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A Nanoscale LowPower Resistorless Voltage Reference with High PSRR
Nanoscale Research Lettersvolume 14, Article number: 33 (2019)
Abstract
In this paper, a nanowatt resistorless subthreshold voltage reference with highpower supply rejection ratio (PSRR) is presented. A selfbiased MOS voltage divider is proposed to provide bias current for whole voltage reference, which is a positive temperature coefficient (TC) current containing threshold voltage characteristics. By injecting the generated current into a transistor with a different threshold voltage, a delta threshold voltage with a greatly reduced negative TC is realized and temperaturecompensated by a generated positive TC item at the same time. Therefore, a temperaturestable voltage reference is achieved in the proposed compacted method with low power consumption and high PSRR. Verification results with 65nm CMOS technology demonstrate that the minimum supply voltage can be as low as 0.35 V with a 0.00182mm^{2} active area. The generated reference voltage is 148 mV, with a TC of 28 ppm/°C for the − 30 to 80 °C temperature range. The line sensitivity is 1.8 mV/V, and the PSRR without any filtering capacitor at 100 Hz is 53 dB with a 2.28nW power consumption.
Introduction
Voltage reference is one of the core modules in electronic systems, which is widely used in medical electronics, power managements, wireless environmental sensors, and communication circuits. As the supply voltage of electronic systems continues to decrease with technology improvement, the requirements for a lowpower voltage reference with nanoscale technology are critically increasing [1, 2].
Conventional voltage references are based on a bandgap reference (BGR) circuit, which is a weighted sum of V_{BE} and thermal voltage [3, 4]. However, due to the nonlinear temperature behavior of V_{BE}, it is essential to use curvature compensation approaches to improve the precision of BGR [5, 6]. Another disadvantage of BGR is the power consumption. The V_{BE} is around 0.7 V without shrinking down with process improvement, which absolutely restricts the supply voltage. These make BGRs unsuitable for lowvoltage and nanoscale applications.
In order to achieve lowpower operation, MOSonly subthreshold voltage references are gradually adopted [7,8,9,10]. As transistors in a weak inversion region have inherent advantages in lowpower applications with quite small current, the power consumption of relative voltage references can be effectively reduced. Besides, since the characteristics of metaloxidesemiconductor fieldeffect transistor (MOSFET) are consistent with process improvement, voltage reference based on MOSFET is more adaptable to advanced technologies. In addition, the usage of resistors should also be avoided in lowpower applications. Since the current in the voltage reference is usually inversely proportional to resistance value, lowpower dissipation means highohmic resistors [10], which can induce large noise occupying a large chip area.
Power supply rejection ratio (PSRR) is another important parameter of voltage reference. Conventional solutions to improve PSRR are at the cost of chip area and power consumption, such as additional amplifiers [11], long channel transistors [12], cascode structures, and additional gain stage [13].
In order to overcome the mentioned issues above, a nanowatt MOSFETbased resistorless subthreshold voltage reference with high PSRR is proposed in this brief, which is suitable for advanced technology, such as nanoscale process. A selfbiased MOSFET voltage divider for PSRR enhancement is adopted in the proposed voltage reference, which can generate a positive temperature coefficient (TC) current containing threshold voltage characteristics. The current serves as bias currents for the whole voltage reference. Besides, the threshold voltage embedded in the bias current is reproduced by injecting bias current into MOSFET with different threshold voltages in the paper. With the proposed method, a delta threshold voltage (∆V_{TH}) with greatly reduced negative TC is obtained. Besides, a weighted proportional to absolute temperature (PTAT) item is also obtained, while a weighted sum of ∆V_{TH} and PTAT voltage is realized at the same time. Due to the mutual TC cancelation of two different threshold voltages, the required PTAT voltage can be greatly reduced for temperature compensation. By this method, a MOSFETonly resistorless voltage reference is achieved by a compacted structure with low power consumption.
Method
As shown in Fig. 1, the proposed voltage reference is composed of a startup circuit, a selfbiased current generator, and a V_{REF} generating circuit. All the nchannel MOSFETs are a medium threshold voltage Ntype metaloxidesemiconductor (mvt NMOS). MP4 is a high threshold voltage transistor Ptype metaloxidesemiconductor (hvt PMOS), and the other pchannel MOSFETs are a medium threshold voltage PMOS (mvt PMOS). All the transistors shown in Fig. 1 operate in the subthreshold region, except those in the startup circuit.
StartUp Circuit
