EP0466717B1 - Precision reference-voltage source - Google Patents

Abstract

Proposed is a monolithic-integrated precision reference-voltage source based on the bandgap principle and suitable for use over a wide temperature range. The parabolic temperature/reference-voltage curve produced by the source is made linear using the processing means available in the monolithic integration without the need for additional active components such as transistors and diodes. The precision voltage-reference source includes two resistors (21, 22) represented by the n-doped emitter diffusion zone.

Description

State of the art

The invention relates to a precision reference voltage source according to the preamble of independent claim 1.

The requirements for the characteristic data of monolithically integrated circuits for the motor vehicle are constantly increasing. Because of the large temperature range of -40 ° CC ≦ T _j ≦ + 150 ° C and above, reference voltage sources with an extremely small or defined temperature coefficient (TK) and low piezo sensitivity are particularly important.

From the article by GCM Meijer, PC Schmale and K. van Zalinge "A New Curvature-Corrected Bandgap Reference" in IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 6, Dec. 1982, a precision Known reference voltage source, which contains 47 components on a chip area of 4 mm² and requires an IC manufacturing process with nickel-chromium resistance technology. Their temperature coefficient is given as 50 ppm in a temperature range of 25 ° C ≦ T _j ≦ 85 ° C.

AP Brokaw's article "A Simple Three-Terminal IC Bandgap-Reference" in IEEE Journal of Solid-State Circuits, Vol. SC-9, No., Dec. 1974 also includes a monolistically integrated one that works according to the bandgap principle Known reference voltage source, which contains 29 components on a chip area of 1.47 mm² and is also manufactured with nickel-chrome resistance technology. Their temperature coefficient is given as 5 to 60 ppm for a temperature range of -55 ° C ≦ T _j ≦ 125 ° C.

US Pat. No. 4,490,670 also discloses a monolithically integrated reference voltage source which operates on the bandgap principle and in which the temperature dependence of the reference voltage is linearized. In this known circuit arrangement, three current paths, each with an associated reference transistor, are required for linearization, and a resistor that is also required for linearization lies outside of these current paths parallel to the emitter-collector path of one of the three reference transistors.

From GB-OS 2 199 677 a reference voltage source according to the preamble of the main claim is also known. For linearization, in addition to the first and the second reference transistor and the first and the second resistor, this requires a further reference transistor and two further resistors which cooperate with the further reference transistor.

From US Pat. No. 4,250,445 a reference voltage source according to the type of the main claim is further known, in which the two resistors are designed as nickel-chromium sheet resistors and consequently have the temperature coefficient "zero" or a very small temperature coefficient. In this known reference voltage source, the quadratic term of the reference voltage can already be compensated for with a resistor whose temperature coefficient is linearly related to the temperature. However, this reference voltage source has the disadvantage that additional process steps, namely the application of the nickel-chromium resistance layer together with the associated photoresist process, are required for the production of the nickel-chromium film resistors, which means a considerable additional cost.

Advantages of the invention

The precision reference voltage source according to the invention with the characterizing features of independent claim 1 has the advantage that the piezo sensitivity is reduced. Further advantages result from the dependent claims 2 to 15.

The temperature coefficient of the bandgap voltage of silicon contains higher-order terms (Tsividis, YP: "Accurate Analysis of Temperature Effects in I _C - V _BE Characteristics with Application to Bandgap Reference Sources", IEEE Journal of Solid-State Circuits, Vol. SC- 15, No. 6, Dec. 1980).

The following zones are available for the monolithically integrated circuit: substrate (P⁻), insulation diffusion (PP⁺), epitaxy (N⁻), buried layer diffusion (N⁺), deep collector diffusion (N⁺) , Base diffusion (P), emitter diffusion (N⁺), metallization and possibly other zones such as doped polysilicon or Cr / Ni resistors (for "fused links"); other zones may also be present depending on the process, such as an upper and a lower insulation diffusion or a base connection diffusion.

dieser Zonen, so finden sich welche mit (nahezu) linearem Temperturkoeffizienten wie die N⁺-dotierten bzw. metallischen Zonen und solche mit einem mehr oder weniger hohen Anteil an Termen höherer Ordnung wie die P-dotierten. Ebenso finden sich Zonen mit mehr oder weniger hoher Piezo-Empfindlichkeit.Consider the temperature coefficients of the specific or area resistances $${\text{R (ΔT) = R}}_{\text{To}}\text{[1 + α (ΔT) + β (ΔT) 2 + γ (ΔT) 3]}$$

of these zones, there are those with (almost) linear temperature coefficients like the N⁺-doped or metallic zones and those with a more or less high proportion of higher order terms like the P-doped. There are also zones with more or less high piezo sensitivity.

