High Accuracy Zener-Based Voltage Reference Circuit

Abstract

A voltage reference circuit is disclosed, comprising: a power supply terminal; a ground terminal; a first current source and a Zener diode connected in series between the power supply terminal and the ground terminal, having a first node therebetween, and configured to supply a Zener voltage at the first node; an output node configured to provide a voltage reference; and a CTAT circuit connected between the first node and the output node; wherein the CTAT circuit comprises: two bipolar transistors, the two bipolar transistors having their respective emitters connected at a second node and configured to have equal collector-emitter currents in operation, a base of a first bipolar transistor connected to the first node, and a base of a second bipolar transistor connected to a center node of a first voltage divider; and wherein the first voltage divider is connected between the emitter of the second bipolar transistor and the output node.

Description

Technical Field

[0001] The present disclosure relates to voltage reference circuits, and in particular, to temperature compensated Zener-based voltage reference circuits. Background Art

[0002] For many electronic applications, it's necessary to provide an accurate, known, fixed reference voltage. The requirements for this reference voltage vary depending on the application. For example, battery management system (BMS) products often require a reference voltage that is relatively insensitive to changes in ambient or operating temperature and remains stable over long periods of time, typically measured in years. In other words, the reference voltage must exhibit low drift. Drift can be caused, for example, by component aging or stress on the IC package.

[0003] Conventionally, a bandgap circuit is used to provide a known reference voltage. However, for applications such as BMS products, where long-term drift is a critical performance requirement, a Zener diode-based reference voltage circuit is an attractive alternative. The voltage across a Zener diode changes only slowly with the current passing through it, and therefore the diode can form the basis of an accurate voltage reference. However, Zener diodes typically have a positive temperature coefficient (TC), meaning that for a fixed current, the voltage across the diode increases as temperature rises, necessitating temperature compensation by adding a complementary to absolute temperature (CTAT) circuit. Summary of the Invention

[0004] According to a first aspect of the present disclosure, a voltage reference circuit is provided, comprising: a power supply terminal configured to be connected to a power supply voltage; a ground terminal configured to be connected to a ground voltage; a first current source and a Zener diode, the first current source and the Zener diode being connected in series between the power supply terminal and the ground terminal, having a first node therebetween, and configured to supply a Zener voltage (Vz) at the first node; an output node (Vref_hv) configured to provide a voltage reference (Vref_hv, Vref); and a complementary to absolute temperature (CTAT) circuit connected between the first node and the output node; wherein the CTAT The circuit includes: a first bipolar transistor (Q1) and a second bipolar transistor (Q2), each having a base, a collector, and an emitter, with their respective emitters connected at a second node (Vs) and configured to have equal collector-emitter currents in operation, wherein the base of the first bipolar transistor is connected to the first node and the base of the second bipolar transistor is connected to a center node of a first voltage divider, and wherein the first voltage divider is composed of a first resistor connected between the output node (Vref_hv) and the center node and a second resistor connected between the center node and the emitter of the second bipolar transistor.

[0005] Therefore, by providing a pair of bipolar transistors arranged to carry the same or nearly the same current, their base-emitter voltages can be accurately matched, which improves the accuracy of the positive temperature coefficient cancellation of the Zener diode voltage. Furthermore, because the transistors are not stacked or partially stacked relative to ground, the minimum supply voltage required at the power supply terminals for proper operation can be lower than the minimum supply voltage in known circuits.

[0006] In one or more embodiments, the CTAT circuit further includes a second current source (I_BIAS_hs) connected between the collector of the first bipolar transistor and a power supply node and configured to provide a bias current to the first bipolar transistor.

[0007] In one or more embodiments, the CTAT circuit further includes a FET having a main terminal connected between the collector of the second bipolar transistor and the power supply node, and a control terminal connected to the collector of the first bipolar transistor, and configured to match the collector-emitter currents through the first and second bipolar transistors. This arrangement can provide a particularly simple method of ensuring matching currents.

