Abstract:
A sub-nW voltage reference is presented that provides inherently low process variation and enables trim-free operation for low-dropout regulators and other applications in nW microsystems. Sixty chips from three different wafers in 180 nm CMOS are measured, showing an untrimmed within-wafer σ/μ of 0.26% and wafer-to-wafer σ/μ of 1.9%. Measurement results also show a temperature coefficient of 48-124 ppm/° C. from −40° C. to 85° C. Outputting a 0.986V reference voltage, the reference operates down to 1.2V and consumes 114 pW at 25° C.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/349,731 filed on Jun. 14, 2016. The entire disclosure of the above application is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to a variation-tolerant voltage reference circuit. 
       BACKGROUND 
       [0003]    Voltage references in low-dropout regulators (LDOs), amplifiers, and analog-to-digital converters (ADCs) for nW systems, such as sensors and IoT devices, can tolerate ˜5% inaccuracy, but they require sub-nW power consumption. Conventional bandgap voltage references achieve excellent uniformity across process variation and temperature, but their complexity leads to μW range power, which is unacceptable for emerging nW microsystems. To achieve low power, one approach is to use a V th -based voltage reference with devices biased in the sub-threshold region. However, these sub-nW voltage references make use of native transistors, which are potentially at different corners than normal devices due to distinct doping processes, making them more sensitive to process variations. Also, native transistors are not provided by all fabrication technologies and the output reference voltage is too low if an NMOS diode is used. Combining the native NMOS with stacked PMOS diodes can increase the reference voltage, but this further enlarges variation across corners. 
         [0004]    For both bandgap references and the aforementioned sub-threshold references, post-fabrication trimming of each chip is required to alleviate the impact of process variations. However, this is a significant expense in cost-sensitive designs because of area overhead and testing complexity. In addition, non-volatile memory such as one-time-programmable (OTP) memory is required to store the trimming configuration information, requiring extra fabrication masks at increased cost. This paper proposes an ultra-low power PMOS-only voltage reference. By using only PMOS transistors, the reference has inherently low process variation. The untrimmed within-wafer σ/μ is 0.26%, and the untrimmed wafer-to-wafer σ/μ of 1.9%, which is sufficient for many applications in nW systems. With a 0.986V output reference voltage, the design can function down to 1.2V and consumes only 114 pW. 
         [0005]    This section provides background information related to the present disclosure which is not necessarily prior art. 
       SUMMARY 
       [0006]    This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
         [0007]    A voltage reference circuit is presented. The voltage reference circuit is comprised of a first MOSFET and a second MOSFET having same type of charge carrier as the first MOSFET. The gate terminal of the first MOSFET is coupled to the source terminal of the first MOSFET and the body terminal of the first MOSFET is biased with a voltage that is different than the voltage at the source terminal and the voltage at the drain terminal. The second MOSFET is configured in a stacked arrangement with the first MOSFET such that a reference voltage is generated at a node interconnecting the first MOSFET to the second MOSFET. Of note, the threshold voltage of the first MOSFET and threshold voltage of the second MOSFET are designed to be the same and the gate terminal of the second MOSFET is coupled to the drain terminal of the second MOSFET. 
         [0008]    In one embodiment, the first MOSFET and the second MOSFET are p-type such that the drain terminal of the first MOSFET is electrically coupled at the node to the source terminal of the second MOSFET. 
         [0009]    In another embodiment, the first MOSFET and the second MOSFET are n-type such that the source terminal of the first MOSFET is electrically coupled at the node to the drain terminal of the second MOSFET. 
         [0010]    The voltage reference circuit may include a bias circuit comprised of transistors only having same type of charge carrier as the first MOSFET and configured to output the voltage that biases the body terminal of the first MOSFET. In one embodiment, the bias circuit includes a third MOSFET in a stacked arrangement with a fourth MOSFET, such that a drain terminal of the third MOSFET is electrically coupled at an output node to a source terminal of the fourth MOSFET. 
         [0011]    In another aspect of this disclosure, the bias circuit is configured to bias the body terminal of the first MOSFET with a bias voltage that changes with temperature changes so that the reference voltage is temperature independent. 
         [0012]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0013]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0014]      FIG. 1  shows a simplified circuit of proposed voltage reference circuit; 
           [0015]      FIG. 2  is a graph showing how V body  tracks V dd  change and creates constant V BS  for the first MOSFET; 
           [0016]      FIG. 3  is a schematic of the proposed voltage reference circuit with stacked PMOS diodes; 
           [0017]      FIG. 4  is a schematic of a variant of the voltage reference circuit implemented with n-type transistors; 
           [0018]      FIG. 5  is a graph showing a comparison of V ref  simulation at all corners among the proposed design; 
           [0019]      FIG. 6  is a graph showing the measured V ref  across temperature for 3 wafers in 3 different corners; 
           [0020]      FIG. 7  is a graph showing the distribution of V ref  on three different wafers; 
           [0021]      FIG. 8  is a graph showing the measured line sensitivity; 
           [0022]      FIG. 9  is a graph showing the measured temperature coefficients at different V dd ; and 
           [0023]      FIG. 10  is a graph showing the measured PSRR; and 
           [0024]      FIGS. 11A and 11B  are graphs showing the measured power across V dd  and temperature, respectively. 
       
