GB2032659A

GB2032659A - Temperature compensated voltage reference

Info

Publication number: GB2032659A
Application number: GB7933524A
Authority: GB
Original assignee: Analog Devices Inc
Current assignee: Analog Devices Inc
Priority date: 1978-09-27
Filing date: 1979-09-27
Publication date: 1980-05-08
Also published as: DE2938849A1; NL7907161A; JPS5556212A; GB2032659B; JPH0135364B2; FR2437656B1; FR2437656A1; DE2938849C2; CA1141820A

Abstract

The invention relates to a circuit providing a temperature-compensated voltage reference. The circuit comprises a Zener diode 10 serving as the principal voltage source, in combination with a compensating voltage source including transistors Q1 & Q2 and their control circuitry 24. The compensating voltage is summed with the Zener voltage by operational amplifier 14 to produce a reference voltage Vo. The compensating voltage source includes adjustment elements R1 or R2 for trimming the reference output to a specified voltage, and the control circuitry operates with the adjustment elements to automatically produce optimum temperature compensation when the output has been adjusted to the specified value. Thus the output reference and the slope of the output-temperature characteristic (i.e. the temperature coefficient) are simultaneously adjusted by a single adjustment, R1 or R2. <IMAGE>

Description

SPECIFICATION Temperature compensated IC voltage reference This invention relates to solid-state voltage references. More particularly, this invention relates to improved means and methods for temperature-compensating such voltage references, and to simplified procedures by which such references may be set for optimum compensation performance.

Solid-state voltage references commonly incorporate a junction voltage source, such as a Zener, which exhibits a significant temperature coefficient requiring compensation. For many reference devices, the voltage-vs-tertiperature relationship can be approximated as: Vdev = Vk + a(TTK) Eq. 1 where Vde is the device terminal voltage at any temperature T, VK and Tk are constants, and a is a coefficient which varies with the processing of the device.

To provide compensation for the changes in voltage with temperature, the output of such a device can be summed with a compensating voltage circuit, such as a band-gap junction source, having a temperature coefficient opposite to the original in sign (slope), and incorporating appropriate scaling to develop the specified output voltage level. The characteristics of such a compensated voltage reference device may be represented by the following relationship: V = AkVGo - T)a + VK + a(T - TK)] Eq. 2 where VGO is the band-gap voltage, ss is the temperature coefficient of a forward-biased junction, a is a proportionality factor between the voltage reference device and the compensating device, and A is an overall scaling factor needed to achieve a specified voltage value.

Such a device has two degrees of freedom for adjustment purposes, represented symbolically by a (slope) and A (scaling) in Equation (2) above. One procedure in adjusting the device for specified operating characteristics is to utilize a computer-operated algorithm to set a at the proper value to minimize temperature-induced variations for a calculated value of a, and then adjust A to achieve the specified output voltage V,,,'. This procedure accordingly requires two separate adjustment steps, one for each of the two degrees of freedom of the control circuit design. Experience has shown however that this procedure is undesirably complex and expensive to carry out, and although useful commercially, it is not fully satisfactory in achieving desired performance. Thus a need for significant improvement has become evident.

In accordance with an important aspect of the present invention, it has been found that importantly superior results can be achieved by a technique wherein adjustment of a single circuit element of the voltage reference is employed to simultaneously alter the two variable factors (represented by A and a in Equation 2) which control the output voltage and temperature characteristics of the voltage reference. More particularly, in a presently preferred embodiment of the invention, the adjustment of a trim resistor to bring the reference output voltage to the specified value serves concurrently to alter the temperature compensating control circuitry to provide for optimum TC at the point where the reference voltage output is equal to the specified value.

To put the matter somewhat differently, it has been discovered that the two degrees of freedom previously utilized to make the complete adjustment of each voltage reference should be reduced to a single degree of freedom, thereby to improve performance of the voltage reference and at the same time simplify the manufacturing procedures. Reducing the adjustment procedure to a single degree of freedom can be understood in a mathematical sense by considering that the variable A is made dependent upon a by the topology of the associated control circuitry for the compensating voltage source. The dependency relationship can be expressed as follows:

where V,O,' is the specified output voltage.

The output voltage can then be expressed as:

The final expression becomes:

where a is the remaining adjustable parameter.

