A New Compact and Tunable CMOS Temperature Sensor

This paper presents a new compact low voltage and low power CMOS integrated temperature sensor for ultra-low power applications. The design is based on cross-coupled MOS transistors operating in the subthreshold region. The new sensor is a negative resistance that varies with temperature and hence it can be used to sense and amplify the signal at the same time. The functionality of the design was confirmed using Tanner T spice in 0.18μm CMOS TSMC process technology. The circuit is operated from ±0.5Volt and consumes 120nW. DOI: 10.29011/BBOA-121. 100021


Introduction
Temperature sensors are widely used in various applications such as wireless sensor networks, Radio Frequency Identification (RFID), automotive systems, microprocessors, DRAM, and energy harvesting systems, Moreover, temperature sensors are needed to monitor the thermal profile of the integrated chip because the temperature affects the performance such as speed, power, and reliability. This fact made the temperature measurement and control mandatory [1][2][3][4][5][6]. There are many sensors design techniques available in the literature. However, emerging technology trends require the development of better sensors in terms of efficiency, low calibration cost and operation at low power supply voltage. The design in [1] is based on using two Op-Amps in addition to a reference generator. In [2] a temperature to pulse generator is used in which two lines are used along with exclusive OR gate. Reference [3] presents an accurate CMOS integrated temperature sensor. This design is complex and based on Proportional to Absolute Temperature (PTAT) source and bandgap reference. The design in [4] presents a bandgap temperature sensor and a temperature sensor based on the delay line. In [5] Substrate PNP transistors are used for temperature sensing and for generating the reference voltages which is almost the same concept as the design in [1]. In [6] a CMOS temperature sensor is presented. The design uses two Op-Amp buffers and large number of transistors to generate PTAT and Vref. In [7] a Resistor Temperature Detector (RTD)-based temperature sensor is presented. The design is based on using planner resistance temperature detector which can be manufacturer with microelectronics processing technique.
To my knowledge, there is no CMOS-based RTD sensor is available in the open literature. In this paper, a new compact low voltage and low power CMOS integrated temperature sensor is presented. The rest of the paper is organized as follows: Section II presents the proposed design and mathematical analysis. The simulation results and discussion are presented in section III. Section IV concludes the paper.

Proposed Temperature Sensor
The proposed design is based on the cross-coupled MOSFETs shown in (Figure 1). The two transistors are biased in subthreshold region. The drain current of MOSFET in subthreshold is given by [8]: Where is the saturation current, n is the slope factor and is the thermal voltage? To keep the MOSFETs operating in the sub-threshold forward saturation region, the following conditions must be satisfied: Where I_Do is the saturation current, n is the slope factor and V_T is the thermal voltage? To keep the MOSFETs operating in the sub-threshold forward saturation region, the following conditions must be satisfied: Using the small signal equivalent circuit, with M1 and M2 match transistors, it is easy to show that the equivalent resistance seen between nodes V1 and V2 is given by: Where T is the temperature in Kelvin. It is clear that equation (4) implements a resistance-type temperature sensor whose resistance varies linearly with temperature. The sensitivity of the design can be tuned using the bias current B I .
The circuit diagram for the proposed design is shown in (Figure 2). The bias current 2 B I =0.5 B I and is copied through MP2 and MP3 to assure same bias current is supplied to M1 and M2.

Simulation Results
To prove the design concept, the proposed circuit was simulated using tanner T spice with 0.18µm TSMC CMOS process technology. The equivalent resistance is connected to an AC source with 20mV amplitude, 10KHz frequency and a 1Mega Ohm load resistor as shown in (Figure 3 a,b).  The temperature was varied from -50°C to 150°C, and the output voltage is given by: Plot of the calculated and simulated resistance as a function of the temperature is shown in (Figure 4). It is clear from the plot that the resistance varies from 2.5-to-4.5Mega ohm as the temperature varies from -50°C to 150°C. The relative error between the calculated and simulated results are shown in (Figure 5). It is evident from the plot that there is a deviation between the calculated and simulated values. One reason could be the fixed value of n used (n=1.5) in equation (4) for the calculated values of e q R .  The bias current is set to 100nA and plot of the calculated and simulated resistances are shown in (Figure 6). It is clear from the figure the resistance is reduced to half its value and this agrees well with equation 4. The % relative error is less than that shown in (Figure 7). The circuit was simulated for transient response with 1 B I =50nA, and 100nA. Simulation results are shown in (Figures 8,9) respectively. It is clear from the two figures that the circuit is functioning properly. Also, it can be seen that the output voltage in (Figures 8, 9) has 180° phase shift which indicates that the sensor resistance is negative and higher than L R . Also, it is evident from figure 9, that the output voltage is higher than the input voltage and according to equation 5 this will happen only if the sensor resistor is negative.  Since the sensor is a negative resistance that varies with temperature, it can be used to amplify the signal at the same time as shown in (Figure 9). It must be noted that, increasing the bias current will drive the transistor out of subthreshold forward saturation and consequently will limit the input range. For 1 B I =100nA, the input signal amplitude is reduced to 5mV to assure proper operating mode. Alternatively, one can increase the aspect ratios of all transistors so that higher biasing current can be used while keeping all transistors in forward saturation. The performance of the proposed design is compared to the existing temperature sensor and is summarized in (Table 1).
It is clear from ( Table 1) that the proposed design is superior in terms of sensitivity, area, temperature range. However, the power consumption is higher than that in [1].

Conclusion
A new compact and tunable CMOS low voltage and low power tunable integrated temperatures sensor is developed. The sensor can be used in many applications where temperature monitoring is crucial to minimize the power consumption especially in batterypowered circuits. The circuit is powered from ±0.5V DC supply and consumes 120nW.