The startup circuit consists of MP5, MP6, and MN4. At the beginning of a poweron stage, the gate potential of MP6 is low and MP6 is turned on. The current generated by MP6 makes the gate potential of MN1 and MN2 rise, and the whole circuit starts to work. At the same time, MP5 charges the startup capacitor, MN4. With the charging procedure of MN4, transistor MP6 is gradually turned off, which makes the startup circuit to be broken away from the core of the proposed voltage reference without additional power dissipation. By this method, the proposed voltage reference can work in a desired operating point while avoiding a degeneration point.
SelfBiased Current Generator
The middle part in Fig. 1 is a selfbiased current generator, which is based on a MOSFETonly voltage divider. The bias current with positive TC for the whole voltage reference is generated in this part, which is relevant to the medium threshold voltage of NMOS. The unique characteristic of the presented bias current is adopted to realize the proposed voltage reference in a convenient way, which will be analyzed in the “Method” section.
With regard to voltage current characteristic of a transistor in the subthreshold region, the drain current of the transistor in the subthreshold becomes almost independent of V_{DS} with V_{DS}> 4V_{T}, where V_{T} = kT/q is the thermal voltage, k is the Boltzmann constant, q is the elementary charge, and T is the absolute temperature. Hence, the current can be expressed as:
where S = W/L is the aspect ratio, m is the subthreshold slope factor, V_{TH} is the threshold voltage, and I_{SQ} represents the specific current and is presented by:
where μ is the carrier mobility and C_{OX} is the oxide capacitance per unit area.
Therefore, the currents through MOSFETonly voltage divider, formed by MN1, MN2, and MN3, can be expressed as follows:
where I_{SQN} is the specific current of NMOS and V_{THN} is the threshold voltage of NMOS.
Since the aspect ratios of MN2 and MN3 are the same and I_{D_MN2} = I_{D_MN3}, V_{GS_MN2} = V_{GS_MN3} is guaranteed. This makes V_{GS_MN1} = 2V_{GS_MN2}. Besides, the PMOS transistors form the current mirrors and define the current ratios K_{1} = S_{MP1}/S_{MP2} and K_{2} = S_{MP3}/S_{MP2}. The relationship of drain currents between MN1 and MN2 can be expressed as:
Combined with Eqs. (3)–(6), the V_{GS_MN2} and I_{D_MN2} can be given by:
For the convenience of analysis, Eq. (8) can be abbreviated as:
where a = S_{MN2}μ_{n0}C_{OX}(m − 1)(k/q)^{2} and b = ln(K_{1}S_{MN2}/S_{MN1}) are independent of temperature, μ_{n0} is a temperatureindependent factor of carrier mobility, and n_{1} is the absolute temperature exponent term of carrier mobility, which is usually around 1.5.
As shown in Eq. (9), threshold voltage V_{THN} is complementary to absolute temperature (CTAT), while thermal voltage V_{T} is proportional to absolute temperature (PTAT). As the temperature increases, V_{THN}/(mV_{T}) will reduce, so that the positive current characteristics of the bias current will be enhanced.
By this method, a positive TC bias current is achieved by MOSFETonly structure, which carries the characteristics of NMOS threshold voltage.
V _{REF} Generating Circuit
The V_{REF} generating circuit is shown in the right part of Fig. 1, which is only formed by two transistors, MP3 and MP4. Due to the subthreshold region operation, I_{D_MP4} can be written as:
where I_{SQP} is the specific current of PMOS and V_{THP} is the V_{TH} of PMOS.
Since I_{D_MP4} = K_{2}I_{D_MN2}, the characteristics of NMOS threshold voltage, V_{THN}, can be transferred to the output node and be superposed with the characteristics of PMOS threshold voltage, V_{THP}. From Eqs. (8) and (10), V_{REF} can be written as:
As shown in the first two items of Eq. (11), a delta threshold voltage is realized. Since V_{TH} = V_{TH0} − βT, where V_{TH0} is the threshold voltage at 0 K and β is the TC of the threshold voltage, the generated delta threshold voltage is a complementary to the absolute temperature (CTAT) voltage with greatly shrunken TC with βV_{THP} > βV_{THN}. Besides, two additional PTAT voltages are simultaneously realized and shown in the last two items of Eq. (11), which are adopted to cancel the reduced TC of delta threshold voltage. Therefore, a compacted temperaturestable reference voltage is achieved without a complicated structure, which is stable at V_{THP0} − V_{THN0}.
Based on the previous analysis, a lowpower MOSFETonly voltage reference is realized in this paper which only requires three branches in the core. With the unique characteristics of a selfbiased current source, one diodeconnected PMOS is adopted to achieve a CTAT voltage with shrunken TC, PTAT voltage generator, and weighted summation at the same time. What is more, the proposed structure is only constructed by MOSFETs, and the generated reference voltage is proportional to the delta threshold voltage. Therefore, the proposed voltage reference is more suitable for low power consumption applications with nanoscale technology, which can be further extended to more advanced technologies.
PSRR of Proposed Voltage Reference
In order to illustrate the PSRR performance, the paths from supply voltage noise to V_{REF} and corresponding equivalent function diagrams are shown in Fig. 2.
Based on Fig. 2, the smallsignal model of path 3 is shown in Fig. 3, and the following equation can be obtained:
From Eq. (12), the expression of the supply noise through path 3 to node A can be given by:
The transconductance of the transistor operating in the subthreshold region is g_{m} = I_{D}/mV_{T}. Therefore, the relationship between g_{m,MP1} and g_{m,MN1} with the same current can be given as g_{m,MP1} = g_{m,MN1}. Then, Eq. (13) can be simplified as:
Node B also has an effect on node A through path 1, but the effect is opposite to path 3, which can be expressed as:
For V_{A} = 2V_{GS,MN2}, the gain of path 2 is given as:
The effect of v_{dd} on node B through path 4 can be written as:
From node A to node B in Fig. 2, two additional equations can be gotten, which are:
According to Eqs.(18) and (19), the noise at V_{B} can be given by:
With the help of the proposed selfbiased current source, the output node of the current generator part, B, can track the smallsignal variation of the supply voltage, which is beneficial for the PSRR improvement of the whole voltage reference.
With a similar method, the supply noise gains of path 5 and path 6 can presented by Eqs. (21) and (22), respectively:
Taken into consideration the noise path connection relationship of the reference generator shown in Fig. 2, the effect of the supply noise at the reference voltage, V_{REF}, can be determined by path 5 and path 6:
For V_{DS} > 4V_{T}, the exponential term in Eq. (23) is very large. This makes the PSRR performance to be greatly enhanced with V_{DS,MP3} increasing. In the proposed design, the minimum V_{DS,MP3} is over 200 mV, which means the change in the supply voltage has little effect on the V_{REF}. Thus, the proposed structure has a good PSRR performance.
Results and Discussion
The voltage reference is implemented in a 65nm CMOS process, whose layout is shown in Fig. 4 occupying a 0.00182mm^{2} active area.
Figure 5 shows the line regulation of the proposed voltage reference at 27 °C. As shown in Fig. 5, the minimum supply voltage can be as low as 350 mV, and the generated reference voltage, V_{REF}, is around 148 mV. The line sensitivity (LS) is 1.8 mV/V.
The temperature performance of V_{REF} with 350 mV supply voltage is shown in Fig. 6. The TC of V_{REF} is 28 ppm/°C from − 30 to 80 °C. V_{REF} shows positive temperature characteristics below − 15 °C and above 25 °C, while negative temperature characteristics at medium temperature region.
Figure 7 shows the current consumption versus temperature with 350 mV supply voltage. The current shows a positive TC. The power consumption at room temperature is around 2.28 nW.
Figure 8 shows the result of PSRR at 27 °C with 350 mV supply voltage, where the PSRR without any output filter capacitor is over 53 dB up to 100 Hz. As mentioned above, the PSRR performance can be further improved with a supply voltage increase, which means the PSRR shown in Fig. 8 is the worst case of the proposed voltage reference.
The distributions of untrimmed V_{REF} at 27 °C with 100 samples is shown in Fig. 9. The mean value and standard deviation of the V_{REF} is 147 mV and 3.97 mV, respectively, which results in a spread (σ/μ) of 2.7%.
Table 1 summarizes the characteristics of the proposed voltage reference and compares it with some previously reported voltage references.
Conclusion
A resistorless lowpower voltage reference with high PSRR is presented in this paper, which is suitable for nanoscale applications and can be extended to more advanced process. With the help of selfbiased current source based on MOSFET voltage divider, the required CTAT voltage, PTAT voltage, and weighted summation can be simultaneously realized in a compacted structure. What is more, a delta threshold voltage is chosen as the CTAT voltage, which has a greatly reduced negative TC. This also makes the required value of PTAT voltage to be shrunken. Therefore, the supply voltage and current consumption can be brought down. All the parts are only constructed by MOSFETs, which has priority in powersensitive highly integrated applications, such as SOC.
Abbreviations
 BGR:

Bandgap reference
 CTAT:

Complementary to absolute temperature
 hvt:

High threshold voltage
 LS:

Line sensitivity
 mvt:

Medium threshold voltage
 PSRR:

Power supply rejection ratio
 PTAT:

Proportional to absolute temperature
 TC:

Temperature coefficient
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Acknowledgements
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Funding
This work was supported by the National Science Foundation of China under Grant No. 61674025 and No. 61306035, the Fundamental Research Funds for the Central Universities under Grant No. ZYGX2016J056, and Open Projects of Sichuan Key Laboratory of Meteorological Information and Signal Processing under Grant No. QXXCSYS201504 and No. QXXCSYS201603.
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ZZ proposed the novel structure and was a major contributor in writing the manuscript. JC improved the design of the circuit. YW verified the theory with a simulation. The others authors offered comments and revised the manuscript. All authors read and approved the final manuscript.
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Correspondence to Yue Shi.
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Keywords
 Subthreshold
 Lowpower
 High PSRR
 Resistorless
 Nanoscale