[0008] In one or more embodiments, the CTAT circuit further includes a third current source connected between the emitters of the first and second bipolar transistors and the ground terminal. The third current source can be configured to sink a current equal to twice the current supplied by the second current source plus the current through the second resistor. This ensures that the emitter currents of the two bipolar transistors are precisely matched. Note that the general term "current source" used herein refers to both current sources and current sinks in a strict sense.

[0009] In one or more embodiments, the voltage reference is provided directly at the output node. In other embodiments, a second voltage divider is provided, comprising two resistors or resistors connected between the output node and ground, with a center node therebetween, wherein the voltage reference (Vref) is at the center node of the second voltage divider. This allows the reference voltage to be scaled to a particular selected value or range.

[0010] In one or more embodiments, the first bipolar transistor and the second bipolar transistor are each NPN transistors.

[0011] In one or more embodiments, the first bipolar transistor and the second bipolar transistor are matched transistors. That is, the transistors can be designed to have the same or very similar characteristics. This ensures that a scaled version of the base-emitter voltage from Q2 is directly applied to the Zener voltage Vz, despite only an indirect connection through Q1.

[0012] In one or more embodiments, the current through the second resistor is less than 100 nA. Using a low current through this voltage divider ensures that the transistor currents are nearly identical.

[0013] In one or more embodiments, the voltage reference circuit is configured to operate with a supply voltage between 6 V and 7 V. This may not be possible in prior art designs.

[0014] In one or more embodiments, the second current source and the third current source are each configured to have a zero temperature coefficient (0TC).

[0015] In one or more embodiments, the third current source is configured to provide a current consisting of a proportional to absolute temperature (PTAT) component and a CTAT component, wherein the CTAT component is a scaled version of the current through the second resistor.

[0016] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments will be described, by way of example only, with reference to the accompanying drawings, in which:

[0018] Figure 1 A conventional voltage reference circuit based on a Zener diode and including temperature compensation is shown;

[0019] Figure 2 An alternative voltage reference circuit based on a Zener diode and including temperature compensation is shown;

[0020] Figure 3 A voltage reference circuit based on a Zener diode and including temperature compensation according to one or more embodiments is shown;

[0021] Figure 4 A voltage reference circuit based on a Zener diode and including a temperature coefficient is shown according to one or more other embodiments; and

[0022] Figure 5 A bias circuit is shown for providing bias current to embodiments of the present disclosure.

[0023] It should be noted that the drawings are diagrammatic and not drawn to scale. For clarity and convenience in the illustrations, the relative sizes and proportions of the various parts of these drawings have been shown exaggerated or reduced in size. The same reference numerals are generally used to refer to corresponding or similar features in modified and different embodiments. DETAILED DESCRIPTION

[0024] Figure 1 A conventional voltage reference circuit 100 based on a Zener diode and including temperature compensation is shown. Circuit 100 has a power supply terminal 10 configured to receive a power supply voltage, and a ground terminal 20 configured to operate at a ground voltage. The circuit includes a Zener diode 30 connected in series with a compensation circuit 40 and a current source 50 between the power supply and ground. Current source 50 supplies a current IZEN through Zener diode 30. The voltage Vz across the diode is relatively stable, but as mentioned, exhibits a positive temperature coefficient. Therefore, compensation circuit 40 is placed in series with the Zener diode to add a temperature-dependent voltage to the Zener voltage Vz. The compensation portion of circuit 40 consists of a bipolar transistor Q1 42 connected in parallel with a voltage divider 44 consisting of a lower resistor R2 46 and an upper resistor R1 48. The base of the bipolar transistor is connected to the center node of the resistor divider, that is, between R2 and R1, while the lower end of the voltage divider is connected to the emitter of the bipolar transistor at the lower node 55 and the upper end of the voltage divider is connected to the collector of the bipolar transistor. The reference voltage Vref_hv at the output node 60 is provided at the collector of the bipolar transistor.

