System and method for mutual capacitance sensing

A display device includes: a display panel configured to display an image at an active area; a touch screen panel overlapping the display panel at the active area, the touch screen panel comprising an input line and an output line; and an operational amplifier having an input electrode coupled to the input line and an output electrode coupled to the output line, wherein the display device is configured to measure a voltage at the output electrode of the operational amplifier for detecting a touch event at an intersection between the input line and the output line.

BACKGROUND

One or more aspects of example embodiments of the present disclosure relate to a touchscreen panel, and a method of driving the same.

2. Description of the Related Art

A touch screen panel is an input device capable of inputting a user's instruction by selecting an instruction content that is displayed on a display device, or the like, with a human hand or an object. To this end, the touch screen panel may be provided on a front surface of the display device to convert information of a touch point (e.g., a contact position) of the human hand or the object into an electrical signal. The instruction content selected at the touch point may then be recognized as an input signal. Because the touch screen panel may be substituted for a separate input device connected to the display device, such as a keyboard or a mouse, its areas of application have been gradually extended.

Implementation types of touch sensors of the touch screen panel include resistive overlay touch sensors, photosensitive touch sensors, capacitive touch sensors, and the like. Among these touch sensors, the capacitive touch sensor converts the touch point information into an electrical signal by sensing a change in capacitance formed between a conductive sensing electrode and an adjacent (or overlapping) sensing electrode or ground electrode when a user's hand or object comes in contact with the touch screen panel.

The above information disclosed in this Background section is for enhancement of understanding of the background of the disclosure, and therefore, it may contain information that does not constitute prior art.

SUMMARY

Aspects of one or more example embodiments of the present disclosure include a system and method for mutual capacitance sensing.

According to some example embodiments of the present disclosure, a display device includes: a display panel configured to display an image at an active area; a touch screen panel overlapping the display panel at the active area, the touch screen panel comprising an input line and an output line; and an operational amplifier having an input electrode coupled to the input line and an output electrode coupled to the output line, wherein the display device is configured to measure a voltage at the output electrode of the operational amplifier for detecting a touch event at an intersection between the input line and the output line.

According to some embodiments, the touch event is detected based on a change in a mutual capacitance between the input line and the output line.

According to some embodiments, the display device further includes a current source coupled to the input line and the input electrode of the operational amplifier.

According to some embodiments, the current source is configured to apply a predetermined charge to the input line and the input electrode of the operational amplifier.

According to some embodiments, the voltage at the output electrode of the operational amplifier is proportional to a ratio of a predetermined input charge divided by a mutual capacitance between the input line and the output line.

According to some embodiments, the display device is configured to detect the touch event in response to a change in the voltage at the output electrode of the operational amplifier.

According to some embodiments, a mutual capacitance between the input line and the output line is directly amplified using miller amplification.

According to some example embodiments, a system includes: a touch screen panel comprising an input line and an output line; an operational amplifier having an input electrode coupled to the input line and an output electrode coupled to the output line; and a current source coupled to the input line and the input electrode of the operational amplifier for applying a predetermined charge to the input line and the input electrode of the operational amplifier, wherein the touch screen panel is configured to measure a voltage at the output electrode of the operational amplifier for detected a touch event at an intersection between the input line and the output line.

According to some embodiments, the system further includes a display panel configured to display an image at an active area.

According to some embodiments, the touch screen panel overlaps the display panel at the active area.

According to some embodiments, the touch event is detected based on a change in mutual capacitance between the input line and the output line.

According to some embodiments, the voltage at the output electrode of the operational amplifier is proportional to a ratio of a predetermined input charge divided by a mutual capacitance between the input line and the output line.

According to some embodiments, the system is configured to detect the touch event in response to a change in the voltage at the output electrode of the operational amplifier.

According to some embodiments, a mutual capacitance between the input line and the output line is directly amplified using miller amplification.

According to some example embodiments, in a method of driving a touch screen panel, the method includes: applying an input charge to an input line and an input electrode of an operational amplifier; measuring an output voltage from an output electrode of the operational amplifier; and detecting a touch event based on a change in the output voltage from the output electrode of the operational amplifier, wherein the input line is connected to the input electrode of the operational amplifier and an output line is connected to the output electrode of the operational amplifier.

