TWI470527B - An integrated circuit that achieves touch capacitance sensing with charge sharing - Google Patents

Description

An accumulative circuit for sensing capacitance sensing by charge sharing

The invention relates to a circuit for sensing a touch capacitance, in particular to an integrated circuit for achieving touch capacitance sensing by charge sharing.

With the popularization of various 3C products, the demand for touch panels is also increasing. Generally speaking, the touch panel can be roughly classified into a resistive type, a capacitive type, an ultrasonic type, or an infrared type according to the sensing principle. At present, the most cost-effective touch panel technology on the market is a resistive system, and the global touch panel market share is also the highest in resistance. The resistive system is a standard glass panel consisting of two layers of ITO conductively laminated; the two layers are separated by a spacer and current can flow between the two layers. In use, the pressure is used to make the upper and lower sets of ITO conductive contacts, and the change of the electric field is regarded as a contact event. Finally, the signal is transmitted to a controller for processing, and the panel voltage is detected by the controller. Calculate the contact point position for input.

On the other hand, the capacitive touch panel uses a capacitive sensor to sense the change in capacitance generated by the electrostatic combination between the transparent electrodes arranged in the human body and the human body, and to detect the coordinates from the induced current generated. When the user's finger touches the panel, current is continuously passed through the sensor, allowing the sensor to store electrons horizontally and vertically to form a precisely controlled capacitive field. When the finger touches a different position, the so-called normal capacitance field of the sensor is changed by another capacitance field, which is set in each corner of the panel. The circuit calculates the degree of change in the electric field and then transmits this contact event signal to the controller for processing. Compared with the resistive touch panel, the capacitive touch panel exhibits better performance and is more convenient to use. However, the capacitive touch panel is disadvantageous in terms of process steps, and its driver IC and circuit are also complicated, so it is disadvantageously applied to small and medium-sized products in terms of cost and technological progress.

The first figure shows a schematic diagram of a circuit that utilizes a relaxation oscillator to achieve capacitive sensing in the prior art. As shown in the first figure, the relaxation oscillator 101 periodically charges and discharges a capacitor Cx. Among them, the oscillation frequency is related to the Cx capacitance value and the charge and discharge current, that is, CdV=I dt. In the case where the current of the charge and discharge is constant, the change of Cx changes the frequency of the relaxation oscillator 101, and the frequency comparator 103 compares the output frequency Fro of the relaxation oscillator 101 with a reference of a fixed reference clock 102. The difference in frequency Fref. Since the relaxation oscillator 101 charges and discharges the capacitor Cx, and the impedance on the Cx is usually not low, the Cx of the method is susceptible to interference from external environmental noise.

The second figure shows a schematic diagram of a circuit that utilizes charge transfer to achieve capacitive sensing in the prior art. As shown in the second figure, the circuit for charge transfer includes a capacitor Csum, a capacitor Cx, a comparator 201, and three switches S1, S2, and S3, respectively connected as follows: one end of the capacitor Cx passes through the switch S1 and a voltage source. VDD is connected and the other end is grounded. One end of the capacitor Csum is grounded, and the other end is connected to the end of the capacitor Cx that is connected to the switch S1 through the switch S2, and the voltage value thereof is represented by Vsum; further, the two ends of the capacitor Csum are respectively connected through the switch S3. The voltage value Vsum is input to the forward input terminal of the comparator 201, and the reverse input terminal is input with a reference voltage value Vref. The output of comparator 201 is represented by Vo.

The operation of the above circuit is as follows: firstly, the capacitor Csum is discharged to the grounding level through the switch S3, and then the switches S1 and S2 are alternately connected with a non-overlapping clock to gradually transfer the charge on the capacitor Cx to the capacitor. On Csum, therefore, the voltage value Vsum gradually increases; when the voltage value Vsum is higher than the voltage value Vref, the output Vo will be shifted from the low level to the high level, as shown in the third figure. The time Tcf to the Vo transition state is calculated from the discharge of Csum. The larger the capacitance value of the capacitor Cx, the shorter the transition time. The advantage of this circuit is that it has better resistance to external environmental noise interference, because the capacitance Cx is kept at a low impedance during charging, and when the charge is transferred, the capacitance value of the capacitance Csum is large, usually the capacitance Cx capacitance value. Hundreds of times, so it is also a low impedance component. Therefore, the capacitor Cx is kept in a low impedance state and has better resistance to radio frequency interference. On the other hand, the disadvantage of this circuit is that the capacitance value of the capacitor Csum must be hundreds to thousands times the capacitance value of the capacitance Cx (for example, a capacitance value of several hundred pF to nF), and thus cannot be integrated. Another disadvantage is that the charge transfer mode, the voltage value Vsum is accumulated and nonlinear, but the voltage value Vsum and the capacitance Cx capacitance value are exponential, in other words, . It is also less linear to calculate the difference in capacitance Cx capacitance.