[0025] from Figure 1 It can be shown that:

[0026] Vref_hv = V z + V be1 + V R1 (1)

[0027] where V R1 is the voltage across resistor R1, and V be1 is the emitter - base voltage of bipolar transistor Q1.

[0028] By setting the resistors in the voltage divider to be large, the base current can be set low enough to be negligible, and then:

[0029]

[0030] From which it can be shown that

[0031]

[0032] Vz has a positive temperature coefficient; however, this is compensated by the negative temperature coefficient of V be1 . Since this negative temperature coefficient is approximately - 2mV / °C, it is scaled by the factor (1 + R1 / R2), where the ratio between R1 and R2 is chosen to cancel the positive temperature coefficient of the Zener diode. It should be understood that this is because (1 + R1 / R2) is always greater than unity. Therefore, when 0mV / °C < TC1_zener < - 2mV / °C, this structure itself cannot produce 0TC Vref_hv.

[0033] Figure 2 Shows an alternative voltage reference circuit 200 based on a Zener diode and including temperature compensation. This circuit is generally similar to Figure 1 's circuit, except that the compensation circuit relies on the V be of a second bipolar transistor, which separates the current through the bipolar transistor from the Zener current.

[0034] The circuit 200 has a power supply terminal 10 configured to receive a power supply voltage, and a ground terminal 20 configured to operate at a ground voltage. The circuit includes a Zener diode 30 connected in series with a compensation circuit 240 and a current source IZEN 50 between the power supply and the ground. The current source 50 supplies a current through the Zener diode 30. Thus, the compensation circuit 40 is arranged in series with the Zener diode to add a temperature-dependent voltage to the Zener voltage Vz. The compensation of the circuit 240 consists of a bipolar transistor Q1 42 connected in parallel with a voltage divider 44 consisting of a lower resistor R2 46 and an upper resistor R1 48. The base of the bipolar transistor is connected to the center node of the resistance divider, that is, between R2 and R1, while the lower end of the voltage divider is connected to the emitter of the bipolar transistor at a lower node 55 and the upper end of the voltage divider is connected to the collector of the bipolar transistor. This circuit differs from Figure 1 , except that the collector of Q1 at voltage Ve is not directly connected to the reference voltage, but is instead connected to the base terminal of a second bipolar transistor Q2 270. This second bipolar transistor Q2 is connected in a second path between the power supply 10 and the ground 20. The second bipolar transistor Q2 is connected in series with a second voltage divider 64, which includes two resistors R4 66 and R3 68, with a node Vref therebetween. The node between the second voltage divider 64 and the emitter of transistor Q2 is denoted as Vref_hv. Analysis of this circuit yields:

[0035]

[0036] Where V be2 is the base-emitter voltage of Q2. Assuming the bias current of each transistor is the same, V be1 Equal to V be2 , and the above equation can be simplified to:

[0037]

[0038] Therefore, this circuit is similar to Figure 1 , but instead of a constant voltage reference Vref_hV, the value of the voltage reference Vref can be selected by appropriately selecting the resistors in the second resistance divider 64.

[0039] However, the above analysis relies on the assumption that the base-emitter voltages of Q1 and Q2 are the same. In practice, this is not necessarily the case. It is necessary to provide a circuit in which the base-emitter voltages of Q1 and Q2 can be made the same, or close enough that the difference is negligible.

[0040] Figure 3FIG. 3 shows a voltage reference circuit 300 based on a Zener diode and including temperature compensation according to one or more embodiments. Conceptually, this circuit differs from Figure 1 and 2 The circuit is similar to the circuit shown in , except that the compensation circuit is not stacked on top of the diode and can therefore operate at a lower supply voltage.

