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VOLTAGE REFERENCES AND DESIGN THEREOF

20200310482 ยท 2020-10-01

    Inventors

    Cpc classification

    International classification

    Abstract

    Embodiments of the disclosure are drawn to voltage reference circuits and methods of designing same. The voltage reference circuit may include a main stage and one or more auxiliary stages. The output of the main stage may be a reference voltage. The auxiliary stages may provide a feedback voltage that reduces a temperature dependence of the reference voltage. Each stage may include two or more transistors. The transistors may operate in a sub-threshold mode to provide the reference voltage.

    Claims

    1. An apparatus comprising: a main stage comprising a first transistor and a second transistor coupled in series between a voltage source and a common voltage, wherein a reference voltage is provided at a node between a source of the first transistor and a drain of the second transistor; and a bulk feedback network coupled to a substrate of the second transistor.

    2. The apparatus of claim 1, wherein the main stage and the bulk feedback network include complementary-to-absolute-temperature circuits.

    3. The apparatus of claim 1, wherein the main stage and the bulk feedback network include proportional-to-absolute temperature circuits.

    4. The apparatus of claim 1, wherein the bulk feedback network includes a first stage comprising a third transistor and a fourth transistor coupled in series between the voltage source and the common voltage, wherein a gate of the third transistor is coupled to the common voltage and a gate of the fourth transistor is coupled to the substrate of the second transistor.

    5. The apparatus of claim 4, wherein the bulk feedback network further includes a second stage comprising a fifth transistor and a sixth transistor coupled in series between the voltage source and the common voltage, wherein a gate of the fifth transistor and a gate of the sixth transistor are coupled to a substrate of the fourth transistor.

    6. The apparatus of claim 5, wherein a source of the fifth transistor is coupled to the substrate of the fourth transistor and a substrate of the sixth transistor is coupled to the common voltage.

    7. The apparatus of claim 5, wherein the fifth transistor and the sixth transistor include I/O devices.

    8. The apparatus of claim 4, wherein the second transistor and the fourth transistor include deep N-well devices.

    9. The apparatus of claim 4, wherein the first transistor and the third transistor include native devices.

    10. The apparatus of claim 4, wherein a source of the third transistor is coupled to the substrate of the second transistor.

    11. The apparatus of claim 1, wherein a width of the first transistor and a width of the second transistor have a same value.

    12. The apparatus of claim 1, wherein a threshold voltage of the first transistor has a value less than a value of a threshold voltage of the second transistor.

    13. The apparatus of claim 1, wherein the source of the first transistor is further coupled to a gate of the second transistor.

    14. A method, comprising: plotting a temperature coefficient as a function of width of a first transistor; plotting the temperature coefficient as a function of width of a second transistor; and based on the plotting, selecting a first width of the first transistor and a second width of the second transistor associated with a lowest value of the temperature coefficient where the first width and the second width have equal values.

    15. The method of claim 14, further comprising: sweeping a bulk voltage of the second transistor across a range of voltages; calculating a desired temperature coefficient based, at least in part, on the bulk voltage of the second transistor across the range of voltages; and selecting a third width of a third transistor and a fourth width of a fourth transistor associated with the desired temperature coefficient.

    16. The method of claim 15, further comprising: sweeping a bulk voltage of the fourth transistor across the range of voltages; calculating a second desired temperature coefficient based, at least in part, on the bulk voltage of the fourth transistor across the range of voltages; and selecting a fifth width of a fifth transistor and a sixth width of a sixth transistor associated with the second desired temperature coefficient.

    17. The method of claim 15, further comprising providing a reference voltage from a node between a source of the first transistor and a drain of the second transistor, wherein the first transistor and the second transistor are coupled in series, the third and the fourth transistor are coupled in series, and wherein a gate of the fourth transistor is coupled to a substrate of the second transistor.

    18. The method of claim 15, wherein the third width is different than the fourth width.

    19. The method of claim 15, wherein lengths of the first transistor, the second transistor, the third transistor, and the fourth transistor have equal values.

    20. The method of claim 15, further comprising scaling the third width and the fourth width by a same factor.

    Description

    BRIEF DESCRIPTION OF THE DRAWINGS

    [0014] FIG. 1 is a circuit diagram of a voltage reference circuit in accordance with examples described herein.

    [0015] FIG. 2 is a plot of width of devices versus temperature coefficients in accordance with examples described herein.