According to some embodiments, the change in the output voltage is based on a change in a mutual capacitance between the input line and an output line.

According to some embodiments, the method further includes applying the input charge by a current source coupled to the input line and the input electrode of the operational amplifier.

According to some embodiments, a mutual capacitance between the input line and an output line is directly amplified using miller amplification.

According to some embodiments, the touch screen panel comprises the input line and the output line, overlaps a display panel at an active area configured to display an image.

According to some embodiments, the voltage at the output electrode of the operational amplifier is proportional to a ratio of the input charge divided by a mutual capacitance between the input line and an output line.

DETAILED DESCRIPTION

Generally, for capacitive touch screen panels, a known (e.g., set or predetermined) signal is input to a touch sensor of the touch screen panel (TSP), and the output of the touch sensor is monitored. Typically, the input signal may be in the form of a clock pattern (e.g., a clock signal) with alternating sequence of ones and zeros, or may be in the form of a sinusoid waveform. While less common, ramp waveforms or saw-tooth waveforms may be used as the input signals. The output signal changes (typically it drops in magnitude) in response to a touch event (e.g. a finger or object such as a stylus contacting the TSP).

For a given touch sensor, there is a narrow band of frequencies where the touch sensor exhibits a difference between a touch and no-touch. This band of frequencies is ideal for transmitting the input signal, but there may also be noise sources that occupy the same band of frequencies. These noise sources may be time varying and/or frequency varying, and may lead to false indications of touch events on the TSP.

According to one or more example embodiments of the present disclosure, the input signal may be spread out over a wideband (or sub-bands within the wideband) of frequencies, and touch events may be detected in the output signal on the same frequencies (or instantaneous frequencies) of the input signal. According to one or more embodiments of the present disclosure, one or more state machines may control one or more transmitters to transmit the input signal to the TSP at set or predetermined frequencies spread over a wideband (or sub-bands within the wideband), and the one or more state machines may calibrate one or more receivers to listen to the output signal for touch events on the same set or predetermined frequencies over the wideband (or sub-bands) as that of the input signal, while filtering out noises at other frequencies. For example, the receivers may be connected to output lines of the TSP, and each of the receivers may include one or more filters (e.g., bandpass filters) to filter the output signal on each of the output lines to detect touch events. The state machines may configure each of the filters according to the same or instantaneous frequency of the input signals at any given time. Accordingly, touch events may be detected on the output signals at the same or instantaneous frequency of the input signal, while noises are filtered out at other frequencies.

Accordingly, undesirable noises may be reduced or eliminated, leading to a more reliable TSP. Further, because the input signal is spread out over the wideband of frequencies, electromagnetic emissions may also be reduced.

FIG. 1is a diagram illustrating a display device including a touch screen panel according to some embodiments of the present disclosure,FIG. 2is a diagram illustrating a transceiver system according to some embodiments of the present disclosure, andFIG. 3is a diagram illustrating an example of the touch screen panel shown inFIGS. 1 and 2according to some embodiments of the present disclosure.

Referring toFIGS. 1 through 3, a touch sensor system100includes a timing controller110, a scan driver120, a data driver130, and a plurality of pixels Px arranged in an active area (e.g., a display area or a touch area) AA. Each of the plurality of pixels Px is coupled to respective ones of scan lines SL1to SLn, where n is a positive integer, and data lines DL1to DLj, where j is a positive integer, at crossing regions of the scan lines SL1to SLn and the data lines DL1to DLj. Each of the pixels Px receives a data signal from the data driver130through the respective one of the data lines DL1to DLj, when a scan signal is received from the scan driver120through a respective one of the scan lines SL1to SLn.

The timing controller110receives an image signal Image, a synchronization signal Sync, and a clock signal CLK from an external source (e.g., external to the timing controller). The timing controller110generates image data DATA, a data driver control signal DCS, and a scan driver control signal SCS. The synchronization signal Sync may include a vertical synchronization signal Vsync and a horizontal synchronization signal Hsync. The timing controller110transmits the image data DATA and the data driver control signal DCS to the data driver130, and transmits the scan driver control signal SCS to the scan driver120.