Therefore, how to improve the shortcomings of the above-mentioned conventional technologies has become an important issue in the development of capacitive touch panel technology.

Based on the above-mentioned shortcomings of the prior art, the main object of the present invention is to provide an integrable circuit that achieves touch capacitance sensing by charge sharing, and by designing a clock to control charging and discharging of a shared capacitor to reduce sharing. The capacitance value of the capacitor is used for the purpose of integration.

An embodiment of the present invention discloses an integrable circuit that achieves touch capacitance sensing by charge sharing, including: a charge sharing circuit and a voltage controlled oscillator (VCO). a reference frequency generator, a switch clock generator, and a frequency comparison circuit; wherein the charge sharing circuit further includes a sensing capacitor and a sharing capacitor. And performing charge sharing by charging the charge of the sensed capacitor and discharging the shared capacitor, and accumulating a voltage on the shared capacitor; the voltage controlled oscillator is coupled to the shared capacitor of the charge sharing circuit, Generating an output frequency linearly related to the magnitude of the sense capacitance of the charge sharing circuit; the reference frequency generator provides a reference frequency; the switch clock generator is coupled to the output of the voltage controlled oscillator, the voltage is The output of the controlled oscillator generates a non-overlapping switching clock; and the frequency comparison circuit is coupled to the output of the voltage controlled oscillator and the reference frequency provided by the reference frequency generator, respectively counting the voltage controlled oscillator The output frequency and the reference frequency, when the output frequency of the voltage controlled oscillator is changed by the frequency of the finger touch, it can be judged that There are touching situation.

A more detailed and clear understanding of the objects, functions and structural features of the present invention is intended to be

101‧‧‧ Relaxation Oscillator

102‧‧‧Reference clock

103‧‧‧frequency comparator

201‧‧‧ Comparator

410‧‧‧Charge sharing circuit

4101‧‧‧Sense Capacitance

4102‧‧‧Touch Capacitance

4103‧‧‧Charge sharing capacitor

4104‧‧‧Charge switch

4105‧‧‧Charge sharing switch

4106‧‧‧Discharge switch

420‧‧‧Variable Control Oscillator

430‧‧‧Reference frequency generator

440‧‧‧Switching Clock Generator

4401‧‧‧Delephone

4402‧‧‧ Non-overlapping clock generator

450‧‧‧frequency comparison circuit

The first figure shows a schematic diagram of a circuit using a relaxation oscillator to achieve capacitive sensing in the prior art.

The second figure shows a schematic diagram of a circuit that utilizes charge transfer in the prior art to achieve capacitive sensing.

The third figure shows a schematic diagram of the time Tcf calculated by the capacitance sensing circuit in the prior art in the second figure from the time after the Csum discharge is calculated to the Vo transition state.

The fourth figure shows an integrable circuit that achieves touch capacitance sensing by charge sharing.

The fifth figure shows the waveform of the switch enable signal in the fourth figure.

The sixth figure shows the structure of the switch clock generator.

Figure 7 shows an embodiment of the waveforms of the switch clock generators Fo, Fod, EN1, EN2, and EN3 of the present invention.