    [0016] FIG. 3 is a circuit diagram illustrating sweeping of bulk voltage in accordance with examples described herein.

    [0017] FIG. 4 shows plots of a reference voltage versus temperature and feedback voltage versus temperature in accordance with examples described herein.

    [0018] FIG. 5 shows a chip layout of a voltage reference circuit in accordance with examples described herein.

    [0019] FIG. 6 is a plot of an output reference voltage as a function of supply and temperature in accordance with examples described herein.

    [0020] FIG. 7 is a plot of the reference voltage of an example reference voltage circuit versus the supply voltage in accordance with examples described herein.

    [0021] FIG. 8 is a plot of current versus temperature in accordance with examples described herein.

    [0022] FIG. 9 is a plot of power supply rejection ratio versus frequency in accordance with examples described herein.

    DETAILED DESCRIPTION

    [0023] The following description of certain embodiments is merely exemplary in nature and is in no way intended to limit the scope of the disclosure or its applications or uses. In the following detailed description of embodiments of the present apparatuses, systems and methods, reference is made to the accompanying drawings which form a part hereof, and which are shown by way of illustration specific embodiments in which the described apparatuses, systems and methods may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice presently disclosed systems and methods, and it is to be understood that other embodiments may be utilized and that structural and logical changes may be made without departing from the spirit and scope of the disclosure. Moreover, for the purpose of clarity, detailed descriptions of certain features will not be discussed when they would be apparent to those with skill in the art so as not to obscure the description of embodiments of the disclosure. The following detailed description is therefore not to be taken in a limiting sense, and the scope of the disclosure is defined only by the appended claims.

    [0024] According to examples of the present disclosure, an ultra-low-power voltage reference which works on the principle of cancellation of temperature dependency on sub-threshold current is disclosed. The feedback control which is based on bulk-voltage compensation, may not only suppresses the second order temperature dependency but may also enable it to achieve a much higher output voltage using supply voltages as low as 0.35 V. The closed-loop control may provide reliable performance over all process corners in sub-threshold operation. A disclosed example design achieves a line sensitivity of 0.004%/V at 200 C and power supply rejection ratio (PSRR) of 45 dB with total power consumption of 12.9 pW. The temperature coefficient is 8.3 ppm/0 C over a temperature range of 10-1100 C and a supply range of 0.35-2.5V. Disclosed herein is a complete feedback-controlled pico-watt voltage reference which may have a stable temperature coefficient and line sensitivity while having an output voltage (293.5 mV) comparable to the supply voltage even if it is operated with a supply voltage as low as 350 mV.

    [0025] FIG. 1 is a circuit diagram of a voltage reference circuit 100 in accordance with examples described herein. The voltage reference circuit 100 is based on sub-threshold operation, but instead of using an open loop configuration or a feedback amplifier, the voltage reference circuit 100 uses a complementary-to-absolute-temperature (CTAT) voltage source to control the bulk voltage of the reference voltage generator. The voltage reference circuit 100 may not only boost the output voltage without using a cascode topology, but the bulk feedback may also reduce the temperature coefficient (TC). The voltage reference circuit 100 may include a main stage 102 for generating a reference voltage and a bulk feedback network (e.g., feedback network) 108, which may reduce temperature variations in the reference voltage.

    [0026] The main (e.g., first) stage 102 may include a first MOSFET device M.sub.1 and a second MOSFET device M.sub.2 coupled in series. The gate of M.sub.1 may be coupled to a common voltage (e.g., ground). The drain of M.sub.1 may be coupled to a voltage source V.sub.DD (e.g., supply voltage). The source of M.sub.1 may be coupled to the drain and the gate of M.sub.2. The source of M.sub.2 may be coupled to the common voltage. A reference voltage V.sub.REF may be provided at a node between the source of M.sub.1 and the gate and drain of M.sub.2. In some examples, M.sub.1 may be a native 2.5V device with a low threshold voltage (V.sub.th). In some examples, M.sub.2 may be a 2.5V I/O device with a higher V.sub.th than M.sub.1. In some examples, the devices M.sub.1 and M.sub.2 may be N-channel MOSFETS.

