Touch sensor readout circuit with comparator threshold self-adjustment

Abstract

This work describes a capacitive type touch sensor readout circuit using a current based capacitance-to-time converter with comparator threshold level self-adjustment. The proposed circuit generates an output pulse with duty-cycle corresponding to the panel capacitance variation using a simple switch control block, a current generator, and a comparator. This does not require additional amplifiers and passive capacitors/resistors. As a result, the proposed scheme can lead to small size and low power on-chip solution with fast detection time compared to conventional touch sensor readout circuits. Furthermore, due to the comparator threshold level self-adjustment, the readout circuit is less sensitive to component mismatch and process variations. The readout circuit is implemented using CMOS 0.35µm technology with core area of 100µm × 27µm and power consumption of 52µW, which can detect the touch panel capacitance ranging from 5pF to 50pF.