Reference Voltage Generator With Programmable Thermal Drift

The present invention relates to a reference voltage generator with programmable thermal drift.

The need is known of having available a voltage generator whose reference voltage is capable of tracking with a high degree of accuracy the voltage drop across a resistance in a temperature interval ranging from -40.degree. C. to +150.degree. C.

Let us in fact suppose that we wish to verify the current flowing through a load. A circuit that accomplishes such an operation provides for the presence of a comparator which at one input is supplied with a reference voltage and at the other input is supplied with a voltage present across a detection resistance arranged in series with the load with the interposition of a switch. A control circuit operated by the output of the comparator opens the switch every time the voltage across the detection resistance is higher than the reference voltage. It is thus possible to calculate the current flowing through the detection resistance and thus the current through the load.

It is evidently important to accomplish a reference voltage generator having a heat coefficient that is identical to that of the detection resistance, so that it is possible to read the value of the current in the load with the same degree of accuracy at all temperatures.

According to the known art such a generator is accomplished through a circuit comprising a so-called bandgap reference generator (as described in the book "Analogue Integrated Circuits, Analysis and Design", Paul R. Gray and Robert Meyer, chapter 4, paragraph A 4.3.2.), that has extremely low thermal drift, and a series of two diodes connected between the output of the bandgap generator and a bias resistance. Across the bias resistance there is then taken a reference voltage, obtained as the difference between the bandgap voltage and the sum of the voltages across the diodes, that has a heat coefficient that is substantially the same as that of the abovementioned detection resistance.

The known art has some drawbacks. In it the reference voltage is given by the expression V.sub.REF =V.sub.BG -2V.sub.d, where V.sub.REF is the reference voltage, V.sub.BG is the bandgap voltage and V.sub.d is the diode voltage.

Analysing the two left-hand terms of the expression it is possible to see that as far as the bandgap voltage V.sub.BG is concerned the error introduced by the variations of its absolute value and connected with the different processing steps is not negligible and it is thus necessary to control the voltage V.sub.BG by means of calibrations that require undesirably high silicon areas.

In addition the heat coefficient of the voltage V.sub.d is a function of the absolute value of the same, which depends logarithmically on the absolute value of the operating current and is subject to variations as a result of mass production processes.

On the basis of these considerations it can be deduced that it is impossible with this art to obtain a high degree of accuracy of the absolute value of the reference voltage V.sub.REF and in particular of its heat coefficient.

The object of the present invention is that of obtaining a reference voltage generator with thermal drift that can be selected in a continuous range of values, that has a high degree of accuracy and a very small size.

According to the invention such object is attained with a generator of a reference voltage, characterized in that it comprises a first generator of voltage with thermal drift of zero, a second generator of voltage with given thermal drift, first means for applying a given load to the voltage generated by the first generator, second means for applying a given load to the voltage generated by the second generator, subtracting means for subtracting one from the other the loaded voltages generated by said first and second generator of voltage.

Preferably, the second generator of the voltage is included inside the first and has in common with it two NPN transistors with bases connected together and with emitters connected together through a resistance and to ground through a further common resistance, said transistors having different emitter areas.

The voltage with thermal drift of zero and the voltage with given thermal drift are applied across respective resistances and said subtracting means comprise a circuit node in which said respective resistances converge together with an output resistance for the generation of said reference voltage.

The features of the present invention shall be made more evident by an embodiment illustrated as a non-limiting example in the only FIGURE of the enclosed drawing.

