Voltage reference circuit with linearized temperature behavior

Abstract: This voltage reference circuit has high thermal stability and minimal bulk and comprises a transistor (Q₂) defining a base-emitter junction having a voltage drop (V BE ) which varies in a non-linear manner as a function of temperature and resistors (R 2a -R 2c ) connected in series to the junction, the junction and the resistors being interposed between a ground line and the output terminal. A compensation transistor (Q R ) generates a compensation current which varies as a function of temperature so as to produce a voltage drop with a behavior substantially opposite to the previous voltage drop upon reaching the switching on temperature of the transistor.

Description: The present invention relates to a voltage reference circuit with linearized temperature behavior. As is known, the voltage reference is an essential block of integrated circuits. Said block can comprise configurations using Zener diodes or a so-called band-gap structure, a typical configuration whereof is shown in figure 1. Said illustrated structure is currently preferred to configurations using Zener diodes, since it has some advantages, among which the low value of its output voltage, typically 1.2 V, which allows to extend its compatibility with power supply sources, and good thermal stability. With reference to the diagram of figure 1, in particular to the transistors Q₁ and Q₂, simple calculations show that in which Δ V BE = η V T lnA where A is the ratio between the emitter areas of Q₁ and Q₂; I S is the inverse saturation current, V T = KT q ϑ η is a corrective parameter which is related to the employed technology and is independent from the temperature. By derivation with respect to the temperature, the following is obtained: By analyzing this last equation, it has been seen that · ∂ΔV BE /∂T is a constant and positive, and therefore the primitive function has a rising linear behavior; while · ∂V BE /∂T is not constant and is negative, and therefore the voltage V BE (T) has a non-linear decreasing behavior. This situation is exemplified in figures 2a and 2b, which respectively illustrate the derivative of the voltage drop on R₂ (directly proportional to the derivative of Δ V BE with respect to the temperature) and the derivative of the base-emitter drop with respect to the temperature. In a significant temperature range (typical for applications in the motor-vehicle field) comprised between -40 o C and 150 o C, three different situations are possible, namely: if ∂V BE /∂T > ∂αΔV BE /∂T in absolute value in the entire range being considered, the voltage V REF (T) will always have an always decreasing behavior; if instead ∂V BE /∂T < ∂αΔV BE /∂T (always in absolute value in the entire range), V REF (T) will always have a rising behavior; if, always within the initially considered range, the second of the two described conditions is true initially and the first one is subsequently true, the derivative of the voltage V REF (T) with respect to the temperature will be initially positive and subsequently negative (see figure 2c) and the primitive function will have a parabolic plot. More generally, it can be said that the voltage V REF has a parabolic behavior in which the position of the maximum value can be internal or external to the temperature range being considered. With a same voltage V BE , the position of this point is linked to the voltage V REF to be obtained at a given reference temperature (environmental temperature is usually considered). This reference voltage value therefore determines the value of the resistor R₂. These conclusions are illustrated in figures 2a, 2b, 2c and 3, in which three different values of the resistor R₂ have been assumed and therefore three different plots have been obtained. In particular, the curves 1, 2 and 3 relate to decreasing values of the resistor R₂ which entail a shift of the sign-change point of the curve ∂V REF / ∂T, i.e. a variation in the slope-change point of the primitive, which will therefore have one of the three behaviors shown in figure 3. This behavior is in any case merely theoretical, as it is determined by solving a mathematical equation; in practice, however, the unavoidable process spreads make such a behavior unattainable. Given this situation, the problem arises of limiting the variation of the reference voltage as a function of temperature by providing means for linearizing the behavior of said voltage. Known from US-A-4,249,122 is a device as defined in precharacterising portion of claim 1. Also known from GB-A-2,040,087 is a device with two transistors operating at different current densities to develop a current with a positive temperature coefficient which passes through a resistor which is series with the transistor that generates the negative temperature coefficient voltage. Further temperature compensation is provided by adding an additional resistor having a positive temperature coefficient. A particular