    [0027] The current through each device, I.sub.D, may be dictated by the sub-threshold current equation of MOSFET devices as shown below

    [00001] I D = k ( f - 1 ) .Math. V T 2 .Math. e ( V G .Math. S - V t .Math. h ( V B .Math. S ) ) f .Math. V T ( e - V S .Math. B VT - e - V D .Math. B V .Math. T + V D .Math. S V A ) . ( 1 )

    [0028] Where k is the mobility of the device, f is the work function, V.sub.T is the thermal voltage, V.sub.GS is the gate-to-source voltage, V.sub.th is the threshold voltage, V.sub.BS is the substrate (e.g., bulk)-to-source voltage, V.sub.SB is the source-to-substrate voltage, V.sub.DB is the drain-to-substrate voltage, V.sub.DS is the drain-to-source voltage, and VA is the early voltage. Equation (1) can be simplified in some examples by assuming that the bulk and source are at the same potential:

    [00002] I D = k ( f - 1 ) .Math. V T 2 .Math. e ( V G .Math. S - V t .Math. h ) fV T ( 1 - e .Math. - V D .Math. S V T + V D .Math. S V A ) . ( 2 )

    [0029] In some examples, it can be assumed that V.sub.DS is large enough so that

    [00003] e - V D .Math. S V T

    can be neglected. In some examples, both devices (M.sub.1 and M.sub.2) may carry equal current, in these examples, the reference voltage V.sub.REF can be simplified by equating both current equations as follows:

    [00004] V REF = ( f 1 .Math. .Math. .Math. .Math. f 2 ) .Math. .Math. V th + ( f 1 .Math. .Math. .Math. .Math. f 2 ) .Math. V T .Math. ln ( k 1 k 2 ) . ( 3 )

    [0030] Where f.sub.1 is the work function for M.sub.1, f.sub.2 is the work function for M.sub.2, k.sub.1 is the mobility of M.sub.1, k.sub.2 is the mobility of M.sub.2, and V.sub.th is the threshold voltage difference between M.sub.1 and M.sub.2. Equation (2) shows that properly sized devices having k.sub.1=k.sub.2 can lead to a fixed reference voltage:


    V.sub.REF(V.sub.th1V.sub.th2)C.sub.01+C.sub.1TC.sub.02+C.sub.2T=C.sub.0+TC.(4)

    [0031] Where V.sub.th1 is the threshold voltage of M.sub.1, V.sub.th2 is the threshold voltage of M.sub.2, T is temperature, C.sub.01 and C.sub.02 are the combined temperature independent factors whereas C.sub.1 and C.sub.2 are the summation of all temperature dependent factors up to the first order in the threshold voltage. Both devices may have identical TCs (C.sub.1 and C.sub.2), which may provide a constant reference voltage output.

    [0032] Equation (3) describes the performance of voltage reference if the desired performance is not too stringent (e.g., TC<20 ppm/ C.). However, some approximations and simplifications may cause deviation from the theoretical calculations in some applications.

    [0033] In some examples, it may be assumed that V.sub.DS>>V.sub.T. Hence, the exponential term within the parenthesis in Equation (2) is reduced, but the linear term may cause a deviation. Moreover, increasing V.sub.DS may mean increasing the minimum supply voltage of the circuit in some applications. In some examples, the device M.sub.1 may suffer from the body effect while the device M.sub.2 may not if both the devices have their bulk (e.g., substrate) connected to ground. In some examples, it may be assumed that both the devices M.sub.1 and M.sub.2 may have significant V.sub.th difference, but may have equal temperature coefficients. All these factors may contribute to either CTAT or PTAT like nature with small non-linear temperature dependency especially if the desired reference voltage is comparable to the supply voltage.

    [0034] Feedback circuits using amplifiers are power hungry and require large voltage headroom to retain the devices in the saturation region. Moreover, they require another stable reference voltage.

    [0035] An energy efficient feedback mechanism is to control the body voltage of the main stage 102 with one or more auxiliary PTAT or CTAT stages in the bulk feedback network 108. Returning to FIG. 1, a first auxiliary stage 104, which may include MOSFET devices M.sub.3 and M.sub.4 coupled in series. The drain of M.sub.3 may be coupled to the voltage source V.sub.DD and the gate of M.sub.3 may be coupled to the substrate of M.sub.1 and the common voltage. The substrate of M.sub.3 may be coupled to the common voltage. The source of M.sub.3 may be coupled to the drain and gate of M.sub.4 as well as the substrate of M.sub.2. The source of M.sub.4 may be coupled to the common voltage. In some examples, M.sub.3 and M.sub.4 may be N-channel devices. In some examples, M.sub.3 may be a native device and M.sub.4 may be a deep N-well device.