In accordance with one important aspect of the invention, adjustment of V,,, to the specified value simultaneously makes the term (a - fia) zero, i.e. by setting ssa = a, thus establishing the desired equality within the limits of the model.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which: Figure 1 is a simplified circuit diagram to illustrate the basic arrangement of a preferred embodiment of the invention; Figure 2 is a circuit diagram showing details of a voltage reference based on the principles illustrated in Fig. 1; and Figure 3 is a graph showing voltage-vs-temperature characteristics of classes of voltage sources.

Turning now to Fig. 1, the voltage reference embodying the principles of this invention includes a Zener diode voltage source 10 with one electrode connected to the output line 1 2 of an operational amplifier 14. The diode is connected through a negative feedback circuit to the inverting input terminal 1 6 of the amplifier, which in turn is connected through a resistor 1 8 to a common line or ground 20.

The Zener diode 10 is formed as part of an IC chip, together with associated control circuitry as shown in Fig. 2. The chip also typically will include further circuitry (not shown) requiring the stabilized reference voltage to be developed as will be explained. Preferably, the Zener diode is formed as a buried-layer device, for example as disclosed in detail in U.S. application Serial No.

801,410, filed on May 27, 1977, by W. K. Tsang, and assigned to the present applicant.

The potential of the non-inverting terminal 22 of the amplifier 14 is fixed by a control circuit generally indicated at 24 comprising a second voltage source means. This circuit includes seriesconnected matched transistors Q, and Q2 each with an emitter resistor R, and R2. The collector of Q2 is connected to the output line 12, and the emitter resistor R, is returned to ground. A 3resistor voltage divider 26, 28, 30 is provided to fix the base voltages of transistors Q1 and Q2.

at predetermined levels as will be explained.

The feedback circuit of the operational amplifier 14 maintains the input terminals 1 6 and 22 at the same potential, so that the amplifier output voltage V, can be viewed as being the sum of the diode voltage V, and the voltage suppled to the non-inverting terminal 22. It may be noted that in the particular bridge-type of circuit shown herein, the voltage on terminal 22 also is dependent upon the output voltage V,. However, such dependency is not a requirement of the invention, and other types of circuitry can be used to combine the Zener voltage with a compensating voltage.

The voltage reference output V, can be represented as a function of circuit element values and significant parameters to be discussed subsequently. A detailed derivation of the relationship is set forth in the Appendix at the end of this specification. As shown in that derivation, the output voltage can be expressed as:

where V, is the Zener diode voltage, Vb" is the base-to emitter voltage (of either Q, or Q2), 8 is the proportionality factor for the base voltage of Q2 (i.e., Vb2 = So,), e is the proportionality factor for the base voltage of Q" and R1, R2 are resistance values.

To determine one set of relationships for zero TC, the derivative of Equation 1 A can be taken with respect to temperature, and set to zero, to produce:

where y is defined as d/dT Vbo which is approximately equal to (V,,--V,,,)/T,; a, as previously described is equal to d/dT Vz; and VGO is the band-gap voltage.

To develop the necessary further constraints for zero TC conditions, Equation 1A may be further elaborated as:

where VK and TK are constants (see Eq. 1 above) and T is the device temperature.

Equation 3A can be further developed through use of Equation 2A to produce:

where VK, Tx, 8, e and y are constants.

Taking the derivative of Equation 4A with respect to a and setting it equal to zero yields:

When this relationship is established, V, will be independent of a. That is, the control circuitry will be effective in achieving the desired result regardless of the particular Zener diode with which it is used.

Since the parameters being established are to be valid for any a, a still further relationship for e and 8 can be found by setting a = O in Equation 4A: Vx --1 -+s Eq. 6A V, Equations 5A and 6A can be solved for e and 8:

These relationships have been derived to provide for zero TC at the specified output voltage.

However, modified relationships can, by the same techniques, be derived for other kinds of desired control of the temperature coefficient dependent upon adjusting the output voltage to a specified value. For example, there are applications requiring a specific non-zero TC at the specified reference voltage, e.g. for the purpose of matching the reference performance to another circuit characteristic. In addition, the control function described herein can be used in applications where different output voltages are required for individual units of a group, with each such output voltage having a corresponding different TC requirement. Thus, the manner in which the invention is embodied will depend upon the particular application problem to be solved.

