one of claims 1 to 5, characterized in that said first and second resistors are made according to the same technology, so as to have the same behavior as a function of variations in operating conditions of said device. Generation device according to claim 6, characterized in that said first and second resistors are polysilicon resistors made on the same wafer. An electronic integrated circuit comprising a device for generating a reference voltage comprising a first and a second current generator respectively delivering a proportional current and a current inversely proportional to the temperature, and means for summing said currents so as to obtain a voltage independent of said temperature, said first current generator comprising at least one operational amplifier (14) and two branches in parallel, a first branch (31) comprising a first current source and a first bipolar transistor, and a second branch ( 32) comprising a second current source, a first resistor (R1) and a second bipolar transistor,
characterized in that said generation device comprises means for reducing the dependence on the value of said first resistance (R1) of the current flowing in said first branch (31), said reduction means comprising at least one second resistance of non-value adjustable (R4).

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