    [0036] Optionally, a second auxiliary stage 106 including devices M.sub.5 and M.sub.6 coupled in series may be included to compensate for the temperature dependency of the main stage (M.sub.1-M.sub.2). In some applications, adding a second auxiliary stage 106 may further reduce temperature sensitive of the reference voltage. However, in some examples, the second auxiliary stage 106 may be omitted, which may save power and/or die area. Second auxiliary stage 106 may include MOSFET devices M.sub.5 and M.sub.6. The gates of M.sub.5 and M.sub.6 may be coupled to the substrate of M.sub.4. The source of M.sub.5 and the drain of M.sub.6 may also be coupled to the substrate of M.sub.4. The drain of M.sub.5 may be coupled to the voltage source V.sub.DD and the substrate of M.sub.5 may be coupled to the common voltage. The substrate and source of M.sub.6 may be coupled to the common voltage. In some examples, M.sub.5 and M.sub.6 may be N-channel devices. In some examples, M.sub.5 and M.sub.6 may be standard I/O devices.

    [0037] In some examples, deep N-well devices may be used for M.sub.2 and M.sub.4 which are available in most modern CMOS processes. M.sub.3 and M.sub.4 the second stage 104 (e.g., first auxiliary stage) may create a negative feedback for the main stage 102. M.sub.5 and M.sub.6 of the third stage 106 (e.g., second auxiliary stage) may be included to reduce temperature sensitivity of the first auxiliary stage 104. As a result, a TC as low as 8.3 ppm/ C. can be achieved for an output voltage of 293.5 mV. In some examples, the maximum supply may be determined by the device break-down rating which may be 2.5V for the I/O devices in the used process.

    [0038] To design a voltage reference circuit according to the present disclosure, such as voltage reference circuit 100, the main stage 102 (M.sub.1-M.sub.2) may be sized with low V.sub.DS so that it can be operated with very low V.sub.DD. Designing the main stage 102 with low V.sub.DS may increase its temperature dependence. This may be compensated for by the bulk feedback network (e.g., the two auxiliary stages 104, 106).

    [0039] In some examples, M.sub.1 and M.sub.2 may have the same W/L ratio which may achieve low temperature dependency. In an example implementation, the length of both the devices were chosen as 80 m which may reduce power consumption. In other examples, shorter lengths may be chosen to reduce size, but may come at the expense of increased power consumption. The widths of the devices M.sub.1 and M.sub.2 may be optimized using a plot of TV versus widths of the devices M.sub.1 and M.sub.2. In the example plot 200 shown in FIG. 2, TCs over a temperature range of 20 to 130 C. are plotted for different widths of M.sub.1 (W.sub.Top) and M.sub.2 (W.sub.Bottom). In some embodiments, equal widths W are chosen for M.sub.1 and M.sub.2. For example, equal widths where the TC is the lowest may be selected. Though any equal width can be chosen for both devices M.sub.1 and M.sub.2, smaller widths may cause larger mismatches between the devices in some applications.

    [0040] Once the main stage 102 is designed, the feedback network 108 including auxiliary stages 104 and 106 may be optimized to reduce the TC of the main stage 102 by controlling the bulk proportional to the output voltage. The main stage 102 in the example implementation has a slightly negative TC (20 ppm/ C.); hence the feedback network may be designed so that it has an overall negative TC.

    [0041] As illustrated in FIG. 3, in some examples, the desired characteristic equation of the feedback network 108 (e.g., auxiliary stages 104, 106) may be found by sweeping the bulk voltage (e.g., substrate voltage) of device M.sub.2. The bulk voltage (V.sub.BULK) is equal to V.sub.oscT where V.sub.os is the offset voltage, T is temperature, and c is a constant, which corresponds to the desired temperature coefficient (TC) of the feedback network 108.