The fourth figure shows an integrable circuit for sensing capacitance sensing by charge sharing, comprising: a charge sharing circuit 410, a voltage controlled oscillator (VCO) 420, and a reference frequency. The generator 430, a switch clock generator 440, and a frequency comparison circuit 450; wherein the charge sharing circuit 410 further includes a sensing capacitor 4101, a touch capacitor 4102, and a charge. Sharing a capacitor 4103, a charging switch 4104, a charge sharing switch 4105, and a discharging switch 4106, and performing charge sharing by charging the charge of the sensed capacitor 4101 and discharging the shared capacitor 4103 through charge sharing. Sharing the voltage on the shared capacitor 4103; the voltage controlled oscillator 420 is coupled to the shared capacitor 4103 of the charge sharing circuit 410, and generates an output frequency Fo linearly related to the magnitude of the sensing capacitor 4101 of the charge sharing circuit 410; the reference frequency The generator 420 is provided with a reference frequency Fref; the switch clock generator 440 is connected to the output end of the voltage controlled oscillator 420, and the voltage controlled oscillation The output frequency Fo of the device 420 generates a non-overlapping switching clock to control the charging switch 4104, the charge sharing switch 4105, and the discharging switch 4106 of the charge sharing circuit 410, wherein the charging switch 4104 is connected to an external power source VDD and The sensing capacitor 4101 is connected to control the charging of the sensing capacitor 4101; the charge sharing switch 4105 is respectively connected between the sensing capacitor 4101 and the ungrounded end of the charge sharing capacitor 4103. To control the sharing of the charge; and the discharge switch 4106 is connected in parallel with the charge sharing capacitor 4103 to control the discharge of the charge sharing capacitor; and the frequency comparison circuit 450 is coupled to the output of the voltage controlled oscillator 420. The reference frequency generator provides a reference frequency, respectively counting the output frequency of the voltage controlled oscillator and the reference frequency 430. When the output frequency Fo of the voltage controlled oscillator 420 is changed by a finger touch, the frequency can be determined. Is there a touch condition?

It should be noted that in the general capacitive touch application, the capacitance value Cx of the sensing capacitor 4101 of the charge sharing circuit 410 ranges from about 5 to 100 pF; therefore, in the present embodiment, the capacitance of the shared capacitor 4103 is shared. Csh is designed as a capacitor with adjustable capacitance (5~100pF) to adjust the appropriate Csh for Cx generated by different touch applications. Moreover, in addition to the sensing capacitance value Cx and the touch capacitance value Cf, no large capacitance is required, and an external large capacitance is also avoided to achieve the goal of integrated circuitization.

The charge sharing circuit 410 operates as follows: when the Fo output is 1, the enable signal EN1 of the charge switch 4104 is 1, so the charge switch 4104 is turned off, and the enable signals EN2, EN3 of the charge share switch 4105 and the discharge switch 4106 are 0, so the charge sharing switch 4105 and the discharge switch 4106 are turned on; in this case, the sensing capacitor 4101 is charged to the level of VDD. The fifth figure shows the waveform of the switch enable signal in the fourth figure. As shown in the fifth figure, most of Fo's period is maintained at the high level (1). Before Fo is turned from the high level (1) to the low level (0), the enable signal EN3 of the discharge switch 4106 is set to 1, so the discharge switch 4106 is turned off. After the sharing capacitor 4103 is discharged to the grounding level, Fo is turned from the high level (1) to the low level (0), EN1, EN3=0, and the charging switch 4104 and the discharging switch 4106 are simultaneously turned on. Then, the enable signal EN2=1, the charge sharing switch 4105 is turned off for charge sharing. When the charge sharing is completed, the enable signal EN2=0, the charge sharing switch 4105 When it is turned on, the enable signal EN1=1 is turned on, and the charging switch 4104 is turned off.

On the other hand, when Fo transitions from the low level (0) to the high level (1), the enable signal EN1=1 is maintained, and the charge switch 4104 is turned off. The charge sharing switch 4105 and the discharge switch 4106 are turned off at a time between one tenth and one ten thousandth of a Fo period. The charging switch 4104, the discharging switch 4106 and the charge sharing switch 4105 are controlled by non-overlapping clocks, the time for turning off the charging switch 4104 is extended, and the time for turning off the charge sharing switch 4105 and the discharging switch 4106 is minimized. Reduce interference from external environmental noise. Therefore, the charge sharing circuit 410 can greatly reduce the interference of external environment noise by shortening the charge sharing time. The charge sharing circuit 410 can complete the charge sharing only at the clock cycle of one Fo, and immediately changes the voltage of Vsh according to the presence or absence of a finger touch, and immediately after the charge sharing is completed. The clock frequency of the next Fo will also change according to the change of Vsh. As shown in the fourth figure, Fo and Vsh will change after touching the finger; in addition, the voltage of Vsh is as shown in the following formula: Vsh=VDD*(Cx+Cf))/((Cx+Cf)+Csh); It can be seen from the above formula that Vsh is linearly related to Cx and Cf. In other words, the sensed capacitance value can be linearly converted into a measured voltage.

