Rotating gain resistors to produce a bandgap voltage with low-drift

In accordance with an embodiment of the present invention, a bandgap voltage reference circuit includes a plurality of circuit branches, a plurality of resistors and a plurality of switches. The plurality of switches are used to selectively change over time which of the resistors are connected to be within a first one of the circuit branches and which of the resistors are connected to be within a second one of the circuit branches, to thereby reduce the effects that long term drift of the resistors have on a bandgap voltage output (VGO) of the bandgap voltage reference circuit.

BACKGROUND

A bandgap voltage reference circuit can be used, e.g., to provide a substantially constant reference voltage for a circuit that operates in an environment where the temperature fluctuates. A bandgap voltage reference circuit typically adds a voltage complimentary to absolute temperature (VCTAT) to a voltage proportional to absolute temperature (VPTAT) to produce a bandgap reference output voltage (VGO). The VCTAT is typically a simple diode voltage, also referred to as a base-to-emitter voltage drop, forward voltage drop, base-emitter voltage, or simply VBE. Such a diode voltage is typically provided by a diode connected transistor (i.e., a BJT transistor having its base and collector connected together). The VPTAT can be derived from one or more VBE, where ΔVBE (delta VBE) is the difference between the VBEs of BJT transistors having different emitter areas and/or currents, and thus, operating at different current densities.

FIG. 1Aillustrates an exemplary conventional bandgap voltage reference circuit100, including transistors Q1through QN connected in parallel (in the “N” branch), a transistor QN+1 (in the “1” branch), and a further transistor QN+2 (in the “CTAT” branch).

The bandgap voltage reference circuit100also includes an amplifier120and three PMOS transistors M1, M2and M3that are configured to function as current sources that supply currents to the “N”, “1”, and “CTAT” branches. Since the gates of the PMOS transistors are tied together, and their source terminals are all connected to the positive voltage rail (VDD), the source-to-gate voltages of these transistors are equal. As a result, the “N”, “1”, and “CTAT” branches receive and operate at approximately the same current, Iptat.

InFIG. 1Athe transistor QN+2 is used to generate the VCTAT, and the transistors Q1through QN in conjunction with transistor QN+1 are used to generate the VPTAT. More specifically, the VCTAT is a function of the base emitter voltage (VBE) of diode connected transistor QN+2, and the VPTAT is a function of ΔVBE, which is a function of the difference between the base-emitter voltage of transistor QN+1 and the base-emitter voltage of diode connected transistors Q1through QN connected in parallel.

Here, the bandgap voltage output (VGO) is as follows:

where Vt is the thermal voltage, which is about 26 mV at room temperature.

If VBE˜0.7V, and R2/R1*VT*ln(N)˜0.5V, then VGO˜1.2V.

The current sources can be implemented using alternative configurations than shown inFIG. 1A. Accordingly,FIG. 1Bis provided to show the more general circuit. As was the case inFIG. 1A, inFIG. 1Bthe amplifier120controls the current sources I1, I2and I3.

The voltage across R2is proportioned to temperature and when it is scaled to about 0.5V at room temperature it makes VGO relatively constant with temperature by compensating the negative temperature coefficient of VBE3(i.e., the base emitter voltage of transistor Q3).

For N=8, which is a common value for N,

R⁢⁢2R⁢⁢1~9
for a good temperature coefficient (tempco) of VGO. R2can be provided by connecting three unit resistors in series, and R1can be provided by connecting another three unit resistors in parallel. This is a common practice and makes the ratio of 9 very accurate in manufactured circuits.

In practice, long term drift in unit resistor values can cause long term drift in VGO, which is undesirable.

SUMMARY

Certain embodiments of the present invention are directed to bandgap voltage reference circuits that reduce the affects that long term drift of resistors have on the bandgap voltage output (VGO) produced by the bandgap voltage reference circuits. In accordance with an embodiment of the present invention, a bandgap voltage reference circuit includes a plurality of resistors, a plurality of circuit branches, and a plurality of switches. The plurality of circuit branches of the bandgap voltage reference circuit (e.g., an “N”, a “1” and a “CTAT” branch) are collectively used to produce the bandgap voltage output (VGO). The plurality of switches (e.g., controlled by a controller) are used to selectively change over time which of the resistors are connected to be within a first one of the circuit branches (e.g., the “N” branch) and which of the resistors are connected to be within a second one of the circuit branches (e.g., the “CTAT” branch).