    [0042] FIG. 4 shows reference voltage versus temperature plot 402 and feedback voltage versus temperature plot 404 in accordance with examples described herein. Plot 402 is a plot of the reference voltage provided by the main stage 102 with and without feedback (FB) (e.g., from feedback network 108) versus temperature. Plot 404 is a plot of the feedback voltage (e.g., the voltage provided by auxiliary stages 104, 106) versus temperature. The desired TC of the feedback network 108 may be determined from plot 404 of FIG. 4 by finding the slope of the voltage versus temperature line. The widths of the devices M.sub.3 and M.sub.4 can then be selected in a similar procedure to M.sub.1 and M.sub.2 of the main stage 102 using a plot similar to the plot shown in FIG. 2, where TC corresponds to the value of the constant c as shown in FIG. 4. Thus widths of M3 and M4 may be selected to provide the desired TC of the first auxiliary stage 104. In some examples, the value of the constant c may determine the performance of the reference voltage over temperature while the value of the offset voltage V.sub.os may determine the output voltage level. Once the length and width of the devices are determined, the value of V.sub.os may be optimized by scaling both the devices M.sub.3 and M.sub.4 by the same factor. A scaling factor of two is used in the example implementation to set the reference voltage close to V.sub.DD. The second auxiliary stage 106 (M.sub.5-M.sub.6) may be included to improve the performance over temperature. The second auxiliary stage 106 may work as local feedback to the first auxiliary stage 104. The devices M.sub.5 and M.sub.6 of the second auxiliary stage 106 may be designed using the same procedure as was used for the first auxiliary stage 104. In the example implementation, the geometry for each device is: M.sub.1: 3.6u/80u, M.sub.2: 3.65u/80u, M.sub.3: 500n/80u, M.sub.4: 100u/80u, M.sub.5: 400n/40u and M.sub.6: 400n/40u.

    [0043] The voltage reference circuit 100 may be implemented in a 65 nm CMOS process. The gate lengths of the devices may be chosen to be large to reduce power consumption. However, the same output voltage performance can be achieved with smaller length devices at the expense of higher power consumption. Hence, there may be a trade-off between area and power consumption. FIG. 5 shows a chip layout 500 of a voltage reference circuit. In some examples, the chip layout 500 may be used to implement the voltage reference circuit 100 shown in FIG. 1.

    [0044] The example implementation of voltage reference circuit 100 having chip layout 500 was simulated over supply voltages from 0.35-2.5V and over temperature from 20 C.-130 C. FIG. 6 shows a plot 600 the output reference voltage as a function of supply and temperature of the simulated voltage reference circuit. The voltage reference circuit described herein varies by approximately only 1 mV over a temperature range of 10 to 110 degrees Celsius. The voltage reference circuit described herein may to reduce the overall TC. FIG. 7 shows a plot 700 of the reference voltage of the example reference voltage circuit versus the supply voltage, which is a measure of the line sensitivity of the reference voltage circuit. As seen in plot 700, the worst line sensitivity (0.012%/V) occurs at 0 C. while the best line sensitivity (0.004%/V) occurs at 60 C.

    [0045] The example implementation of the voltage reference circuit was designed in 65 nm CMOS; hence it may have a much higher leakage current compared to wider line CMOS devices or more advanced technology nodes using high-K gate dielectrics or FinFETs. Thick gate-oxide, high-voltage (2.5 V) devices may be used to increase the supply voltage range. FIG. 8 is a plot 800 of current versus temperature in accordance with examples described herein. Plot 800 shows the total system current including I/O pads. The current increases exponentially with temperature. The minimum power consumption is approximately 4.8 pW at 10 C.

    [0046] FIG. 9 is a plot 900 of power supply rejection ratio versus frequency in accordance with examples described herein. As shown in plot 900, the example implementation of the voltage reference circuit has <43 dB power supply rejection ratio even without external output capacitors up to 10 MHz, with a resonance around 1 MHz due to modelled bondwire inductance.

    [0047] An ultra-low power voltage reference circuit has been disclosed herein which in some examples may exhibit an output voltage of 293 mV even after being supplied by voltage as low as 350 mV. Instead of using conventional amplifiers, in some examples, properly sized CTAT or PTAT stage may be used as feedback to the main stage. This may facilitates operations at lower V.sub.DD with ultra-low power consumption. The voltage reference circuit described herein may achieve a temperature coefficient of 7.3 ppm/ C. with line sensitivity of 0.004%/V with a total power consumption of 13.6 pW without any trimming in some examples.

    [0048] Of course, it is to be appreciated that any one of the examples, embodiments or processes described herein may be combined with one or more other examples, embodiments and/or processes or be separated and/or performed amongst separate devices or device portions in accordance with the present systems, devices and methods.

    [0049] Finally, the above-discussion is intended to be merely illustrative of the present system and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Thus, while the present system has been described in particular detail with reference to exemplary embodiments, it should also be appreciated that numerous modifications and alternative embodiments may be devised by those having ordinary skill in the art without departing from the broader and intended spirit and scope of the present system as set forth in the claims that follow. Accordingly, the specification and drawings are to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims.