A LOW-POWER LOW-VOLTAGE MOSFET-ONLY VOLTAGE REFERENCE F. Bedeschi 1 , E. Bonizzoni 2 , A. Fantini 2 , C. Resta 3 , and G. Torelli 2 1 STMicroelectronics, Memory Products Group R&D, via Olivetti, 2 - 20041 Agrate Brianza, Italy 2 Department of Electronics, University of Pavia, via Ferrata, 1 - 27100 Pavia, Italy 3 Studio di Microelettronica, STMicroelectronics, via Ferrata, 1 - 27100 Pavia, Italy ABSTRACT A low-power low-voltage MOSFET-only voltage reference featuring very good temperature stability and referred to the positive power supply is proposed. It compensates for the temperature dependence of a gate-to- source voltage of an MOS transistor working in the weak inversion region with a proportional-to-absolute- temperature voltage generated by a pair of MOS devices operating in the same region. The circuit, designed for a 0.35- m (0.18- m in the memory array) CMOS Flash memory technology, can operate with a supply voltage as low as 1 V and draws a current of 3 A. The simulated variation of the reference voltage is within 0.2% over the range from –20 to 80 ºC. 1. INTRODUCTION Voltage references are a key element in a number of integrated applications, including analog-to-digital and digital-to-analog converters, signal processing, and voltage regulators. The most widely used schemes of integrated voltage references with low temperature dependence are based on the bandgap approach. Several bandgap voltage references [1] have been implemented in bipolar and CMOS technology, where they achieve high levels of accuracy and stability. Recently, requirements of low-power and low-voltage devices are more and more vital as a consequence of the technology scaling down and the widespread applications of portable equipment (such as cellular phones, notebooks, personal data assistants, audio players, digital cameras, portable medical diagnosis systems, etc.). In voltage reference design, the above requirements can be easier met by using fully CMOS circuits. In particular, the use of MOS transistors working in the subthreshold region (hereinafter referred to as subthreshold MOS transistors) dramatically reduces power consumption, and also leads to silicon area saving [2-5]. Typically, bandgap voltage references provide an output voltage referred to ground. However, in some applications, a stable voltage referred to the positive supply V dd is desirable. For instance, Flash memories require high negative gate voltages for the erase operation [6]. In addition, recently, negative-bulk programming [7] has been demonstrated to increase hot-electron injection efficiency, thereby allowing lower power consumption and faster programming. A typical value of the bulk bias voltage for this purpose is on the order of –1.4 V. Fig. 1 – Negative voltage regulator topology. A negative voltage can be obtained by using the regulator topology depicted in Fig. 1, where the operational amplifier (op-amp) is operated from a positive (V dd ) and a negative (–V ss ) power supply (the latter is provided by an on-chip negative charge pump voltage multiplier). The voltage V neg turns out to be V r (1 + R 2 /R 1 ) – V dd R 2 /R 1 , which shows a large dependence on V dd as generally R 2 /R 1 is larger than unit. To limit the sensitivity of the generated voltage to V dd variations, a good choice is using a reference voltage negative to the positive supply V dd , namely setting V r = V dd + V ref (where V ref has a negative value, as is clear from Fig. 1). This way, V neg can be expressed as V dd + V ref (1 + R 2 /R 1 ), which reduces the output voltage dependence on V dd by a factor of R 2 /R 1 . In this paper, a low-power, low-voltage, MOSFET-only voltage reference negative to the power supply is proposed. This scheme, which is based on a current-mode approach, can operate with a supply voltage as low as 1 V, and draws a current of 3 A. Circuit design has been optimized for a p-substrate 0.35- m (0.18- m in the memory array) CMOS Flash technology. Moreover, design considerations regarding some critical aspects in the design of voltage references based on subthreshold MOS transistors are presented. 2. CIRCUIT DESCRIPTION The operating principle of the proposed voltage reference is depicted in Fig. 2. A current-mode approach has been adopted to meet low-voltage requirements. The block