[0041] The circuit 300 has a power supply terminal 10 configured to be connected to a power supply voltage, and a ground terminal 20 configured to be connected to a ground voltage. The circuit includes a first current source 50 supplying a current IZEN and a Zener diode 30, the first current source 50 and the Zener diode 30 being connected in series between the power supply terminal and the ground terminal. Between the current source 50 and the Zener diode 30 is a first node 355, at which a Zener voltage (Vz) is present.

[0042] A Zener voltage node with a voltage Vz is associated with an output node 360, which is configured to provide a first voltage reference Vref_hv via a compensation circuit. The first voltage reference 360 ​​can also be considered a high voltage reference, as will become more apparent from the discussion of lower voltage references below. The compensation circuit takes the form of a complementary to absolute temperature (CTAT) circuit 340 connected between the first node and the output node. That is, the voltage difference (between the Zener voltage Vz and the voltage Vref_hv at the output node) decreases as the temperature or absolute temperature increases.

[0043] Similar to Figure 2 In the circuit shown in FIG. 3 , the CTAT circuit 240 includes two bipolar transistors Q1 370 and Q2 380. However, unlike Figure 2 Thus, in the embodiment shown, the CTAT circuit is based on a first bipolar transistor Q1 370 and a second bipolar transistor Q2 380, each having a base, a collector, and an emitter, with their respective emitters connected at a voltage Vs at a second node 365, and configured to have equal or similar collector-emitter currents in operation.

[0044] The base of the first bipolar transistor is connected to a first node, i.e., a Zener voltage node, and the base of the second bipolar transistor is connected to a center node 305 of a voltage divider consisting of two resistors or resistors R1 315 and R2 325. The voltage divider is connected between the emitter of the second bipolar transistor and the output node (Vref_hv), that is, it is connected in parallel with the second bipolar transistor Q2.

[0045] Current is supplied to each of the two bipolar transistors 370 Q1 and 380 Q2 via two additional current sources 335 and 345 (separate from the first current source 50 providing the Zener current). Specifically, the second current source 345 is connected between the collector of the first bipolar transistor and the power supply node and is configured to provide a bias current I_BIAS_hs to the first bipolar transistor. Furthermore, the third current source 335 is connected between the common emitter of the first and second bipolar transistors and ground. This current source is configured to sink a current I_BIAS_ls from the transistor pair. Transistors Q1 and Q2 are arranged in parallel, i.e., transistors Q1 and Q2 each form part of two separate branches between a node 365 at voltage Vs (i.e., the high side of the first current source 335) and the voltage terminal I0.

[0046] The two branches carry similar or equal currents. Various ways of ensuring proper current sharing between the two branches will be apparent to those skilled in the art. Figure 3 In the depicted embodiment, this is achieved by FET 390. The FET can be a p-channel mode FET. The FET is included in the same branch as the branch of the second bipolar transistor, and the main terminal of the FET is connected between the collector of the second bipolar transistor and the power supply node, and the control terminal is connected to the collector of the first bipolar transistor. The FET is configured to match the collector-emitter current through the first branch and the second branch - that is, through the first bipolar transistor and the second bipolar transistor. (In practice, the current through the second branch is shared between the second bipolar transistor Q2 and the voltage divider R1 R2. However, as will be discussed in more detail below, the current through the voltage divider R1 R2 is selected to be several orders of magnitude lower than the current through Q2 and is therefore practically negligible.)

[0047] Figure 4 A voltage reference circuit based on a Zener diode and including a temperature coefficient according to one or more other embodiments of the present disclosure is shown. This circuit is generally similar to Figure 3 Circuit: Directly corresponding parts do not need to be described in further detail. However, this circuit has an additional second voltage divider 410. This second voltage divider 410 includes two resistors R3 415 and R4 425 connected between the output node 305 and ground. The second voltage divider has a center node 405 between resistors R1 and R2. A voltage reference 405 having a voltage Vref is located at the center node of the second voltage divider. As will be immediately apparent to those skilled in the art, this voltage divider is used to scale the high voltage output reference Vref_hv to a selected reference voltage Vref.