object of the present invention is to improve the stability and reduce the temperature-dependence of the reference voltage with a circuit having minimum bulk. Another object of the present invention is to provide a simple compensation system which can be easily integrated in the voltage reference circuit and operates reliably. The above-mentioned objects and others which will become apparent hereinafter are achieved by a voltage reference circuit with linearized temperature behavior, as defined in the accompanying claims. The characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a simplified circuit diagram of a known voltage reference in band-gap configuration; figures 2a, 2b, 2c and 3 represent possible plots of the voltages and derivatives thereof of the circuit of figure 1; figure 4 is a simplified circuit diagram of the structure of figure 1, modified according to the invention; figure 5 is a simplified circuit diagram of another configuration of a known voltage reference; and figure 6 is a modification, according to the invention, of the diagram of figure 5. Figures 1 to 3 are not described hereafter; reference is made for said figures to the introductory section of the present patent (for the diagram of figure 1, see also A. Paul BROKAW, "Single terminal three IC reference", I.E.E.E. Journal of Solid State Circuits, Vol. SC9, No. 6, December 1974, pages 389-393). Reference is thus made to figure 4, which illustrates a voltage reference circuit in band-gap configuration according to the invention. Such circuit substantially corresponds to the one of figure 1, except for the fact that the resistor R₂ has been replaced by three resistors R 2a , R 2b , and R 2c arranged mutually in series and connected to the emitter of Q₂ and to a terminal of R₁ on one side and to the ground on the other. The circuit furthermore comprises an NPN-type transistor Q R which has its base connected to the common point between R 2a and R 2b , its collector connected to the supply voltage V CC and its emitter connected through a resistor R R to the common point between R 2b and R 2c . If the resistor R R is temporarily ignored, the collector current I C and the base to emitter voltage drop V BE of transistor Q R are as follows: from which This current has a parabolic temperature-dependence which is due exclusively to the current I S , the value whereof doubles approximately every 10 o C. By injecting this current in the resistor R 2c , an additional term for the voltage V REF is obtained, able to compensate the natural behavior. By employing this current it is therefore possible to compensate the variation of the reference voltage V REF starting from a given temperature. Infact, on the basis of the above, the voltage on the resistors R₂ and therefore in particular on R 2b , which is proportional to Δ V BE , rises with the temperature, while the voltage on the base-emitter junction of Q R decreases with the temperature. At a given temperature, therefore, V R2b = V BE , i.e. the transistor Q R is switched on. With the illustrated diagram, the transistor Q R will tend to conduct increasingly as the temperature rises. Therefore the resistor R R has been inserted in order to make switching on of said transistor more gradual. Since the action of the transistor Q R is limited to high temperatures, the compensation is optimized starting from a reference voltage behavior having its maximum value at low temperatures, i.e. in the condition shown by the curve 3 of figure 3, which can be obtained, as mentioned, by appropriately setting the value of R 2b . The same inventive concept can be applied to a reference voltage arranged according to Widlar's theory, of which figure 5 illustrates a typical non-linearized structure. For this known configuration, the output voltage V REF is given by the base-emitter drop on the transistor Q₅ plus the drop on R₄, and therefore: The considerations presented above are therefore valid for this circuit, and in general the output reference voltage will have a parabolic plot which can be compensated at high temperatures by using the diagram shown in figure 6. As can be seen in said figure, similarly to the solution illustrated in figure 4, the resistor R₄ has been divided into the two resistors R 4a and R 4b , and the PNP-type transistor Q′ R has been inserted; said transistor has its collector connected to the ground, its base connected to the common point between R 4a and R 4b and its emitter connected to the resistor R′ R which has its other terminal connected to the upper portion of the circuit, which has been schematically represented by the current source I. The temperature compensation of the circuit of figure 6 operates similarly to the one shown in figure 4; specifically, R 4a is the equivalent of R 2b and sets the switching on temperature of the recovery or compensation transistor Q′ R ; R 4b is the equivalent of the resistor R 2c and therefore sets the recovery voltage (as it receives the base current