A touch screen panel TSP may be arranged to overlap with the plurality of pixels Px in the active area AA. The touch screen panel TSP may detect a touch point (e.g., a contact position or a touch event) TP of a user's hand or object at the active area AA, and may convert information of the touch point TP into an electrical signal. The electrical signal may be transmitted to a corresponding receiver connected to a corresponding output line of the touch screen panel TSP as an output signal of the touch screen panel TSP.

In this regard, a touch transceiver (or touch driver)140may be connected to the touch screen panel TSP. The touch transceiver140may include one or more transmitters210, one or more receivers220, and one or more state machines280to control and configure the transmitters210and the receivers220. According to one or more example embodiments of the present disclosure, the state machines280may control the transmitter210to transmit an input signal over wideband frequencies (or sub-bands within the wideband) to the touch screen panel TSP, and may calibrate the receiver220to listen to the output signal at the same frequencies of the input signal, while filtering out noise at other frequencies. Accordingly, the state machines280may keep the one or more transmitters210in sync with the one or more receivers220, so that touch events may be detected at any given time at a corresponding frequency over the wide band frequencies (or sub-bands).

The touch screen panel TSP may include a plurality of touch sensors. In some embodiments, the plurality of touch sensors may include, for example, a plurality of first lines (e.g., first electrodes or input lines) X1to Xm, and a plurality of second lines (e.g., second electrodes or output lines) Y1to Yi crossing the plurality of first lines X1to Xm, where m and i are natural numbers. For example, the plurality of first lines X1to Xm may extend in a row direction, and the plurality of second lines Y1to Yi may extend in a column direction.

A mutual parasitic capacitor Cp may be formed at each crossing region of the first lines X1to Xm and the second lines Y1to Yi. Each crossing region at which the mutual parasitic capacitor Cp is formed may operate as a touch sensor (or sensing cell) that may recognize a touch event.

The touch transceiver140may supply input signals SEN to the first lines X1to Xm through control of the timing controller110. For example, the one or more transmitters210may be connected to the first lines X1to Xm to supply the input signals SEN to the first lines X1to Xm sequentially or concurrently (e.g., simultaneously or at the same time). In an embodiment in which the input signals SEN are supplied to the first lines X1to Xm, the mutual capacitance generated in each touch sensor generates an output signal (e.g., a sensing signal) DET on corresponding second lines Y1to Yi. Thus, in the event that a user's finger or object comes in contact with the touch screen panel TSP, for example, at the touchpoint TP inFIG. 2, a change in mutual capacitance occurs at the touchpoint TP. Accordingly, the output signal DET on a corresponding one of the second lines Y1to Yi (e.g., at the second output line Y2in the example shown inFIG. 2) is also changed, thereby registering (or recognizing) a touch. In this regard, the one or more receivers220may be connected to the second lines Y1to Yi to receive and filter the output signals DET from the second lines Y1to Yi.

Embodiments of the present disclosure may provide a relatively less complex touch sensing system and method, with fewer components for sensing and processing touch events, compared to alternative systems, and may also have a relatively higher signal to noise ratio per unit of power consumption.

FIG. 4is a diagram illustrating a touch cell according to some example embodiments of the present disclosure. As shown inFIG. 4, a touch cell or sensor400includes a transmission node or input line TX (e.g., from among the input lines X1-Xm) and a receiver node or output line RX (e.g., from among the output lines Y1-Yi). Throughout the present disclosure and the claims, the terms “transmission node” and “input line” may be used synonymously. Similarly, throughout the present disclosure and the claims the terms “receiver node” and “output line” may be use synonymously.

A touch event may be detected based on changes in a mutual capacitance, Cm, between the transmission node TX and the receiver node RX. As illustrated inFIG. 4, the input line and output line may include some inherent resistance R_tx and R_rx, respectively. Similarly, parasitic capacitances C_tx and C_rx may exist between the transmission node TX and a reference node402, and between the receiver node RX and the reference node402.