The voltage controlled oscillator 420 is a converter whose output frequency is linearly related to the input voltage. The voltage Vsh generated by charge sharing can be generated by the voltage controlled oscillator 420 to produce a linearly correlated frequency. In other words, it is through the voltage / frequency conversion to achieve the goal of measuring the change in capacitance value. The capacitance value when there is no finger touch is Cx, the voltage Vsh accumulated on Csh after charge sharing; the capacitance value Cx+Cf when there is a finger touch, and the voltage Vsh' accumulated on Csh after charge sharing, which is known by the formula Vsh'>Vsh, and then through the voltage controlled oscillator 420 will also produce a frequency of Fo'>Fo. The difference between Fo and Fo' can be used as a reference using the reference frequency Fref generated by the reference frequency generator 430. The frequency comparison circuit 450 determines the difference in frequency.

The sixth diagram shows the structure of a switch clock generator 440. As shown in the sixth figure, the switch clock generator 440 further includes a frequency divider 4401 and a non-overlapping clock generator 4402. The function of the frequency divider 4401 is to let the frequency of the voltage-controlled oscillator 420 change the frequency of the charge-sharing circuit 410 from the original Fo cycle by one time (the frequency divider is divided by one). ), convert to charge sharing every N Fo cycles (divider divided by N). In this way, by using the frequency divider to switch the clock generator, the overall circuit can achieve the goal of low power consumption in the application of slow touch under the condition that the voltage of the voltage controlled oscillator is constant. The non-overlapping clock generator 4402 can generate the control enable clock of the charging switch 4104, the charge sharing switch 4105, and the discharging switch 4106 from the clock of the frequency Fod after the frequency division, and achieve the time when the charging switch 4104 is turned off as much as possible. When elongated, the charge sharing switch 4105 is turned off as much as possible in a state in which charge sharing can be completed. The discharge switch 4106 is turned off as short as possible to completely discharge Csh to the ground level to achieve the goal of minimizing interference from external environmental noise. And the charge sharing switch 4105 and the charging switch 4104 and the discharging switch 4106 must be a non-overlapping clock. Among them, the waveforms of Fo, Fod, EN1, EN2, and EN3 can be as shown in the seventh figure and the frequency divider in the figure is divided by two.

Therefore, the integrated circuit for performing touch capacitance sensing by charge sharing disclosed in the present invention can achieve the intended purpose and effect by the disclosed technology, and conforms to the novelty and progress of the invention patent. The nature of sexuality and industrial use.

The illustrations and descriptions of the present invention are merely preferred embodiments of the present invention, and are not intended to limit the implementation of the present invention. The changes and modifications of the spirit of the present invention are intended to be included in the scope of the claims of the present invention.

410‧‧‧Charge sharing circuit

4101‧‧‧Sense Capacitance

4102‧‧‧Touch Capacitance

4103‧‧‧Charge sharing capacitor

4104‧‧‧Charge switch

4105‧‧‧Charge sharing switch

4106‧‧‧Discharge switch

420‧‧‧Variable Control Oscillator

430‧‧‧Reference frequency generator

440‧‧‧Switching Clock Generator

450‧‧‧frequency comparison circuit

Claims (14)

