A 110 nA Voltage Regulator System With Dynamic Bandwidth Boosting for RFID Systems

This paper describes a voltage regulator system for ultra-low-power RFID tags (also called passive tags) in a 0.15 mum analog CMOS technology. These tags derive their power supply from the incoming RF energy through rectification instead of from a battery. The regulator is functional with just 110 nA current. Owing to the huge variation of the rectified voltage (by as much as tens of volts), voltage limiters and clamps are employed at various points along the regulation path. A limiter at the rectifier output clamps the rectifier voltage to a narrower range of 1.4 V. A fine-regulator, then, regulates the supply voltage close to a bandgap reference value of 1.25 V. The key aspect of this regulator is the dynamic bandwidth boosting that takes place in the regulator by sensing the excess current that is bypassed in the limter (during periods of excess energy) and increasing its bias current and hence bandwidth, accordingly. A higher bandwidth is necessary for quick recovery from line transients due to the burst nature of RF transmission, with a larger energy burst requiring a higher bandwidth to settle quickly without large line transients. The challenge of compensating such a regulator across various load currents and RF energy levels is described in this paper

I. Introduction

There is great interest in the area of radio frequency identification (RFID) tags [1], [2] due to a large number of applications where they could be used to collect information about a large number of assets quickly, easily and without human error. RFID tags can be read from and written to without the need for direct line-of-sight. Tags fall into two categories—active and passive. Active tags use a battery to power the circuits. Passive tags, on the other hand, are used for high-volume commercial applications. Due to cost reasons, the die area for passive tags is small, usually well within 1 , and they do not use a battery or any other external components, but for the antenna. The power management strategy for these two, thus, greatly differ. The entire passive tag is powered by rectifying the incoming energy to a DC voltage. This energy source can be a RF signal obtained through an antenna or a low frequency (tens of kHz to several tens of MHz) AC signal introduced by a coupling mechanism, like a magnetic coupler. This paper focuses on a passive tag operating in the ultra-high-frequency (UHF) range around 900 MHz, using an antenna to pick up RF signals. The next paragraph describes the method of communication to and from the tag and the various constraints it poses on the power management scheme.