Design of touch signal readout IC for capacitive touch screen using dual mode sensing algorithm

Abstract

This work presents a high performance capacitive touch screen readout IC for medium-large size touch screen panel applications. Among the technical challenges that must be solved in medium-large size touch screen panel, touch signal sensing speed and power consumption are the most critical problems because the more increasing touch screen panel size is, the more increasing touch screen panel pixels to readout signal information is. Also, External noises in harsh environment are critical problems. Conventional capacitive touch screen readout ICs solve touch signal sensing speed issue in medium-large size with many readout circuit per each column line of touch screen panel, causing high power consumption. The relationship of speed and power is trade off. To solve this problem, Dual Mode Sensing Algorithm is proposed. The trade off relationship is broken by this algorithm. Also it is required to solve external noise issues like peak noise from touch screen panel bottom plate, sine noise from finger capacitance. So, continuous time integration and signal limitation concept using gm cell is proposed. Speed and power consumption performance can be greatly enhanced applying Dual Mode Sensing Algorithm. Also this algorithm operation is not all but influenced by external noise using gm cell. This work is implemented with 0.35 $\mu m$ CMOS technology. Target specification of touch screen panel size is 44(Rx) by 27(Tx). Total scanning speed is 100Hz at 1~2 touch and 50Hz at 9~10 touch. Scanning frequency for driving touch screen panel is 200Hz.