Design of Low Power Analog Front-End of Touch-Screen Controller With Adiabatic Signal Generation and Optimized Sensor Modes

Abstract

In this thesis, a touch sensing analog front-end (AFE) for capacitive touch-screen panel (TSP) is presented. Reduced distance between the touch-screen and display causes large capacitive coupling, resulting in increased parasitic capacitance and reduced touch sensitivity. Display noise interference is worse due to the large coupling capacitance. Therefore, it is a challenge to design an AFE capable of accurate and energy-efficient sensing of a touch input in the integrated TSP. To provide power efficient stimulation on the TSP, an adiabatic signal generation based on charge recycling is newly proposed as an AC-signal generator. Furthermore, in order to cancel out the display noise interference through the large parasitic capacitance, a fully differential touch sensing module is incorporated in the AFE. A correlated noise sampling is employed in the decoder stage for the multi-driving demodulation process. To further improve power efficiency, the sensing module is multiplexed in 4 ways while achieving an optimal conversion time per sample. The proposed AFE was implemented in a 180-nm CMOS process. The fabricated AFE shows 57.0-dB SNR at 120 fps while consuming 17.8 mW. Compared with power consumption of 19.9 mW expected with a conventional signal generation, the proposed adiabatic signal generator dissipates only 7.1 mW, exhibiting a power reduction of 64 % owing to the adiabatic driving method. In addition, the self-capacitance sensing mode is also implemented as low power, coarse touch detection mode. The newly proposed current mirroring voltage-level regulation (CM-VLR) circuit senses the transition of self-capacitance of the TSP and detects motions of the touch-object. While keeping a voltage level, a certain amount of electric charge is additionally required when a capacitance is newly added at the sensing channel, and the amount of charge is used for the detection. Only one CM-VLR cell is used to scan the entire TSP, and the touch-screen channels are multiplexed and selected in order, and the AFE does not require the offset-calibration step from the additional coverage capacitance attached to either channel. The AFE was fabricated in a 180-nm CMOS process, and the CM-VLR cell occupies 0.12 mm2. In 120-Hz report rate, the AFE detects touch-object in 32-dB SNR while dissipating 2.1-mW of power.