low voltage temperature sensor front end project report

ECE 5321, Design and Analysis of Analog IC, Final Project Report, December 8th

2010

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Abstract—In this brief, a Low Voltage Low Power CMOS

based Temperature Sensor is presented. It is based on

subthreshold MOSFETs and on compensating a PTAT based

source with a gate-source voltage of a subthreshold MOSFET.

The circuit is designed using a standard 0.5-µm CMOS

technology, exhibiting an average voltage of 106mV with an

average temperature coefficient of 511ppm/ºC in the range of 0 to

120 ºC, and voltage sensitivity of 666Kppm/V. A brief study of

PTAT based current Source and subthreshold MOSFET is also

reported.

Index Terms—PTAT, Compensation, Reference, Subthreshold

MOSFET.

I. INTRODUCTION

N THIS document a Low Voltage Low Power Temperature

Sensor is realized exploiting the behavior of subthreshold

MOSFETs. Temperature Sensors are important in monitoring

the effective working of many electronic devices. Earlier BJT

based references were used requiring large area and large

power. Hence use of Subthreshold MOSFETs came into

picture, which helps to get a voltage reference independent of

temperature and power supply, reducing power dissipation and

chip area. The voltage reference used to design this

temperature sensor is similar to [1].The former uses 0.5-µm

CMOS process and the later 0.35-µm CMOS.

II. DESIGN PRINCIPLES

A. Subthreshold MOSFETs

The basic principle of working of a subthreshold MOSFET

is Vgs<Vth. When Vgs<Vth a small amount of Drain

Current flows. Its relationship with respect to Vgs is

exponential.

Id=Ioexp (Vgs/ζVt). Where ζ>1 is a Non-ideal factor and Vt=KT/q.

The equation as a function of drain current can be written as

Vgs= ζVtln (Id/Io). (1)

If (Id/Io) remains constant then Vgs is a positive

temperature co-efficient.

In Subthreshold Region the Vgs equation in terms of

Kg [1] and Temperature can be written as

Vgs (T) =Vgs (To) +Kg {(T/To)-1} (2)

The quantity Kg is negative [1] and hence Vgs

decreases by temperature. We can observe that by

combining equation (1) and (2) a constant voltage

independent of temperature can be obtained.

B. PTAT Current Source

Fig.1. PTAT current source varying linearly with temperature.

The PTAT current source is designed using two NMOS in

Subthreshold region and two PMOS in active region. The Vgs

is kept less than Vth to design the Subthreshold NMOS. The

MOSFETs are diode connected to mirror the current to be

used in later half of the circuit. PTAT is designed by

minimizing the dependence with respect to power supply. The

output of a PTAT current source represents a f(x)=kx straight

line, where the y varies linearly with temperature.

The Slope of PTAT vs T between (20,490) and (60,505) is

Slope={(505-490)/(60-20)}= 3/8=0.375

Low Voltage Smart Temperature Sensor Front End

Shreyas Rao (R10357881), Satyabh Mishra (R10322331), ECE 5321

I


2010

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Fig.2. Output of PTAT Current Source.

C. The Compensation Circuit.

Fig.3. Compensation Circuit

The temperature compensation circuit is designed and tested

using a Constant Current Source of 1.022µA. The current

Mirrored Transistor from Fig.3 mirrors the PTAT current to

set Vgs of the NMOS without external resistance, which is

used to produce Icom= Vgs/R1. (3)

The point to be noted is this transistor works in

subthreshold region. From equation (2) and (3) we can see that

Icom has a negative temperature co-efficient, and hence as

temperature increases this subthreshold transistor produces a

Compensating Current Icom. The testing is done using the

constant current source because this amount of current was

required to cancel out the PTAT effect while designing the

Constant Voltage Reference. In the Final Design the Constant

Current Source is replaced by the Current Mirror of PTAT

circuit.

Fig.4. Output of Compensation Circuit.

III. PROPOSED CIRCUIT, LAYOUT AND ANALYSIS

Fig.5. Final Design Schematic

Fig.6. Final Layout (167.250µm * 56.850µm)


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Fig.7. Extracted View

The final circuit is designed using the PTAT and

Compensation Circuit. The Layout area is minimized by

doing several iteration. The Final and best possible Layout

area is 9508.1625µm². The following three sample figure

demonstrates the basic design principle and desired output

at various stages of design. It shows that we need two basic

output waveform which when added gives the final constant

output with respect to temperature.

Fig.8. Desired PTAT output

Fig.9. Desired Compensation Output

Fig.10. Desired Final Vref Output

The above graph demonstrates the desired final output

which we will compare and see with the actual output of the

circuit in the following section. The challenge for designing

this type of circuit is proper configuration of MOSFETs,

i.e., their (W/L) ratios and Resistance Value. By applying

the concept of subthreshold MOSFETs most of the

transistor size is determined, but to make it temperature and

power supply independent lot of design iteration has been

done.

IV. OUTPUT WAVEFORMS

Fig.11. Vref vs. Temperature

The average value of Vref is 106.5mV

0

2

4

6

8

10

Temperature

PTAT

PTAT

0

5

10

Temperature

Compensation

Voltage

0

1

2

3

4

5

Temperature

Vref

Vref


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Fig.12. Vref vs. VDD.

The Voltage swing from 0V to 2V is shown with maximum

swing equal to 79mV, which is the point of concern of the

design. Various alternative where tried but decrease of

variation over VDD resulted in Increase of Variation over

Temperature.

V. PERFORMANCE AT WORST CORNERS

Fig.13. Vref vs. Temp at 0.8V

Fig.14. PTAT vs. Temp at 0.8V

Fig.15. Vref vs. Temp at 3V

Fig.15. PTAT vs. Temp at 3V


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Fig.16. Vref vs. Temp at 5V

Fig.17. PTAT vs. Temp at 5V

VI. LVS MATCH

The following figure shows the net-list match of schematic

and Layout.

VII. CONCLUSION

A Low Voltage Front End Temperature Sensor is

realized exploiting the subthreshold characteristics of

MOSFETs. The Voltage Reference obtained is of

106.5mV with voltage variation of 79mV over the

range of 1.2V to 2V and 4mV peak to peak for

temperature variation over 0 ºC to 120 ºC in the chip

area of 9508.1625µm². A Temperature Sensitivity of

511ppm/ºC and Voltage Sensitivity of 666K ppm/V

are obtained.

ACKNOWLEDGMENT

The authors acknowledge Dr Changzhi Li, Dept. of

ECE TTU, Lubbock, Texas-USA for valuable design

input and constant support throughout the course

“Design and Analysis of Analog IC”.

REFERENCES

[1] Po-Hsuan Huang, Hongchin Lin and Yen-Tai Lin, “A Simple

Subthreshold CMOS Voltage Reference Circuit with Channel-

Length Modulation Compensation,” IEEE Transactions on

Circuits and Systems-II, September 2006.

[2] B. Razavi, “Design and Analysis of CMOS Integrated Circuits”.

(Text Book)

[3] G.Giustolisi, G.Palumbo, M.Criscione and F. Cutri,”A Low-

Voltage Low-Power Voltage Reference Based on Subthreshold

MOSFETs,” IEEE Journal of Solid State Circuits, VOL.38,

January 2003.

low voltage temperature sensor front end project report

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