Low Voltage Adjustable CMOS Schmitt Trigger Pratchayaporn Singhanath 1 , Apirak Suadet 1 , Arnon Kanjanop 1 , Thawatchai Thongleam 2 , Sanya Kuankid 2 and Varakorn Kasemsuwan 1 1 School of Electronics Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Rd., Ladkrabang Dist., Bangkok 10520, THAILAND. e-mail: yuan_hwan@hotmail.com and kkvarako@kmitl.ac.th. 2 School of Electronics Engineering, Faculty of Science and Technology, Nakhon Pathom Rajabhat University, 85 Maraiman Rd., Muang Dist., Nakhon Pathom 73000, THAILAND. e-mail: tony_tct@yahoo.com and sanya@npru.ac.th. Abstract This paper presents a low voltage adjustable CMOS Schmitt trigger using dynamic threshold MOS (DTMOS). Cross-coupled inverter with body control is employed to speed up the switching process, and control the intensity of the feedback. The proposed Schmitt trigger has been designed using 0.18 µm 0.4 V CMOS technology and analyzed using PSPICE with BSIM3V3 device models. The simulation results show rail-to-rail operation and independently adjustable switching voltages for both low-to-high (V T(LH) ) and high-to-low (V T(HL) ) as high as 15 % of the supply voltage. The power dissipation is 0.13 µW. 1. Introduction It is well known that CMOS Schmitt triggers are circuits that convert a varying voltage into a stable logical signal (one or zero). They have been used extensively to improve on/off control, and reduce the sensitivity to noises and disturbances. Examples of their applications include buffer [1], sub-threshold SRAM [2], sensors [3], [4] and pulse width modulation circuits [5]. The main difference between Schmitt triggers and comparators lies in the DC transfer characteristics. The comparator shows only one switching threshold, while Schmitt trigger shows different switching thresholds for positive-going and negative-going input signals. This characteristic is called hysteresis. If the noise magnitude of the input signal is less than the switching threshold difference, Schmitt trigger will not respond, thus making Schmitt trigger immune to the undesired noise. Several approaches have been proposed to implement Schmitt trigger. The conventional Schmitt trigger was implemented using operation amplifiers with two resistors connected in the positive feedback configuration [6]. This type of Schmitt trigger is not suitable for the integration in CMOS technology, primarily due to the accuracy limitation of resistors and op-amp design challenges, e.g. high DC gain and low offset requirements. Special implementation of Schmitt triggers that can exploit the potential advantages of CMOS technology has been proposed [7]-[18]. [7] proposed a Schmitt trigger which used only six transistors without a latch. [8] proposed a Schmitt trigger based on [7]. The circuit operated under a 3.3 V supply without suffering high-voltage gate-oxide overstress. Larger voltage difference between the two switching threshold voltages was achieved by using a multilayer Schmitt trigger as reported in [9]. Unfortunately, their design required the stack of four transistors between the power and ground rails, thus precluding its use in low voltage applications. [10] employed standard CMOS inverter along with positive feedback. The circuit used only two stacked transistor between the power and ground. The operation was based on an extra active pull down path of the first inverter, when the input is changing from high to low. The circuit demonstrated only one triggering voltage. Two switching threshold voltages can be easily achieved by adding an additional active pull up path. [11] proposed a logical threshold voltage control circuit to set the logical threshold voltage of the Schmitt trigger. However, this logical threshold voltage control circuit is constructed with variable channel size MOS transistors which need a careful design of a mask. Furthermore, the logical threshold voltage can be tuned with a very narrow range. [12] proposed a Schmitt trigger based on [10]. Two feedback inverters were used to control the switching threshold. Very Low voltage Schmitt triggers based on CMOS inverter were proposed [13]-[14]. [13] used body biasing technique to achieve different switching thresholds for positive- going and negative-going input signals, while two static inverters with distinct threshold voltages were employed in [14]. A common drawback of the proceeding Schmitt triggers is that the hysteresis is determined by the device 978-1-4577-0005-7/11/$26.00 ©2011 IEEE