The voltage at the receiver node RX may be applied to a first input terminal (e.g., a negative input terminal)404of an operational amplifier406, by way of control switches Φ1and Φ2. A second input terminal408of the operational amplifier406may be connected to a low voltage (e.g., ground). A feedback capacitor Cfmay be coupled in parallel to the operational amplifier406, which may force the charge to accumulate onto Cf, as opposed to C_tx and C_rx. In some instances, a switch412may be coupled between the electrodes of the feedback capacitor Cfto control whether or not a charge is stored in the feedback capacitor Cf. The output voltage Voutat an output terminal410of the operational amplifier406may be applied to the driver140to detect whether or not a touch input has occurred based on changes in the output voltage Vout(and the mutual capacitance Cm).

In order to detect a touch input, the unknown mutual capacitance Cmbetween the transmission node TX and the receiver node RX may be charged to a known voltage (e.g., by the transceiver or driver140). Then, the charge stored as the mutual capacitance Cmmay be transferred and integrated onto a sampling capacitor Csto hold the value of the output signal (e.g., at the output electrode410) and/or transferred to the touch receiver140.

The operations of charging the unknown mutual capacitance Cmto a known voltage, and transferring and integrating the charge Cmto the sampling capacitor Csmay be repeated or cycled several times. In some instances, once the voltage on the sampling capacitor Csreaches a reference voltage, the touch sensor system100may stop and measure the time and/or number of cycles to reach the reference voltage in order to determine whether or not a touch event has occurred. In other instances, the voltage on the sampling capacitor Csmay be digitized after a set or predetermined number of cycles to determine whether or not a touch event has occurred.

The power utilized for driving the transmission node TX in embodiments according toFIG. 4may be calculated according to equation (1), below:

FIG. 5is a diagram illustrating another touch cell according to some example embodiments of the present disclosure. As shown inFIG. 5, according to some example embodiments of the present disclosure, a touch cell or sensor500includes a transmission node or input line TX (e.g., from among the input lines X1-Xm) and a receiver node or output line RX (e.g., from among the output lines Y1-Yi).

As in the embodiment illustrated inFIG. 4, a touch event may be detected based on changes in a mutual capacitance, Cm, between the transmission node TX and the receiver node RX. Like the embodiment illustrated inFIG. 4, the input line and output line may include some inherent resistance R_tx and R_rx, respectively. Similarly, parasitic capacitances C_tx and C_rx may exist between the transmission node TX and a reference node502, and between the receiver node RX and the reference node502.

In contrast to the embodiment illustrated inFIG. 4, rather than amplifying the output voltage at the receiver node RX by applying the voltage to an input terminal of an operational amplifier, some example embodiments of the present disclosure may directly amplify the mutual capacitance, by connecting an input terminal504of an operational amplifier506to the transmission node TX, and connecting an output terminal508of the operational amplifier506to the receiver node RX.

A known amount of charge Qinis applied to the transmission node Tx (e.g., by way of a current or voltage source510controlled, for example, by the driver140). The voltage Voutat the output terminal508of the operational amplifier506and/or the receiver node RX may be measured to detect touch events based on changes in the voltage Vout. The voltage Voutmay be proportional to a ratio of the known charge Qindivided by the mutual capacitance Cm. That is, the voltage Vout(and/or the voltage VRXat the receiver node RX) is proportional (or equal) to Qin/Cm.

According to some example embodiments of the present disclosure, by directly amplifying the mutual capacitance Cm, as illustrated inFIG. 5, a large voltage may be achieved at Voutand/or VRX. Additionally, embodiments of the present disclosure may provide relatively lower sensitivity to noise at the receiver node RX and capacitance CRX. According to some example embodiments, the same operational amplifier506may be connected to and utilized for multiple input lines and output lines but, according to some embodiments, each transmission node TX and receiver node RX combination that is read at the same time may utilize a different operational amplifier506.

Additionally, in contrast to the embodiment illustrated inFIG. 4, the power for driving the transmission node TX according to the embodiment illustrated inFIG. 5may be calculated according to the following equation (2), below:
Power=Vdd·Qin·f=Vdd·Vout·Cm·f(2)

Thus, the ratio of transmission powers for the same output voltage and frequency is shown in equation (3), below:

In some embodiments, the above ratio in equation (3) may be on the order of 103.