An accumulative circuit for sensing capacitance sensing by charge sharing comprises: a charge sharing circuit, further comprising a sensing capacitor and a shared capacitor, the charged sensing capacitor is charged by way of charge sharing The charge is shared with the shared capacitor that has been discharged to accumulate voltage on the shared capacitor; a voltage controlled oscillator coupled to the shared capacitor of the charge sharing circuit produces an output linearly related to the magnitude of the sense capacitor a reference frequency generator that generates a reference frequency; a frequency comparison circuit coupled to the output frequency of the voltage controlled oscillator and the reference frequency, respectively counting the output frequency and the reference frequency, when the output frequency is due to a finger A change in frequency by touch can determine whether there is a touch condition; and a switching clock generator generates a set of non-overlapping switching clocks according to the output frequency of the voltage controlled oscillator. An integrated circuit for achieving touch capacitance sensing by charge sharing as described in claim 1 wherein the shared capacitor is a capacitor capable of adjusting a capacitance value to generate a sense for different touch applications. Measure the capacitance value to adjust the appropriate shared capacitance value. The power sharing circuit further includes a touch capacitor, a charging switch, a charge sharing switch, and a discharge, as described in claim 1, wherein the charge sharing circuit achieves touch capacitance sensing. a switch capacitor is connected in parallel with the sensing capacitor, and the charging switch is connected between an external power source and the sensing capacitor to control charging of the sensing capacitor; the charge sharing relationship is respectively connected to the sensing A capacitor is coupled between the ungrounded end of the charge-sharing capacitor to control charge sharing; and the discharge-on relationship is coupled in parallel with the charge-sharing capacitor to control discharge of the charge-sharing capacitor. An integrated circuit for achieving touch capacitance sensing by charge sharing as described in claim 3, wherein the switch clock generator is coupled to the output of the voltage controlled oscillator, according to the voltage The output frequency of the controlled oscillator produces a set of non-overlapping switching clocks to control the charge sharing circuit, the charge sharing switch, and the discharge switch of the charge sharing circuit. An integrable circuit for achieving charge-capacitance sensing by charge sharing as described in claim 4, wherein the charge sharing circuit operates as follows: when the output of the voltage controlled oscillator is at a high level ( 1), the enabling signal of the charging switch is 1, so the charging switch is turned off, and the enabling signal of the charge sharing switch and the discharging switch is 0, so the charge sharing switch and the discharging switch are turned on; in this case The sensing capacitor is charged to the level of an external power supply; when the output of the voltage controlled oscillator is changed from the low level (0) to the high level (1), the enable signal is 1, The charging switch is turned off; before the output of the voltage controlled oscillator is changed from the high level (1) to the low level (0), the enabling signal of the discharging switch is 1, so the discharging switch is turned off; After discharging to the grounding level, the output of the voltage controlled oscillator is changed from a high level (1) to a low level (0), and the enabling signal of the charging switch and the discharging switch is 0, the charging switch and the discharging The switch is turned on at the same time; then the enable signal is 1, and the charge sharing switch is turned off to Line charge sharing; when the charge sharing is completed, the enable signal is 0, the charge sharing switch is turned on, and then the enable signal is 1, and the charge switch is turned off. An accumulative circuit for achieving charge-capacitance sensing by charge sharing as described in claim 5, wherein the charge-distributing switch and the discharge switch are turned off at a time interval of a voltage-controlled oscillator. One tenth to one ten thousandth. An integrable circuit capable of achieving touch capacitance sensing by charge sharing as described in claim 5, wherein the charge sharing circuit only needs to complete the charge at a clock cycle of the output of a voltage controlled oscillator. Share and immediately stand behind the charge sharing That is, the voltage of the shared capacitor is changed; and the clock frequency of the output of the next voltage controlled oscillator is also changed according to the change of the voltage of the shared capacitor. The accumulative circuit for achieving touch capacitance sensing by charge sharing as described in claim 5, wherein the sensed capacitance value can be linearly converted into a measurement voltage. The accumulative circuit for achieving touch capacitance sensing by charge sharing as described in claim 6 wherein the charge switch, the discharge switch and the charge sharing switch are controlled by non-overlapping clocks. An accumulative circuit for achieving touch capacitance sensing by charge sharing as described in claim 6 wherein the charging switch is turned off, and the charge sharing switch and the discharging switch are turned off. The shortest time is to reduce the interference of external environmental noise. The accumulative circuit for achieving touch capacitance sensing by charge sharing as described in claim 5, wherein the charging switch remains in a closed state for most of the clock period. The accumulative circuit for achieving touch capacitance sensing by charge sharing as described in claim 1 is wherein the frequency comparison circuit counts the difference in output frequency of the voltage controlled oscillator based on the reference frequency. Determine if there is a finger touch. An integrable circuit for achieving charge-capacitance sensing by charge sharing, as described in claim 1, wherein the switch clock generator further includes a frequency divider and a non-overlapping clock generator. An accumulative circuit for achieving charge-capacitance sensing by charge sharing as described in claim 13 wherein the frequency of the voltage-controlled oscillator is such that the frequency of the voltage-controlled oscillator does not change. And the charge sharing circuit performs charge sharing from the cycle of each of the original output voltages of the voltage controlled oscillator, and converts into a cycle of output frequency of each of the N voltage controlled oscillators to perform charge sharing, so that the whole electric The road's slow touch applications achieve low power consumption and low noise.