In some embodiments, the plurality of resistors include a first group of resistors and a second group of resistors, and the plurality of switches include a first group of switches and a second group of switches. In such embodiments, the first group of switches can be used to selectively connect the first group of resistors in parallel with one another within the first one of the circuit branches at some times, and to selectively connect the first group of resistors in series with one another within the second one of the circuit branches at other times. Similarly, the second group of switches can be used to selectively connect the second group of resistors in series with one another within the second one of the circuit branches at some times, and to selectively connect the second group of resistors in parallel with one another within the first one of the circuit branches at other times.

In specific embodiments, each of the resistors within the first and second groups of resistors is a unit resistor that is substantially the same size as the other ones of the unit resistors within the first and second groups of resistors.

In certain embodiments, each of the resistors within the first and second groups of resistors spends about a same amount of time connected in parallel within the first one of the circuit branches as connected in series within the second one of the circuit branches.

In accordance with specific embodiments, at least some of the resistors spend at least some time not connected within any of the plurality of circuit branches which are collectively used to produce the bandgap voltage output (VGO), even though at other times the same resistors spend time connected within one or more of the plurality of circuit branches which are collectively used to produce the bandgap voltage output (VGO).

Embodiments of the present invention are also directed to methods for use with bandgap reference circuits that produce a bandgap voltage output (VGO), where the bandgap voltage reference circuits include a plurality of circuit branches that are collectively used to produce the bandgap voltage output (VGO). Such methods can include selectively changing over time which of a plurality of resistors are connected to be within a first one of the circuit branches, and selectively changing over time which of the resistors are connected to be within a second one of the circuit branches.

Embodiments of the present invention are also directed to voltage regulators that include a bandgap voltage reference circuit, such as the one described above, but not limited thereto. The voltage regulators can be, e.g., fixed output or adjustable output linear voltage regulators, but are not limited thereto.

This summary is not intended to summarize all of the embodiments of the present invention. Further and alternative embodiments, and the features, aspects, and advantages of the various embodiments will become more apparent from the detailed description set forth below, the drawings and the claims.

DETAILED DESCRIPTION

Embodiments of the present invention can be used to reduce long term drift in VGO that is due to drift long term drift in resistor values. Certain embodiments of the present invention, as can be appreciated from the discussion below, can also be used to compensate for imperfect resistor values.

In accordance with an embodiment of the present invention, a bandgap voltage reference circuit includes two groups of unit resistors all of substantially identical size. Referring, for example, to resistor values R1and R2inFIGS. 1A and 1B, in accordance with an embodiment, one of the groups of unit resistors is alternately connected in parallel to provide R1, then reconfigured (e.g., switched) to be connected in series to provide R2. The other group of unit resistors is similarly alternatively connected in series to provide R2, then reconfigured (e.g., switched) to be connected in parallel to provide R1. When a unit resistor is being used to provide R1, that unit resistor can be said to be in the R1position. Similarly, when a unit resistor is being used to provide R2, that unit resistor can be said to be in the R2position.

If a first group of unit resistors is used to provide R1and R2for equal amounts of time, and a second group of unit resistors is used to provide R2and R1for equal amounts of time, then excellent rejection of individual resistor error and drift over time occurs, as will be appreciated from the discussion below.

Assume that six unit resistors (i.e., two groups of unit resistors, with three unit resistors in each group) are used to provide R1and R2, and that all except one of the six unit resistors are perfect and provide a resistance exactly equal to a value R. Also assume that the resistance value for the imperfect unit resistor is R+ΔR. Under these assumptions, when the imperfect unit resistor is connected in parallel with two of the perfect unit resistors, then the resistance value for R1is as follows:

For ΔR<<R, then

When the three unit resistors (of the group that includes the imperfect unit resistor) are switched to be in series with one another in the R2position, their value is R2=3R+ΔR.

If the two group of unit resistors are each used half the time to provide R1, and are used the other half of the time to provide R2, then the time average of the imperfect group and the perfect group is as follows:

Similarly, the average value of R2is as follows:

The average value of

As can be appreciated from the above, so as long as ΔR<<R, any one unit resistor variation from the group cancels out, as long as the amount of time the first group is used to provide R1equals the amount of time the first group is used to provide R2, and the amount of time the second group is used to provide R1equals the amount of time the second group is used to provide R2. Further, it is noted that more than two groups may be employed to provide R1and R2over time. Specific embodiments that benefit from the use of more than two groups of unit resistors are discussed below.