    
    
       [0025]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0026]    Example embodiments will now be described more fully with reference to the accompanying drawings. 
         [0027]      FIG. 1  shows a simplified structure of the proposed voltage reference circuit  10 . The proposed voltage reference circuit is comprised of two metal-oxide semiconductor field-effect transistors (MOSFETs) M 1 , M 2  in a stacked arrangement and a bias circuit  12 . In the stacked arrangement, the two MOSFETs M 1 , M 2  are configured with one of the source terminal and the drain terminal of the first MOSFET M 1  coupled to a first supply voltage VDD, and the other of the source terminal and the drain terminal of the first MOSFET is coupled to one of the source terminal and the drain terminal of the second MOSFET M 2 . The other of the source terminal and the drain terminal of the second MOSFET M 2  is coupled to a second supply voltage (e.g., ground). The first MOSFET M 1  and the second MOSFET M 2  have the same type of charge carrier and the particular terminal connections will depend upon the type of charge carrier as further described below. 
         [0028]    Additionally, the gate terminal of the first MOSFET M 1  is coupled to the source terminal of the first MOSFET M 1  and the gate terminal of the second MOSFET M 2  is coupled to the drain terminal of the second MOSFET M 2 . Of note, the body terminal of the first MOSFET M 1  is biased with a voltage that is different than the voltage at either the source terminal or the drain terminal of the first MOSFET M 1 . It is also noted that the threshold voltage V th1  of the first MOSFET M 1  and threshold voltage V th2  of the second MOSFET M 2  are designed and manufactured to be the same. 
         [0029]    In a first example embodiment, the first MOSFET M 1  and second MOSFET M 2  are p-type as seen in  FIG. 1 . In the stacked arrangement, source terminal of the first MOSFET M 1  is electrically coupled to the upper supply voltage (V dd ) while the drain terminal of the first MOSFET M 1  is electrically coupled to the source terminal of the second MOSFET M 2  and the drain terminal of the second MOSFET M 2  is electrically coupled to the lower supply voltage (e.g., ground). The reference voltage V ref  is generated at an output node interconnecting the first MOSFET M 1  to the second MOSFET M 2 . 
         [0030]    In operation, the first MOSFET M 1  is forward-biased and provides sub-threshold current flowing through the second MOSFET (i.e., bottom PMOS diode) M 2 . The second MOSFET M 2  is in an off state. The current equations of M 1  and M 2  are expressed as in equation (1). By solving equation (1), V ref  can be expressed as equation (3). As M 1  and M 2  are the same type of charge carrier (i.e, PMOS), the difference between V th1  and V th2  comes solely from the body bias effect of M 1 . Random V th  mismatch is kept negligible by upsizing (e.g., &gt;20 μm 2 ) of all 4 devices in this reference circuit. 
         [0000]    
       
         
           
             
               
                 
                   
                       
                   