In the case of the Fig. 1 circuit to be used to achieve zero TC, the numerical values for E and 8 can be obtained by inserting into Equation 7A and 8A experimentally determined values for Vx and Tx, together with the known value of V,,, a calculated value for y (using the definition in Equation 1 A with a known value of V5,,,), and the desired value for V0. Vx and Tx have been determined experimentally by voltage-vs-temperature measurements on a large number of buried Zener diodes, and typical extrapolated values are: VK = 4.74 and Tx = - 383 K. The value of Vdeo is 0.655 at T, = 300 K.Using a specified value V,= = 10, the proportionality factors become: = .1960 S = .7220 Accordingly, by corresponding selection of the resistors 26, 28 and 30 to achieve base voltages Vb, and V52 of 1.960 and 7.220 volts, the circuit arrangement of Fig. 1 will provide optimum temperature compensation when one or the other emitter resistor R1 or R2 has been adjusted to achieve the specified output voltage of 10 volts. Which resistor R2 or R, is trimmed depends upon whether the initially measured output voltage is above or below 1 0 volts.

For an experimentally measured range of a for a large number of units of the class of Zener diodes produced with an iC process, as described hereinabove, the corresponding values of R2/R1 are appropriately practical. Reverting to Equation 2A, and substituting the measured range of values for a corresponding to measured Zener voltages of V, (at 300 K) of 6.0 to 6.6, it is found that: minimum R2/R2 = 1.966 (for V, = 6.0) maximum R,/R, = 2.426 (for V, = 6.6) Fig. 2 shows details of a presently preferred voltage reference incorporating the arrangement of Fig. 1, and which performs as described above. In Fig. 2, Q..2 and Q"3 form the basic elements of the operational amplifier 14.The Zener diode Dz has Kelvin connections, with force and sense electrodes essentially at the same potential. One is connected to inverting input terminal 1 6 and the other is connected through a resistor R,43 (reference 1 8 in Fig. 1) to common line 20. Transistors Q"5 and Q..6 correspond to Q2 and Q, of Fig. 1, resistors R,38 and R,39 correspond ta resistors Rz, R1, and resistors R,35 R,36 and R,37 correspond to resistors 26, 28 and 30.

The amplifier circuitry of Fig. 2 is arranged with an essentially symmetrical balanced configuration. Q,07 supplies collector current to Q..2 and 0113. The collector of Q,14 receives the emitter currents of Q112 and Q113, and provides adjustment to make the total current correct. The base of Q,14 is controlled through voltage translation transistor Qios and pinch resistor R,40 by current from the left-hand collector of Q107.

Q109 and Q110 are buffer transistors. The current in Q109 is controlled by Q105 which is matched to Q104 to provide for equal currents. The Q,04 current passes through Q,06 which is matched to Q10" so that the Q107 current and the Q109 current will be equal, and equal to the Q"4 current.

Thus, although the base currents of Q109 and Q"4 may represent errors, such errors are balanced with respect to Q,12, Q1,3, so that they tend to cancel due to the circuit symmetry.

Q103 carries any additional current required by Q"5, Olis. Q", provides protection for the output buffer Q11O. The left-hand emitter of Q109 serves to aid start-up of the circuitry.

Fig. 3 illustrates graphically the voltage and temperature relationships discussed above with reference to Fig. 1, for achieving optimum temperature compensation through adjustment of the reference output voltage to its specified value. The presentation includes two straight lines Z and Z2 representing the outer limits of the range of variation for measured voltage-vstemperature characteristic curves of a large number of buried Zener diodes. The slope of these lines (a, and a2) represent the derivative of the voltage-vs-temperature relationship as discussed above.Extrapolation of these lines (and lines for intervening data, not shown) to the left results in an intersection in a common region centered about a particular voltage VK and a corresponding temperature TK. (Note: For the measured data presented herein, the intersection occurs at a temperature below absolute zero, and thus has no physical counterpart, but does have conceptual significance.) With a common intersection point, and at least approximately straightline characteristics, the voltage-vs-temperature characteristics of this Zener-diode class of voltage sources can be represented, as previously stated, as: Vd,iv = VK + a(T - TK) where a represents the slope of each curve.