There are numerous ways in which a group of resistor units can be configured to be selectively changed from being connected in parallel to provide R1to being connected in series to provide R2.FIG. 2Aillustrates one such way. Referring toFIG. 2A, when the switches S are in their left positions, a first group of unit resistors Ra, Rb and Rc (labeled2021) are connected in parallel and are used to provide R1; and when the switches S are in their right positions the group of unit resistors Ra, Rb and Rc are connected in series and are used to provide R2. InFIG. 2A, the second group of unit resistors Rd, Re and Rf (labeled2022) can similarly be switched from being connected in series in the R2position to being connected in parallel in the R1position.

FIG. 2Billustrates how the groups of unit resistors2021and2022ofFIG. 2Acan be used in place of the resistors R1and R2inFIGS. 1A and 1Bto provide a low-drift bandgap voltage reference circuit200, in according with an embodiment of the present invention.

InFIGS. 2A and 2B, a controller210controls with switches S to change how each group of resistors is configured and connected. For example, referring toFIGS. 2A and 2B, the controller210can control the switches such that the three unit resistors (Ra, Rb and Rc) within the group of resistors2021are connected in parallel and within the “N” branch one-half of the time, and such that the three unit resistors (Ra, Rb and Rc) within the group of resistors2021are connected in series and within the “CTAT” branch the other half of the time. Similarly, the controller210can control the switches such that the three unit resistors (Rd, Re and Rf) within the group of resistors2022are connected in series and within the “CTAT” branch one-half of the time, and such that the three unit resistors (Rd, Re and Rf) within the group of resistors2022are connected in parallel and within the “N” branch the other half of the time.

InFIG. 2Aeach switch is shown as a single-pole-double-throw switch, but embodiments of the present invention are not limited thereto. For example, in place of each single-pole-double-throw switch, two single-pole-single-throw switches can be used, but two such switches will still be referred to collectively as a switch. The switches can be implemented, e.g., using CMOS transistors, but are not limited thereto. The controller210can be implemented by a simple counter, a state machine, a micro-controller, or a processor, but is not limited thereto.

In accordance with certain embodiments, there can be more groups of resistors than branches in the bandgap reference voltage circuit. For a specific example, there can be X groups of resistors (e.g., similar to groups2021and2022), where X≧2, and each of the X groups of unit resistors spends 1/Xthof their time connected in parallel within the “N” branch, and 1/Xth of the time connected in series in the “CTAT” branch. Where X>2, at any give time at least one of the X groups of resistors may not be connected within the bandgap voltage reference circuit and not used to produce the bandgap voltage output (VGO), even though at other times the resistors in that group are connected within the bandgap voltage reference circuit and used to produce the bandgap voltage output (VGO). The resistors not used to produce VGO (i.e., the resistors temporarily switched out of the bandgap voltage reference circuit) may not be used, may be used in one or more other circuit, or may be used in some other manner.

In some embodiments, at any given time X unit resistors (which change over time) are connected in parallel within the “N” branch to provide the resistance R1, and Y unit resistors (which also change over time) are connected in series within the “CTAT” branch to provide the resistance R2, where X≠Y. In such embodiments, each unit resistor may spend more time in one of the branches than in the other branch, yet still provide for low drift.

In certain embodiments, the collection of resistors that are connected in the R1position (to provide the resistance value R1) at any given time can include some resistors connected in parallel, and other resistors connected in series. Similarly, the collection of resistors that are connected in the R2position (to provide the resistance value R2) at any given time can include some resistors connected in parallel, and other resistors connected in series. As was the case in the embodiments described above, switches that are controlled by a controller can be used to selectively change over time which of the resistors are connected to be in the R1position, and which of the resistors are connected to be within the R2position. In these embodiments, the controller can also change over time which resistors in the R1position are in parallel and which are in series, and change over time which resistors in the R2position are in parallel and which are in series. In accordance with an embodiment, a ratio of the resistance provided by the resistors in the R2position (which can be referred to as resistance R2) over the resistance provided by the resistors in the R1position (which can be referred to as resistance R1) should always be substantially constant (e.g., R2/R1=9).

Where multiple groups of resistors are used to provide the resistances R1and R2, one group of resistors may be used to provide R1at some times and R2at other times, while another group of resistors may be used to provide R2at some times and R1at other times, e.g., by changing whether resistors within the groups are connected in series or parallel, and changing which branch the group of resistors is connected into. In some such embodiments, each resistor (e.g., resistor unit) may always stay within a same group, even though how and where the resistor is connected can change. In other embodiments, a resistor can be moved (e.g., switched) into and out of different groups.