drawing

The invention will be explained with reference to Figures 1 to 11. Figure 1 shows the basic circuit of a band gap reference according to Brokaw, supplemented by a starter circuit. FIGS. 2 to 4 show the temperature responses of the reference voltages of an exemplary circuit for resistors with three different temperature coefficients in the temperature range from -40 ° C ≦ Tj ≦ + 160 ° C. Figures 5 and 6 represent modifications of the circuit of Figure 1, Figure 7 shows the temperature response of the reference voltage generated. In FIG. 8 the circuit and in FIG. 9 the layout of cross-coupled lateral transistors to reduce their piezo sensitivity are shown, likewise in FIGS. 10 and 11 the arrangement in the layout for the critical NPN reference transistors.

Description of the embodiments

The bandgap reference according to FIG. 1 consists of the two reference transistors 23 and 24, the transistor 24 generally being produced by connecting K identical transistors 23 with 2 ≦ K ≦ 16 in parallel. Because of the formal dependence on In K, K = 4 is already sufficient and K above 8 is hardly used. Together with the resistor 22, the arrangement at the resistor 21 generates a temperature-proportional voltage which, when properly designed, compensates for the negative temperature response of the base-emitter voltage of the transistor 23 quite well. The potential difference 17/15 represents the total voltage. It corresponds exactly to the potential of the bandgap (of silicon).

The two reference transistors 23, 24 operate on the current mirror with the two lateral PNP transistors 25, 26, the common base of which lies on the collector 24 via the PNP emitter follower 27. Correspondingly, the PNP emitter follower 6 is coupled out from the collector of the transistor 23, the emitter of which is connected to the base of the NPN emitter follower 7. In order to obtain voltages greater than the bandgap voltage, the emitter of transistor 7 is not connected directly at point 17, but via resistor 8 at point 17. The reference voltage to be tapped at terminal 18 is thus higher in accordance with the transformation ratio of resistors 8, 9. The transistors 25, 26, 27, 6, 7 form an operational amplifier which is dynamically stabilized by means of the capacitor 10. The transistor 4 with resistor 5, which also works as a current mirror, delivers a sufficiently small “starting current” into the circuit. The positive pole of the operating voltage is connected to terminal 16, the negative to terminal 15.

The temperature curve of the reference voltage of an example in the circuit according to FIG. 1 is shown in FIG. 2. There, the band gap voltage is shown as a function of the temperature between -40 ° C. and + 160 ° C. for an embodiment in which the horizontal tangent is in the middle of the Temperature range is set and the resistors 21 and 22, as usual with simple references, are shown by means of the base diffusion. As can be seen from this, the reference voltage has a fairly parabolic temperature profile, which is known to depend on the manufacturing process, that is to say on doping and doping profiles, and can therefore also contain higher-order terms in other embodiments. The storage at the two corner temperatures is slightly more than - 5 mV, corresponding to an average temperature coefficient of - 4%.

In this example, the temperature response can already be significantly improved by using emitter diffusion instead of basic diffusion for resistors 21, 22, as can be seen in FIG. If, in our example, the resistors 21 and 22 are also given the temperature coefficient "0" in a purely theoretical manner, the calculation shown in FIG. 4 still shows a deviation of approx. -2.3 mV with higher-order components.

This always approximately parabolic course can now be compensated for in that the resistance 21 in FIG. 1 is given a temperature coefficient with larger proportions of terms of higher order than the resistor 22.

Figure 5 shows a modification of the circuit for an execution of the resistors with a zone of the process, which contains a larger square term β₂₁. Since β₂₂ must always be smaller than β₂₁, in this case the resistor 22 is split into at least two partial resistors 32, 42 and a zone with a smaller β is to be used for the compensation resistor 42. A sufficiently good compensation for this example results if the difference between the coefficients of the quadratic terms β₂₁ and β₂₂ is 0.74 ^· 10⁻⁶. If the resistors 21, 32 are implemented by means of the base diffusion and the resistor 42 by means of the emitter diffusion, the temperature profile according to FIG. 7 is 3 435 Ω for the resistor 21, 393Ω for the resistor 32 and 60Ω for the resistor 42.

As already mentioned, the resistors should be formed with zones that have the smallest possible piezo effect, such as emitter diffusion or other heavily n-doped zones. In this case the temperature coefficient of the square resistance contains practically no higher order terms. The solution to this is shown in FIG. 6. So that the resistor 21 with a higher square portion than the resistor 22, it is split into the partial resistors 31 and 41 and the compensation resistor 41 by means of a zone with a larger square term. The difference β₂₁ - β₂₂ should now be 0.49 ^· 10⁻⁶. If the emitter diffusion zone does not contain any higher-order terms and if the base diffusion used for the compensation resistor 41 has the same square term as in the previous example, the resistor 31 receives the value 3 135Ω and the resistor 22 the value 453Ω, the correction in base diffusion 41 receives the value 300Ω. The course of the temperature response also corresponds to that of FIG. 7.