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         [0031]    The bias circuit  12  is configured to output the voltage that biases the body terminal of the first MOSFET M 1 . In the example embodiment, the bias circuit  12  is comprised of transistors having the same type of charge carrier as the first MOSFET M 1  and the second MOSFET M 2 . That is, the third MOSFET M 3  and the fourth MOSFET M 4  are p-type as well. More specifically, the third MOSFET M 3  and the fourth MOSFET M 4  are in a stacked arrangement, such that the drain terminal of the third MOSFET M 3  is electrically coupled at a bias node to the source terminal of the fourth MOSFET M 4 . The bias node is also electrically coupled to the body terminal of the first MOSFET to supply the bias voltage thereto. 
         [0032]    In operation, the third MOSFET M 3  and the fourth MOSFET M 4  generate the required body bias for first MOSFET M 1 . More specifically, the fourth MOSFET M 4  is an off-state PMOS; whereas, the third MOSFET is a PMOS diode. The current equations of M 3  and M 4  are expressed above in equation (2). As the third MOSFET and the fourth MOSFET M 3  and M 4  are also the same type of PMOS, V th3  and V th4  are essentially identical. The combination of the third MOSFET M 3  and the fourth MOSFET M 4  provides a body-bias voltage V body  that tracks V dd  and creates a constant V BS  (V body −V dd ) for first MOSFET M 1  as shown in  FIG. 2 . If the current through third MOSFET M 3  (I L ) is much larger than the parasitic diode current (I dio ) from the source to the N-well of M 1 , V ref  can be expressed by equation (4). The left term of Equation (4) is complementary to temperature; whereas, the right term is proportional to temperature. With proper sizing of the four transistors, the first-order temperature dependency can be cancelled out. That is, the bias is circuit can be configured to bias the body terminal of the first MOSFET with a bias voltage that changes with temperature changes so that the reference voltage is temperature independent. Moreover, the threshold voltage V th  does not play a role in equation (4) because each pair (M 1 /M 2  and M 3 /M 4 ) uses the same type of PMOS, thus significantly reducing process variation. Since I dio  is not well modeled, I L  is designed to be three orders of magnitude larger than I dio  to minimize the effect of I dio . Proper sizing of these transistors can be determined using a global optimization tool. 
         [0033]    Variants of this proposed design are contemplated by this disclosure. Referring to  FIG. 3 , two or more stacked diodes can replace the second MOSFET M 2  and the third MOSFET M 3  to generate a higher reference voltage. Multiple voltage reference levels can be generated in this manner. In  FIG. 3 , three stages of PMOS diodes are used in the design to realize an approximately 1V output reference voltage. MIM capacitors C 0  and C 1  (both set to 1.78 pF) are used to isolate the reference voltage from high-frequency power supply noise. Except with respect to the differences discussed herein, the voltage reference circuit  30  may be substantially the same voltage reference circuit  10  described above. 
         [0034]      FIG. 4  depicts a variant of the voltage reference circuit  40  employing n-type transistors. In this variant, the drain terminal of the first MOSFET M 1  is electrically coupled to the upper supply voltage while the source terminal of the first MOSFET M 1  is electrically coupled to the drain terminal of the second MOSFET M 2 . The source terminal of the second MOSFET M 2  is electrically coupled to the lower supply voltage, where magnitude of the upper supply voltage is larger than the lower supply voltage. Except with respect to the differences discussed herein, the voltage reference circuit  40  may be substantially the same voltage reference circuit  10  described above. 
         [0035]    With continued reference to the voltage reference circuit  10  in  FIG. 1 ,  FIG. 5  compares simulated reference voltage distributions across corners for the proposed reference circuit  10  to designs presented by M. Seok et al. in JSSC, 2012 [3] and by I. Lee et al. in VLSI, 2014 [4]. The proposed design achieves &lt;4% inaccuracy across all corners; whereas, the other designs vary up to 10% and 19%, respectively. 
         [0036]    For verification, sixty chips from 3 different wafers in 180 nm CMOS were tested. One wafer was in a typical corner with thin top-metal, another was found to be at a slow corner with ultra-thick top-metal, and the third was at a fast corner with ultra-thick top-metal. All measurements are reported without trimming. 
         [0037]      FIG. 6  shows the measured reference voltage across temperature for all 60 chips. From −40° C. to 85° C., the temperature coefficient of the typical wafer ranges from 48 ppm/° C. to 104 ppm/° C., and those of the fast and slow wafers are 55.2-124 ppm/° C. and 56.1-117 ppm/° C., respectively. The reference voltage distributions at 25° C. of the 3 different wafers are shown in  FIG. 7 . Without trimming, the typical wafer shows a mean value of 986.2 mV and standard deviation of 2.6 mV. The average voltage difference between the fast and slow wafers is 3.6% (1.9% σ/μ), matching simulation and providing sufficient accuracy for many key circuit applications within nW systems. 
         [0038]      FIG. 8  shows the measured sensitivity of reference voltage to power supply voltage. Line sensitivity is 0.38% N from 1.2V to 2.2V.  FIG. 9  shows the measured temperature coefficients at different supply voltages.  FIG. 10  shows the measured power supply rejection ratio (PSRR) from 10 Hz to 10 MHz. High-frequency PSRR is −56 dB, which can be further improved with larger loading caps C 0  and C 1 . 
         [0039]      FIGS. 11A and 11B  show the measured power consumption across supply voltage and temperature, respectively. The output reference voltage is approximately 1V with 3 stages of stacked PMOS diodes. The power supply can be reduced to 1.2V while maintaining this approximately 1V reference voltage. To lower the minimum power supply, fewer stages of PMOS diodes can be used, but the output reference voltage will be lowered as well. At 25° C. and 1.2V, the power consumption is 114 pW, which is suitable for low-power sensor and IoT applications. 
         [0040]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.