Touch Sensing Circuit Using Current Mirrors Against Parasitic Capacitance
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Solution Overview
Problem
The presence of parasitic capacitance in touch sensing devices, particularly in pOLED displays, leads to reduced touch sensitivity, increased design area requirements for sensing amplifiers, and degradation in signal noise ratio (SNR) due to high parasitic capacitance and display noise penetration.
Innovation Solution
A touch sensing device incorporating buffers, current mirror units, and integrators to process signals from touch electrodes, utilizing cascode configurations and mirroring operations to generate and integrate mirror currents, thereby reducing the impact of parasitic capacitance and enhancing signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If a touch screen panel is embedded into a display panel, then integration and compactness are improved, but parasitic capacitance increases causing touch sensitivity to deteriorate
Solution Approach 1:
A guard electrode is introduced as an intermediary element between the touch electrode and the cathode electrode. The guard electrode is configured to have the same potential as the touch electrode, creating an electric field shield that redirects electric field lines and reduces parasitic capacitance between the touch electrode and cathode electrode, thereby maintaining touch sensitivity in embedded configurations
2Length of moving object
If parasitic capacitance increases due to thinning of smartphone, then device thickness is reduced, but touch sensitivity and SNR characteristic deteriorate
Solution Approach 1:
The guard electrode serves as a protective intermediary that shields the touch electrode from parasitic capacitance effects. By maintaining the same potential as the touch electrode, it creates a controlled electric field environment that reduces coupling with the cathode electrode, thereby preserving touch sensitivity and SNR characteristics even in thinned device configurations
Solution Approach 2:
The invention converts the harmful effect of proximity to the cathode electrode (which generates parasitic capacitance) into a beneficial configuration. The guard electrode is strategically positioned and biased to exploit the electric field interaction, transforming the potentially harmful capacitive coupling into a controlled shielding effect that protects the touch sensing function
3Reliability
If feedback capacitor is increased to compensate for parasitic capacitance, then touch sensing coverage is improved, but design area increases and output signal is reduced
Solution Approach 1:
The guard electrode acts as an intermediary that reduces the effective parasitic capacitance seen by the sensing amplifier. This reduction allows the feedback capacitor to be smaller while still achieving adequate touch sensing coverage, thereby reducing the design area requirement and maintaining output signal level
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively mitigates the reduction in touch sensitivity and SNR degradation by directly connecting touch sensing lines to buffers, allowing simultaneous signal processing across channels, minimizing sensing time, and reducing design complexity and area requirements due to parasitic capacitance, while maintaining high signal quality.
Implementation Method 1
a first PMOS cascode mirror current generating circuit connected to the first buffer to generate a first mirror current by performing a mirroring operation on a first current flowing through the first pull-up circuit; a first NMOS cascode mirror current generating circuit connected to the first buffer to generate a third mirror current by performing a mirroring operation on a second current flowing through the first pull-down circuit
Implementation Method 2
an integrator integrating a difference between a predetermined reference signal and a first output signal generated by using the first mirror current and the third mirror current
Data Source
AI summary
Disclosed is a touch sensing device for preventing touch sensing performance from being reduced by a parasitic capacitance. The touch sensing device includes a plurality of buffers buffering a difference between a reference signal and a reception signal received from a touch electrode and generating first and second currents corresponding to a buffered signal, a plurality of current mirror units generating a first output signal using a first mirror current generated through mirroring of the first current and a third mirror current generated through mirroring of the second current and generate a second output signal using a second mirror current generated through mirroring of the first current and a fourth mirror current generated through mirroring of the second current, and a plurality of integrators integrating a difference between the first output signal from an nth current mirror unit and the second output signal from an (n−1)th current mirror unit.