Additionally, the embodiment illustrated inFIG. 5may have relatively less sensitivity to noise (e.g., noise coupled to the transmission node TX, the receiver node RX, amplifier noise, thermal noise, etc.) compared to alternative systems, for example, the embodiment illustrated inFIG. 4. For example, in the embodiments illustrated in bothFIG. 4andFIG. 5, noise sources such as display noise (e.g., voltage Vn) may be coupled through parasitic capacitance Cnto the transmission node TX and the receiver node RX.

In the embodiment illustrated inFIG. 4, the noise at the output electrode410may be calculated according to equation (4), below:

By contrast, in the embodiment illustrated inFIG. 5, the noise at the output electrode508may be calculated according to equation (5), below:

In the case of an input-referred voltage noise and an input-referred current noise for the operational amplifier, the transfer functions for each noise source to the output of the operational amplifier in the embodiments shown inFIGS. 4 and 5are shown below in Table 1.

Additionally, sensor resistors (e.g., RTXand RRX) may contribute nose to the system output. The transfer functions for each noise source to the output of the amplifier in the embodiments ofFIGS. 4 and 5are illustrated below in Table 2.

Table 3, below, shows the signal-to-noise ratio (SNR) for the signal to each noise source separately (assuming the same amount of power is consumed in the transmission node TX for both systems).

The embodiment ofFIG. 5may have a higher SNR for the transmission node TX coupling, RTXthermal noise, and RRXthermal noise. The embodiment ofFIG. 4, by contrast may have a higher SNR for operational amplifier noise sources. In many instances, the transmission node TX and receiver node RX coupling that comes from display noise is the dominant source of noise, thus the embodiment illustrated inFIG. 5may have a larger overall SNR.

Table 4, below, shows simulation results comparing the SNR of the embodiments illustrated inFIGS. 4 and 5, in which the transmission node TX receives the same amount of power. As shown in table 4, the embodiment illustrated inFIG. 5may have a relatively higher SNR (e.g., 25 dB higher) compared to the embodiment illustrated inFIG. 4.

FIG. 6is a flow diagram of a method of driving a touch screen panel according to some example embodiments of the present disclosure. However, the present disclosure is not limited to the sequence or number of operations of the method shown inFIG. 6, and various embodiments of the present disclosure may include additional or fewer operations, and the sequence of operations may vary.

Referring toFIG. 6, the method starts and, at600, an input line or transmission node of a touch screen panel is connected to an input electrode of an operational amplifier. At602, an output line or receiver node of the touch screen panel is connected to an output electrode of the operational amplifier. Then, at604, a known input charge is applied to input line or transmission node and the input electrode of the operational amplifier. At606, a touch event may be detected by measuring an output voltage from the output electrode of the operational amplifier (e.g., based on changes in the output voltage and/or the mutual capacitance between the transmission node and the receiver node).

Thus, one or more embodiments of the present disclosure, may be configured to drive a touch screen panel by directly amplifying the mutual capacitance between an input line and an output line using miller amplification. The transmission node or input line of a touch cell is connected to the input electrode of an operational amplifier, and the receiver node or output line of the touch cell is connected to the output electrode of the operational amplifier. A known amount of charge is applied to the transmission node or input line of the touch cell, and the voltage, VRX, at the receiver node or output line is proportional to the charge, Qin, applied to the transmission node or input line of the touch cell, divided by the mutual capacitance, Cm, between the transmission node or input line and the receiver node or output line. That is, VRXis proportional (or equal) to Qin/Cm.

Although the present disclosure has been described with reference to the example embodiments, those skilled in the art will recognize that various changes and modifications to the described embodiments may be performed, all without departing from the spirit and scope of the present disclosure. Furthermore, those skilled in the various arts will recognize that the present disclosure described herein will suggest solutions to other tasks and adaptations for other applications. It is the applicant's intention to cover by the claims herein, all such uses of the present disclosure, and those changes and modifications which could be made to the example embodiments of the present disclosure herein chosen for the purpose of disclosure, all without departing from the spirit and scope of the present disclosure. Thus, the example embodiments of the present disclosure should be considered in all respects as illustrative and not restrictive, with the spirit and scope of the present disclosure being indicated by the appended claims, and their equivalents.