Two-Feedback Loop Shunt Regulator Based on PWM RF Power Detector Aiming RFID Applications

The paper presents a novel shunt voltage regulation architecture that uses an RF power detector, based on Pulse Width Modulation (PWM) technique, aiming at magnetic coupled Radio-Frequency Identification (RFID) transponders. The proposed regulator architecture implements two feedback loops: a voltage clamping circuit that composes a fast correction loop and a slow but accurate power detector loop. The first clamping loop guarantees fast over-voltage protection, mainly in high RF power condition (short reading distance), but degrades power efficiency when in low power condition (large reading distance). The second loop corrects the imprecision of the first loop according to the detected RF input power. Therefore, it increases power efficiency and reading distance when in low RF power condition. The RF power detector architecture, based on a PWM technique, and the shunt regulator design are presented and discussed in detail. RFID power regulation issues are also discussed, followed by a brief review of RF power detector circuits. The complete regulator architecture was implemented in a 180nm CMOS low cost process and prototyped as part of a commercial low-frequency (134 kHz) RFID transponder. The complete transponder area is 870 \times 870 \mu \text{m}^{2} , being 130 \times 230 \mu \text{m}^{2} related to the regulator circuit only. Both resonant and supply capacitors are implemented on chip. The analog and digital functional blocks of the RFID system consume 4.5 \mu \text{W} . Performance of the transponder is measured with the shunt regulator enabled and compared with the power limiting approach using only clamping diodes. Measured results show an improvement of 16.7% in the maximum communication distance between the transponder and the reader because of the proposed regulation strategy.