Capacitive Touch Charge Transfer Lateral Mode Technology 4

Addressing Application Challenges with Charge Transfer Technology

Due to the flaws and limitations of resistive touchscreens, many designers have shifted to projected capacitive sensing technology. This technology is categorized into several circuit types in IC form, primarily including RC time constant measurement circuits such as relaxation oscillators, DC (AC) current measurement components, and charge-transfer components. The charge-transfer components further divide into single-ended mode and transverse mode. By selecting any of these methods and utilizing electrode row and column arrays on two or more layers, touchscreen functionality can be achieved. diode

The fundamental principle of the RC time constant technique is that when the capacitance component C changes with finger touch, the time required for charging or discharging the electrode area also changes. Measuring the variations during charging/discharging allows the determination of C's changes. Since C is unknown, it is assumed as Cx. This method has various variations, enabling the measurement of frequency or time, and can operate either freely or based on a single cycle. The drawbacks of RC time constant measurement include slower speed, susceptibility to leakage current interference, and a very limited dynamic range. Calibration is difficult, and it is prone to constant drift issues. Additionally, due to the high-impedance characteristics of its circuit, it is highly vulnerable to external noise interference. Despite these shortcomings, some touchscreens still employ this approach.

As for the AC current measurement method, an AC voltage source drives the impedance, which in turn drives Cx. Therefore, measuring the voltage generated by the impedance can determine the value of Cx. These circuits also share many limitations with RC circuits, but the former typically has a lower driving impedance. However, it requires amplifiers to restore the small voltage generated by the series impedance, leading to issues such as signal-to-noise ratio. This method has already been applied in touchscreens, particularly in front panels with low-impedance edges.

Similar to RC and AC technologies, the single-ended charge transfer capacitive sensor employs a single electrode plate in each sensing channel. Instead of relying on timing measurements or amplifiers, it utilizes a complementary metal-oxide-semiconductor (CMOS) switch to pump charges into the sensing capacitor (Cx) and transfer them to a reference sampling capacitor (Cs). By calculating the number of cycles required for Cs to reach a preset voltage level, the charge level can be easily determined, and this cycle count is inversely proportional to Cx. It is well known that the charge transfer method helps mitigate the effects of leakage current. Additionally, since it employs a large Cs as the detector—which acts as a low-impedance interface to the external environment—the sensor exhibits strong resistance to external electrical noise.diode