[0048] It can be analyzed as follows Figure 3 and 4The circuit:

[0049] Vref hv =Vz-V be1 +V be2 +V R1 (5),

[0050] Where V R1 is the voltage across R1 in the first voltage divider.

[0051] But since Q1 and Q2 carry almost the same current, V be1 and V be2 are equal and can cancel each other out. Then,

[0052] Vref hv =Vz+V R1 (6).

[0053] The base current in Q2 can be considered negligible because the values ​​of resistors R1 and R2 are set large. Then,

[0054] therefore

[0055]

[0056] Substituting this into equation (6) yields:

[0057]

[0058] And applying the voltage divider discussed above:

[0059]

[0060] The above equations first show (and as already mentioned) that the voltage reference Vref can be scaled compared to Vref_hv according to the following formula:

[0061]

[0062] And secondly, through the base-emitter voltage V be2 The negative temperature coefficient of the Zener voltage Vz is used to compensate for the positive temperature coefficient of the Zener voltage Vz. As will be understood by those skilled in the art, V be2 The temperature coefficient of the Zener diode is typically -2mV / °C. Adjusting the ratio of R1 to R2 can then almost completely eliminate the positive voltage coefficient of the Zener diode.

[0063] exist Figure 2 In the voltage reference circuit shown in , the supply voltage must be sufficient to provide the sum of the Zener voltage, the collector-emitter voltage of Q1, and the collector-base voltage of Q2. Figure 3 and4 The embodiment of the present disclosure shown in FIG can operate at a lower supply voltage. Specifically, because the bipolar transistors have interconnected emitters, only the supply voltage needs to be provided to provide sufficient margin for the Zener voltage, the base-collector voltage of Q1, and any voltage drop required to provide the high-side current source IBIAS_hs 345. Thus, compared to a typical bipolar transistor that requires a minimum supply voltage of approximately 7V, such as Figure 2 Compared to the previous circuit shown in , embodiments of the present disclosure can be configured to operate with a minimum supply voltage of approximately 6V.

[0064] Now turn Figure 5 , this figure shows a bias circuit 500 for providing bias current to embodiments of the present disclosure using techniques familiar to those skilled in the art. The bias circuit provides a temperature-compensated constant current IOTC. The temperature-compensated constant current IOTC can be used, for example, as the current I_BIAS_hs in the high-side current source 345. In summary, the temperature-compensated constant current is provided as the sum of two currents, which are determined using a scaled current mirror based on a current complementary to absolute temperature (CTAT) ICTAT and a current proportional to absolute temperature (PTAT) IPTAT, respectively.

[0065] The current complementary to absolute temperature is determined as the current through resistor R5 520 connected between the base and emitter of a first NPN bipolar transistor Q3 530. The first NPN bipolar transistor is connected in series with a first FET M1 542 between a power supply voltage 505 and a ground voltage 515. The current through R5 is supplied by a first branch M3 562 of a first scaling current mirror 560. A second branch M4 564 of the scaling current mirror provides a factored current. <a:1>This current is scaled as the current I 0TC The first part of .

[0066] A current proportional to the absolute temperature is determined as the current through a second resistor R6 570 connected between the emitter of a second NPN bipolar transistor Q4 580 and ground. The collector of the second NPN bipolar transistor is connected in series with a second FET M2 544 to the supply voltage 505. The second FET M2 544 is in a current mirror configuration 540 with the first FET 542. This current mirror 540 includes another scaled copy of the current through another FET M5 546. This branch provides a proportional current through R6. <b:1>The current replica is combined with the first scaling factor current to provide the current I 0TC The second part.

