Highly stable analog front-end design for NFC smart card

In order to increase user experience in using near field communication smartcard, analog front-end (AFE) module is required to provide a sufficient and a well-regulated voltage regardless the distance between the card and the reader. A highly stable AFE design for energy harvesting purpose is introduced in this paper. The design consists of antenna, rectifier, voltage limiter, bandgap reference, and low-dropout (LDO) voltage regulator circuit. The antenna is designed to resonate at 13.56 MHz as regulated by ISO/IEC 14443-2. In order to simplify the implementation using 0.18 μm CMOS process, a full-wave rectifier circuit is built of all low-threshold-voltage diode-connected PMOS transistors. To protect the system from undesired excessive input voltages, a voltage limiter circuit is included in the module. Moreover, control and maintain a stable supply voltage for the whole system, a robust LDO voltage regulator and bandgap circuits are specially designed for this purpose. The LDO is able to provide a stable 1.8 V of supply voltage with a sub-1% ripple factor even under a low input current as low as 20 mA.