With reference to the illustrated figure, the circuit as a whole comprises a resistance R6 interposed between a power supply V.sub.dd and the emitter of a transistor T4 of the type PNP. The collector of the transistor T4 is grounded, the base is connected to the collector of a transistor T6 of the PNP. The latter, together with transistors T7, T8 of the type PNP and with respective emitter resistances R7, R5, R8 connected to the supply voltage V.sub.dd, constitutes a current mirror. The bases of the transistors T6, T7, T8 are connected to an intermediate node B between the resistance R6 and the transistor T4 so that they are biased. The collectors of the transistors T6, T7 are connected to respective collectors of two transistors T1, T2 of the type NPN with different emitter area (that of T1 equal to n times that of T2). The base of the transistor T1 is connected to the base of the transistor T2. The emitters of the two transistors T1, T2 are connected together through a resistance R1. A capacity C1 is interposed between the base and the collector of the transistor T2. Taken as a whole the transistors T1, T2 together with the resistance R1 constitute generating means 2 of a current I which, due to the effect of the presence of the abovementioned current mirror, is taken back on the emitter and thus on the collector of the transistor T8 as current I.sub.C8. The circuit also comprises a transistor T5 of the type PNP, whose base is connected to the collector of the transistor T7. The collector of the transistor T5 is grounded, the emitter is supplied by the current of a current generator I1 connected to the voltage V.sub.dd and to the base of a transistor T3 of the type NPN. To a circuit node A interposed between the collector of the transistor T8 and the collector of the transistor T3 there is connected a resistance R4 which is grounded at its other extremity. The reference voltage V.sub.REF is taken across it. The emitter of the transistor T3 is connected to ground through a resistance R3, which has the function of setting the operating current of the transistor T3. Across the resistance R3, between an intermediate node C connected to the base of the transistors T1, T2 and ground, there is a bandgap voltage V.sub.BG, that is generated by the circuit unit indicated with 1 and that has thermal drift equal to zero as it is originated as the sum of a component having negative thermal drift (base-emitter voltage) of T2) and of a component having positive thermal drift (voltage across R2, function of the difference between the base-emitter voltages of the two transistors T1 and T2 having different emitter area).

The circuit described operates as follows.

Applying Kirchoff's equation to the mesh formed by the transistors T1, T2 and by the resistance R1 there is obtained that across the resistance R1 there is a voltage V.sub.BE equal to the difference between the base-emitter voltages of the transistors T2 and T1 and thus with given constant thermal drift. In the resistance R1 there flows a current I equal to V.sub.BE /R1. This current, due to the effect of the current mirror 4, is taken back as current I.sub.C8 on the emitter of the transistors T8 and thus, in the hypothesis that the base current of the transistor T8 is negligible, on the collector of the transistor T8. On the emitter of the transistor T3 there is a current given by the ratio between the voltage V.sub.BG across the resistance R3 and the resistance R3 itself. It follows that, applying Kirchoff's law to the intermediate node A between the collectors of the transistors T8 and T3, it is obtained that the current through the resistance R4 is given by the difference between the collector current I.sub.C8 on the collector of the transistor T8 and the collector current I.sub.C3 on the collector of the transistor T3. The reference voltage K.sub.REF is thus given by the expression: ##EQU1##

In order to be able to assess the dependence on temperature of equation (3), it is necessary to express the dependence of the individual terms: `.DELTA.V.sub.Be

starting from the equation that expresses the voltage difference V.sub.BE between the two transistors T1, T2, it is possible to write:

where V.sub.T is the voltage equivalent of the temperature defined by the relation V.sub.T =kT/q (k=Boltzmann's constant, T=absolute temperature, q=electronic charge) and on the basis of Einstein's equation it is given by the ratio between diffusion and electronic mobility.

If

I.sub.C1 =I.sub.C2

I.sub.S1 =AI.sub.S2

with I.sub.C1, I.sub.C2 equal to the collector currents of the transistors T1, T2, I.sub.S1, I.sub.S2 saturation currents of the transistors T1, T2 and A the ratio between the emitter areas of the transistors T1, T2 we get:

where:

T=absolute temperature

k=Boltzmann's constant

q=electron charge

n=technological parameter independent of temperature.

Equation 6 can also be written:

with To=reference temperature.

Carrying on, from equation 7 we get:

Equation 8 highlights the law of variation of the voltage .DELTA.V.sub.BE as a function of temperature with:

.DELTA.V.sub.BEO =value calculated at the reference temperature;

.alpha.=heat coefficient

-V.sub.BG /K:

it is assumed as a first approximation that the voltage V.sub.BG is independent of temperature;

-R4/R1:

if the two resistances are coupled, their ratio is independent of temperature.

Substituting in equation 3 we get:

where:

Equation 12 identifies a voltage with linear thermal drift, wherein the value of the heat coefficient depends on the absolute value of the voltage Vo and thus of the voltage V.sub.BG :

This determines the possibility of selecting the value of the heat coefficient on the basis of one's requirements, with a high degree of accuracy and with no need for calibrations.