A Low Power Sub-1 V CMOS Voltage Reference Sameer Somvanshi 1 , Santhosh Kasavajjala 2 1 EEE Department, BITS-Pilani, Rajasthan,India 2 Stanford University, USA ABSTRACT This work describes the circuit which generates a stable voltage of 466.5 mV for 1.8 V power supply in 0.18 µm technology. Circuit uses MOSFETs in linear region and in subthreshold region to generate PTAT and CTAT respectively intended to replace resistor and BJT. The temperature coefficient of circuit is 28.4 ppm/ 0 C in the range of -20 to +120 0 C. The power supply rejection is measured as -30 dB at 8 KHz. Low power consumption of 3.98 µW is an important attribute of this circuit. I. INTRODUCTION Bandgap reference circuit is widely used to provide stable current and voltage references in analog circuits as well as in mixed signal CMOS circuits. A stable reference circuit should be robust against temperature, power supply and process variations. Sub-1-V reference generation has got importance due to scaling resulting in shrinkage of MOS dimensions and reduction of power supply to minimize power consumption. The BGR generators that can be operated under 1-V supply have been widely used in ADCs, DRAM’s, flash memories and various analog devices. Owing to follow Moore's law, it has become necessarily important to decrease the total power in the chip. This puts the constraint on the power dissipation of Reference generators. The design of this bandgap also considers for low power operation. In traditional BGR circuit, bipolar transistors and one or more resistors are used. BJTs that are used in BGR are in parasitic form in CMOS. Resistors occupy large area on the chip and hence increase the cost. On chip tolerance of resistors vary from 20% to 30% [6]. So we have replaced these components with MOS transistors to improve performance of BGR and to save chip area. The combination of different operating regions like subthreshold, linear and saturation of MOS suppresses the temperature dependence of voltage reference. II. TRADTIONAL BANDGAP REFERENCE Fig. 1 shows the traditional BGR. It comprises of both PTAT and CTAT sections. Reference voltage is given as V 0 = V BE2 + IR 1 (1) = V BE3 + M I 2 R 2 (2) where M is the multiplication factor between the W/Ls of M 4 and M 5 . V BE gives CTAT with roughly -1.5 mV/°K at room temperature. PTAT voltage is generated across resistor R 1 and also R 2 . Thus, Vo (Vref) is approximately 1.25V which is nearly equal to bandgap of silicon (1.12V) at room temperature. We replaced the resistors and BJTs of the following circuit by MOS transistors to achieve efficient performance of reference and save area. Fig 1 Traditional Bandgap Voltage Reference Circuit III. STUDY OF MOS IN DIFFERENT REGIONS Of OPERATION: In subthreshold region, for V gs < V th , there flows a finite current but it exhibits exponential dependence on V gs which is given by[3] I d =µ ௐ ௅ V T 2 ට ௤ఌே௖௛ ସః௕ expሺ ௏௚௦௏௧௛௏௖ ௡VT ) (3) where V c is the correction term which gives the difference between V th in the strong inversion and subthreshold region. Φ b is bulk Fermi level which is dependent on temperature given by [3] Φ b = V T ln( ே௖௛ ୬୧ ) (4) n i is the intrinsic carrier concentration which is also dependent on temperature given by [3] n i 2 α T 3 exp( ா௚ ௄ ) (5) In triode region, MOSFET behaves like a resistor whose resistance value is controlled by W/L of the transistor and by the input bias applied so long as V DS <<2(V GS - V th ) [1]. It is given by following expression R on = ଵ µC୭୶ሺ W L ሻሺV୥ୱV୲୦ሻ (6) without considering the early effect. Threshold voltage decreases by 2mV for every 1 o C rise in temperature which results in increase in the drain current[12]. However, mobility decreases with temperature given by µ α T -1.5 due to lattice 271 978-1-4244-2596-9/08/$25.00 ©2008 IEEE Authorized licensed use limited to: MIT Libraries. Downloaded on March 12,2010 at 17:14:48 EST from IEEE Xplore. Restrictions apply.