If process-related scatter is taken into account to compensate for the quadratic term of the reference voltage, the difference between the resulting quadratic terms in the case of compensation in resistor 22 by means of resistor 42 is in the range 0.3 ^· 10⁻⁶ ≦ β₂₁ - β₂₂ ≦ 1.2 ^· 10⁻ ⁶. If, however, is compensated in the resistor 21 by means of the resistor 41, the range is to be set at 0.2 ^· 10⁻⁶ ≦ β₂₁ ≦ 0.8 ^· 10⁻⁶.

bzw. für eine Kompensation im Bereich des Widerstands 22 $$\text{β₂₂ = (β₃₂.R₃₂ + β₄₂.R₄₂).(R₃₂ + R₄₂)⁻¹.}$$

The resulting terms β₂₁ and β₂₂ can be calculated from the known terms of the zones used for the resistors. For compensation in the area of the resistor 21 is general $$\text{β₂₁ = (β₃₁.R₃₁ + β₄₁.R₄₁). (R₃₁ + R₄₁) ⁻¹}$$

or for compensation in the area of the resistor 22 $$\text{β₂₂ = (β₃₂.R₃₂ + β₄₂.R₄₂). (R₃₂ + R₄₂) ⁻¹.}$$

If, as can be seen from the literature, higher-order terms also occur in the temperature response of the reference voltage, it is advantageous to also take these into account.

Resistors with differing temperature coefficients can also be represented by modulating the width of the resistors in the design due to the different amount of lateral underdiffusion in the overall resistance, especially since only minor differences can be generated in the quadratic term or a third order term has to be generated . Observations according to third-order terms appear to occur with particularly narrow resistances. Due to the general dependence of the temperature coefficients on the manufacturing process, no specific information can be given.

The specified compensations can only be adhered to to a certain extent if the actual value of the maximum of the band gap tension is also at the temperature on which the calculation is based. It is therefore advantageous to adjust to this maximum.

In the proposed solutions, the resistors 21 and 22 are represented by more than one zone. This means that different process variations, i.e. resistance variations, are to be expected, which lead to a variation in the division ratio. In the case of a precision reference voltage source, the division ratio is to be adjusted to its setpoint by changing the compensation resistor 41 or 42. Methods for matching resistor networks in wafer samples are described in AB Grebene: "Bipolar and MOS Analog Integrated Circuit Design" by John Wiley & Sons , 1984, pages 155 to 159 and not the subject of the invention.

Although the precision reference voltage source requires only a chip area of approximately 0.3 mm 2, despite resistors 31 and 22 including a four-stage matching network, which are shown by means of the relatively low-resistance emitter diffusion, measures for reducing the piezo sensitivity are advantageous. The collectors of the two PNP lateral transistors 25 and 26 are therefore split into two identical sub-collectors in accordance with the circuit according to FIG. 8 and cross-connected to one another. A further transistor 11 is inserted between the transistors 25 and 26 to derive any base currents in order to achieve higher operating temperatures.

A possible layout for this is shown in FIG. 9. The NPN reference transistors 23 and 24 are also arranged symmetrically to one another, specifically for an emitter ratio of 1: 2 and 1: 4 according to FIG. 10 and for an emitter ratio of 1: 4 and 1: 8 according to Figure 11. Only four sub-transistors 24 are shown in the latter. The approximately piezocompensated ratio 1: 8 can easily be established by filling up the free spaces with another four partial transistors. Wiring is not a problem even with eight sub-transistors 24 arranged around transistor 23, since the eight sub-transistors can be accommodated in a single collector trough.

Precision reference voltage sources can hardly be produced specifically with the previous methods even with complex technologies and are therefore usually expensive selection types from a larger production lot. In contrast, according to the proposals of the invention, they can be produced specifically using standard technologies. Their area requirement is hardly larger than that of ordinary reference voltage sources.