[0067] As already mentioned, this temperature compensated current source I 0TC can be used directly to provide current I_BIAS_hs to the high-side current source. A similar circuit can be used to supply current I_bias_ls to the low-side bias current source 335. This current should be roughly twice the high-side current. In more detail, the current should include current IR2, which, as mentioned above, can be so low that it can be ignored. However, it is possible to take this into account and by setting this current to the current I_bias_ls complementary to the absolute temperature mentioned above CTAT to provide improved accuracy, i.e. IR2 = cI CTAT .

[0068] From reading this disclosure, those skilled in the art will recognize other variations and modifications. Such variations and modifications may involve equivalent and other features that are already known in the art of voltage references at that time and may be used as a replacement or supplement to the features already described herein.

[0069] Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features or any generalization of such novel features disclosed herein, whether or not the novel feature relates to the same invention as the present invention currently claimed in any claim or whether the novel feature alleviates any or all of the same technical problems as those alleviated by the present invention.

[0070] Features described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features described in the context of a single embodiment for the sake of brevity may also be provided individually or in any suitable subcombination. Applicants hereby caution that new claims may be formulated during the prosecution of this application or any further application derived therefrom to such features and / or combinations of such features.

[0071] For the sake of completeness, it is also pointed out that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality and reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

1. A voltage reference circuit, characterized in that: include: a power supply terminal configured to be connected to a power supply voltage; a ground terminal configured to be connected to a ground voltage; a first current source and a Zener diode, the first current source and the Zener diode being connected in series between the power supply terminal and the ground terminal, having a first node therebetween, and being configured to supply a Zener voltage (Vz) at the first node; an output node (Vref_hv) configured to provide a voltage reference (Vref_hv, Vref); as well as a complementary to absolute temperature (CTAT) circuit connected between the first node and the output node; The CTAT circuit comprises: a first bipolar transistor (Q1) and a second bipolar transistor (Q2), each having a base, a collector, and an emitter, with respective emitters of the first bipolar transistor and the second bipolar transistor connected at a second node (Vs) and configured to have equal collector-emitter currents in operation, wherein the base of the first bipolar transistor is connected to the first node, and The base of the second bipolar transistor is connected to the center node of the first voltage divider, and The first voltage divider is composed of a first resistor connected between the output node (Vref_hv) and the central node and a second resistor connected between the central node and the emitter of the second bipolar transistor.

2. The voltage reference circuit according to claim 1, wherein: The CTAT circuit additionally includes a second current source (I_BIAS_hs) connected between the collector of the first bipolar transistor and a power supply node and configured to provide a bias current to the first bipolar transistor.

3. The voltage reference circuit according to claim 1 or 2, characterized in that: The CTAT circuit further includes a FET having a main terminal connected between the collector of the second bipolar transistor and the power supply node, and a control terminal connected to the collector of the first bipolar transistor, and configured to match the collector-emitter currents through the first bipolar transistor and the second bipolar transistor.

4. The voltage reference circuit according to claim 1 or 2, characterized in that: The CTAT circuit further includes a third current source connected between the emitters of the first and second bipolar transistors and the ground terminal.

5. The voltage reference circuit according to claim 4, characterized in that: The third current source is configured to sink a current equal to twice the current supplied by the second current source plus a current through the second resistor.

6. The voltage reference circuit according to claim 1 or 2, characterized in that: The voltage reference is provided directly at the output node.

7. The voltage reference circuit according to claim 1 or 2, characterized in that: Additionally included is a second voltage divider comprising two resistors connected between the output node and ground and having a center node between the two resistors, wherein the voltage reference (Vref) is at the center node of the second voltage divider.

8. The voltage reference circuit according to claim 1 or 2, characterized in that: The first bipolar transistor and the second bipolar transistor are each an NPN transistor.

9. The voltage reference circuit according to claim 1 or 2, characterized in that: The first bipolar transistor and the second bipolar transistor are matched transistors.

10. The voltage reference circuit according to claim 1 or 2, characterized in that: The current passing through the first voltage divider is less than 100 nA.

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