FIG. 3is a block diagram of an exemplary fixed output linear voltage regulator302that includes a bandgap voltage reference circuit300(e.g.,200inFIG. 2B, but not limited thereto) according to an embodiment of the present invention described above. The bandgap voltage reference circuit300produces a bandgap voltage output (VGO), which is provided to an input (e.g., a non-inverting input) of an operational-amplifier306, which is connected as a buffer. The other input (e.g., the inverting input) of the operation-amplifier306receives an amplifier output voltage (VOUT) as a feedback signal. The output voltage (VOUT), through use of the feedback, remains substantially fixed, +/−a tolerance (e.g., +/−1%).

FIG. 4is a block diagram of an exemplary adjustable output linear voltage regulator402that includes a bandgap voltage reference circuit300(e.g.,200inFIG. 2B, but not limited thereto) according to an embodiment of the present invention described above. As can be appreciated fromFIG. 4, VOUT≈VGO*(1+R3/R4). Thus, by selecting the appropriate values for resistors R3and R4, the desired VOUT can be selected. The resistors R3and R4can be within the regulator, or external to the regulator. One or both resistors can be programmable or otherwise adjustable.

FIG. 5is a high level flow diagram that is used to summarize a method for providing a low-drift bandgap voltage reference circuit according to an embodiment of the present invention. Such a method is for use with a bandgap voltage reference circuit that produces a bandgap voltage output (VGO), wherein the bandgap voltage reference circuit includes a plurality of circuit branches (e.g., an “N” branch, a “1” branch and a “CTAT” branch) that are collectively used to produce the bandgap voltage output (VGO). Referring toFIG. 5, as indicated at step502, there is a selective changing over time of which of the resistors are connected to be within a first one of the circuit branches (e.g., the “N” branch). Also, as indicated at step504, there is a selectively changing over time of which of the resistors are connected to be within a second one of the circuit branches (e.g., the “CTAT” branch).

In accordance with specific embodiments, steps502and504can be performed such that the resistors that are connected within the first one of the circuit branches (e.g., the “N” branch) should always collectively provide a substantially constant first resistance (R1), and the resistors that are connected within the second one of the circuit branches should always collectively provide a substantially constant second resistance (R2). This will ensure that the ratio of the second resistance over the first resistance should always be substantially constant. However, there are other ways to ensure that this ratio remains constant that are also within the scope of the present invention.

As was described above with reference toFIGS. 2A and 2B, step502can be accomplished by connecting a first group of resistors in parallel with one another within the first one of the circuit branches at some times, and connecting a second group of resistors in parallel with one another within the first one of the circuit branches at other times. Similarly, step504can be accomplished by connecting the second group of resistors in series with one another within the second one of the circuit branches at some times, and connecting the first group of resistors in series with one another within the second one of the circuit branches at other times. Additional and alternative details of methods of the present invention can be appreciated from the description set forth above.

The foregoing description is of the preferred embodiments of the present invention. These embodiments have been provided for the purposes of illustration and description, but are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to a practitioner skilled in the art. For example, embodiments of the present invention can be used with various other bandgap voltage reference circuits that include gain resistors R1and R2. Thus, embodiments of the present invention are not intended to be limited to use with only the bandgap reference circuits shown inFIGS. 1A and 1B.

While in the FIGS. the diode connected transistors are shown as being NPN transistors, they can alternatively be diode connected PNP transistors.

Further, while inFIG. 1Aeach current source is shown as being implemented using a single PMOS transistor, the current sources can alternatively be implemented using PNP transistors, or cascoded current sources including multiple PMOS or PNP transistors, as can be appreciated from the more generalFIGS. 1B and 2B. These are just a few examples, which are not meant to be limiting.

While in the FIGS. the current sources are shown as being connected to the high voltage rail, that is not necessary. For example, in alternative embodiments, the current sources can be connected between the diode connected transistors and the low voltage rail, e.g., ground, to thereby cause Iptat to equivalently flow through each branch. Such embodiments are also within the scope of the present invention. Further, even though in these alternative embodiments the current Iptat may be considered to be “sunk” instead of “sourced”, the devices used to cause the flow of Iptat will still be referred to as current sources.

Embodiments were chosen and described in order to best describe the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention. Slight modifications and variations are believed to be within the spirit and scope of the present invention. It is intended that the scope of the invention be defined by the following claims and their equivalents.