Claims (15)

1. Monolithically integrated precision reference voltage source employing the bandgap principle, having a first N-P-N reference transistor (23) and a second N-P-N reference transistor (24) which are connected in parallel with one another in order to divide a current into two current paths, and each of which has an emitter electrode, a collector electrode and a base electrode, the base electrodes of the two reference transistors (23, 24) being connected to one another and to an output terminal (18) at which the reference voltage is taken off and a series circuit comprising a first resistor (21) and a second resistor (22) further leading from a supply potential (15) to the emitter electrode of the second N-P-N reference transistor (24) and the emitter electrode of the first N-P-N reference transistor (23) being connected to the node between the first (21) and the second (22) resistor, and having a first P-N-P current-balance transistor (25) and a second P-N-P current-balance transistor (26) for impressing the currents on the current paths of the two N-P-N reference transistors (23, 24), characterised in that, to reduce the influence of the piezoelectric effect on the two P-N-P current-balance transistors (25, 26) mentioned, which are constructed as lateral transistors, their collectors are halved in size and the halves are each interconnected crosswise (Figure 8, Figure 9).
2. Precision reference voltage source according to Claim 1, characterised in that, to reduce the influence of the piezoelectric effect on the N-P-N reference transistors (23, 24) driven at different current density, the at least two identical component transistors of the second reference transistor (24) are arranged symmetrically with respect to the first reference transistor (23) in relation to the piezoelectric effect.
3. Precision reference voltage source according to Claim 1 or 2, characterised in that, to compensate for the temperature coefficient of higher order remaining in the two reference transistors (23, 24), the two resistors (21, 22) are formed as least partly by zones having different temperature coefficients, and in that the quadratic term of the temperature coefficient of the first resistor (21) is greater than the quadratic term of the temperature coefficient of the second resistor (22).
4. Precision reference voltage source according to Claim 3, characterised in that, in a production of the first resistor (21) by means of a zone having a larger quadratic term of the temperature coefficient, the second resistor (22) is split up into the series circuit comprising a first component resistor (32) and a second component resistor (42), the first component (32) being constructed by means of the same zone as the first resistor (21) and the second component resistor (42), serving as compensation resistor, being constructed by means of a zone with a smaller quadratic term (Figure 5).
5. Precision reference voltage source according to Claim 4, characterised in that the difference in the quadratic terms of the temperature coefficients β₂₁ of the first resistor (21) and β₂₂ of the resulting second resistor (22) produced by the sum of the component resistors (32, 42) is in the range of 0.3 x 10⁻⁶ ≦ β₂₁ - β₂₂ ≦ 1.2 × 10⁻⁶.
6. Precision reference voltage source according to Claim 4, characterised in that the first resistor (21) and the first component resistor (32) are produced by means of the base diffusion zone and the second component resistor (42), serving as compensation resistor, by means of the emitter diffusion zone (Figure 5).
7. Precision reference voltage source according to Claim 3, characterised in that, in a production of the second resistor (22) by means of a zone having a smaller quadratic term, the first resistor (21) is split up into the series circuit comprising a third component resistor (31) and a fourth component resistor (41), the third component resistor (31) being constructed by means of the same zone as the second resistor (22) and the fourth component resistor (41), serving as compensation resistor, by means of a zone having a larger quadratic term (Figure 6).
8. Precision reference voltage source according to Claim 7, characterised in that the difference in the quadratic terms of the temperature coefficients β₂₁ of the resulting first resistor (21) produced by the sum of the component resistors (31, 41 ) and β₂₂ of the second resistor (22) is in the range of 0.2 x 10⁻⁶ ≦ β₂₁ - β₂₂ ≦ 0.8 × 10⁻⁶.
9. Precision reference voltage source according to Claim 7, characterised in that the second resistor (22) and the third component resistor (31) are produced by means of the emitter diffusion zone and the fourth component resistor (41), serving as compensation resistor, by means of the base diffusion zone (Figure 6).
10. Precision reference voltage source according to one of Claims 3 to 9, characterised by an alignment of the actual value of the reference voltage, which actual value deviates from the reference value as a result of unavoidable manufacturing tolerances, with the reference value.
11. Precision reference voltage source according to Claim 10, characterised by an alignment by altering at least one of the two component resistors (41 or 42, respectively), serving a compensation resistors.
12. Precision reference voltage source according to Claim 2, characterised in that the second reference transistor (24) comprises four or eight, respectively, identical component resistors (Figure 10, Figure 11).
13. Precision reference voltage source according to one of Claims 3 to 12, characterised in that the third-order term for the correction of the temperature coefficients of higher order remaining in the two reference transistors (23, 24) driven with different current density is also concomitantly taken into account.
14. Precision reference voltage source according to Claim 13, characterised in that the temperature coefficient of at least one component resistor of the resistor combinations (21 and 22; 31, 41 and 22; or 21, 32 and 42, respectively) can be altered by altering its width in the design.
15. Precision reference voltage source according to one of Claims 3 to 14, characterised in that a defined temperature coefficient, deviating from the temperature coefficient "0", of the reference voltage is adjusted by altering the divider ratio of the resistors [21, 22; 21, (32 + 42); or 22, (31 + 41)] referred to the value of the divider ratio to achieve the temperature coefficient "0".

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