Also shown on Fig. 3 are two additional straight lines J, and J2 representing limits of the range of voltage-vs-temperature characteristic curves for the voltage which is combined with the Zener voltage, and which is derived from the compensating voltage source means 24 comprising a band-gap junction.These lines also intersect at a common region, and the control circuitry of the compensating voltage source means is arranged to locate this common region at a temperature of TK, i.e. on the same vertical line as the common region of intersection of the Zener characteristic curves Z, and Z2 The control circuitry is further arranged to locate the common intersection at the compensating voltage Yj having a magnitude such that when Vj is combined with VK, the composite voltage will be equal to the specified reference output voltage, i.e. in this case 10 volts.

Accordingly, with this arrangement the adjustment of the voltage reference to provide a specified output of 10 volts, by in effect changing the slope of the compensating voltage source line within the range between J, and J2, will automatically result in the final adjusted slope of the curve JN having an inversely matching (i.e., complementary) relationship with respect to the slope of the characteristic curve line ZN of the particular Zener diode forming the basic source of the voltage reference. Thus, the temperature coefficient of the voltage reference will be optimized at or very near zero, as a result of trimming the output voltage to its specified value.

Although a specific preferred embodiment of the invention has been set forth hereinabove in detail, it is desired to emphasize that this is for the purpose of illustrating the principles of the invention, and is not to be considered in limitation of the scope of the invention. Thus it will be understood that the invention can be used to compensate various types of basic voltage sources, and that the compensation means can utilize various kinds of compensating voltage source means to be operated with the basic voltage source. Moreover, a wide variety of control circuits can be employed to implement the basic concepts of the invention. Accordingly, it will be appreciated that the present disclosure is provided to aid those skilled in this art in adapting the invention in various forms best suited to particular applications.

APPENDIX Since the input terminals of the amplifier 14 are at the same potential, the following equality can be written:

Substituting in Eq. 1A for Vz and R2/R1 gives:

expanding the numerator gives: VK - aT + a/y VgO so the voltage as a function of a is:

Taking the derivative with respect to a gives:

Setting the derivative equal to zero and solving yields:

Claims

1. A temperature-compensated solid-state supply signal reference comprising: first supply signal source means for producing a first signal following a signal-vs-temperature characteristic curve with a first slope; second supply signal source means for producing a second signal which, in operation, is combined with said first signal to produce a composite reference output signal responsive to said first and second signals, said second signal following a signal-vs-temperature characteristic curve with a second slope; control circuit means operable with said second supply signal source means and including adjustable means to vary said second signal to alter correspondingly said composite reference output signal to a specified value;; said control circuit means including means under the control of said adjustable means to vary said second slope as said second signal is changed to provide a predetermined temperature coefficient for said composite signal when it has been adjusted to said specified value.

2. Apparatus as claimed in Claim 1, wherein said control circuit means is operative to provide an effective zero temperature coefficient for said composite signal when it reaches said specified value.

3. Apparatus as claimed in Claim 2, wherein said control circuit means is operable to produce an effective inverse match between said two slopes when said composite signal reaches said specified value, to achieve a zero temperature coefficient at that level.

4. Apparatus as claimed in Claim 1, 2 or 3, wherein said first source means comprises a Zener diode, and said second source means comprises means for producing a compensating signal which is a function of the base-to-emitter voltage of a semiconductor junction.

5. Apparatus as claimed in Claim 4, wherein said adjustment means comprises a changeable resistor in series with said junction.

6. In the art of temperature-compensating the signal of a solid-state supply signal source by connecting thereto second signal source means producing a second signal so as to develop a reference signal responsive to both said first and second signals, and wherein the signal-vstemperature characteristic curves of said two signals have opposite signs so that the temperature effects tend to cancel in said reference signal; the improved method for providing temperature compensation for a reference signal of a specified value, comprising the steps of: adjusting a circuit element coupled to said second signal source means so as to vary said second signal and thereby alter said reference signal to said specified value; and controlling through said adjustment of said circuit element the slope of the characteristic signal-vs-temperature curve of said second signal to produce a predetermined relationship between the effects on said reference signal of the temperature characteristics of said first and second signals so as to provide a pre-selected temperature coefficient for said reference signal when it has been adjusted to said specified value.

7. A temperature-compensated, solid-state, supply signal reference circuit, substantially as hereinbefore described with reference to the accompanying drawings.

8. A method for providing temperature reference compensation substantially as hereinbefore described with reference to the accompanying drawings.