of the transistor Q′ R after switching on thereof) and R′ R makes the action of the recovery transistor more gradual. As can be seen from the previous description, the invention achieves the proposed aims. In particular, by virtue of the insertion of the compensation transistors, in known circuits, a current source is inserted which injects a recovery current starting from a voltage which can be set by appropriately dimensioning of the circuit. Said current, injected in R 2c or R 4b , allows to compensate or at least reduce the negative slope of the reference voltage as the temperature rises. The illustrated solution is furthermore extremely simple, since it consists in inserting a transistor and resistors without further modification of the known circuit, entails a reduced bulk and can be easily integrated. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: A band gap constant voltage reference circuit with linearized temperature behavior, comprising an output transistor (Q₂, Q₅) including a base-emitter junction having a voltage drop (V BE ) which is variable in a non-linear manner as a function of temperature, resistive means (R 2a -R 2c ; R 4a , R 4b ) connected in series to said junction, compensation means comprising another transistor (Q₁; Q₃,Q₄) for compensating said voltage drop, said junction and said resistive means being interposed between a reference potential line and an output terminal, and furthermore variable current source means, generating a compensation current (I c ), and comprising a compensation transistor (Q R , Q' R ) generating a compensation current which is variable as a function of the temperature so as to produce a voltage drop with a behavior which is substantially opposite to the non linear behavior of said junction voltage drop in at least one operating range of the said circuit, thus further compensating said junction voltage drop in said at least one operating range, characterized in that said current source means feed directly said resistive means (R 2a -R 2c ; R 4a , R 4b ), without the interposition of a transistor, with said compensation current (I C ) so as to produce said voltage drop in said resistive means. A circuit according to claim 1, characterized in that it comprises circuit means (R 2b ,R R ; R 4a ,R' R ) for switching on said compensation transistor at a given operating temperature. A circuit according to preceding claims, characterized in that it comprises a first and a second transistors (Q₁, Q₂) having their collector terminals connected to a respective current source, their base terminals connected together and their emitter terminals connected together through a first resistive element (R₁), the emitter terminals of said first and second transistors being connected to said resistive means (R 2a -R 2c ), in that said resistive means comprise at least one second and one third resistive elements (R 2b ,R 2c ) mutually connected in series with a first terminal thereof, the other terminal of said second resistive element (R 2b ) being connected to said emitter terminals of said first and second transistors (Q₁,Q₂) and the other terminal of said third resistive element (R 2c ) being connected to a ground line, and in that said second resistive element (R 2b ) is furthermore connected in parallel to the base-emitter junction of said compensation transistor (Q R ) and sets the switching on temperature thereof. A circuit according to the preceding claims, characterized by a fourth resistive element (R R ) connected in series to the emitter of said compensation transistor (Q R ). A circuit according to claim 1 or 2, of the Widlar type, characterized in that it comprises a transistor (Q₅) connected between said output terminal and said reference potential line with its own collector and emitter terminals to a terminal of said resistive means (R 4a ,R 4b ) with its own base terminal, said resistive means being connected with their other terminal to said output terminal, in that said resistive means comprise a first and a second resistive elements (R 4a ,R 4b ) mutually connected with a first terminal thereof, the other terminal of said first resistive element (R 4b ) being connected to the base terminal of said transistor (Q₅), the other terminal of said second resistive element (R 4a ) being connected to said output terminal, and in that said second resistive element (R 4a ) is connected in parallel to the base-emitter junction of said compensation transistor (Q' R ) and sets the switching on temperature thereof. A circuit according to the preceding claim, characterized by a third resistive element (R' R ) connected in series to the emitter terminal of said compensation transistor (Q' R ).

Inventors: BRAMBILLA MASSIMILIANO, MAGGIONI GIAMPIETRO, MENEGOLI PAOLO, MORELLI MARCO, BRAMBILLA, MASSIMILIANO, MAGGIONI, GIAMPIETRO, MENEGOLI, PAOLO, MORELLI, MARCO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1989
Main classification: G05F3
IPC codes: H01L21, H01L27, G05F3