Capacitive touch sensing circuit and charge compensation method thereof

1. Field of the Invention

The invention relates to a touch panel; in particular, to a capacitive touch sensing circuit and a charge compensation method thereof applied to a capacitive touch panel.

2. Description of the Prior Art

In general, the capacitive touch panel can be designed in different types such as in-cell, on-cell or out-cell. In the process of using a capacitive touch panel, it is generally necessary to offset the external capacitor to improve the internal charge detection range.

However, as the external capacitance increases, the internal pre-filled capacitor must also increase with the same amount, resulting in a large increase in the area of the compensation capacitor required in the capacitive touch sensing circuit, which needs to be overcome.

Therefore, the invention provides a capacitive touch sensing circuit and a charge compensation method thereof applied to a capacitive touch panel to solve the above-mentioned problems of the prior arts.

A preferred embodiment of the invention is a capacitive touch sensing circuit. In this embodiment, the capacitive touch sensing circuit includes a first switch, a second switch, a third switch, a fourth switch, a fifth switch, an internal capacitor, an integrator and a capacitor. The first switch and an external capacitor are coupled in series between a first voltage and a ground voltage, one terminal of the second switch is coupled between the first switch and the external capacitor, and one terminal of the third switch is coupled to the other terminal of the second switch. The fourth switch and the internal capacitor are coupled in series between a second voltage and the ground voltage, wherein the second voltage is a negative value of the first voltage, and one terminal of the fifth switch is coupled to the other terminal of the second switch and the other terminal of the fifth switch is coupled between the fourth switch and the internal capacitor. One input terminal of the integrator is coupled to the other terminal of the third switch, the other input terminal of the integrator is coupled to a reference voltage, and the output terminal of the integrator outputs an output voltage. The capacitor is coupled between the input terminal and the output terminal of the integrator.

In an embodiment, the capacitive touch sensing circuit is operated in a first phase, a second phase, a third phase and a fourth phase in order.

In an embodiment, when the capacitive touch sensing circuit is operated in the first phase, the first switch and the fourth switch are conducted and the second switch, the third switch and the fifth switch are not conducted.

In an embodiment, when the capacitive touch sensing circuit is operated in the second phase, the second switch and the fifth switch are conducted and the first switch, the third switch and the fourth switch are not conducted.

In an embodiment, when the capacitive touch sensing circuit is operated in the third phase, the second switch and the fourth switch are conducted and the first switch, the third switch and the fifth switch are not conducted.

In an embodiment, when the capacitive touch sensing circuit is operated in the fourth phase, the second switch and the fifth switch are conducted and the first switch, the third switch and the fourth switch are not conducted.

In an embodiment, the capacitive touch sensing circuit is operated in the third phase and the fourth phase repeatedly until a voltage between the second switch and the fifth switch approaches the reference voltage and then the third switch is conducted and the first switch, the second switch, the fourth switch and the fifth switch are not conducted.

Another preferred embodiment of the invention is a charge compensation method. In this embodiment, the charge compensation method is applied to a capacitive touch sensing circuit. The capacitive touch sensing circuit includes a first switch, a second switch, a third switch, a fourth switch, a fifth switch, an internal capacitor, an integrator and a capacitor. The first switch and an external capacitor are coupled in series between a first voltage and a ground voltage, one terminal of the second switch is coupled between the first switch and the external capacitor, and one terminal of the third switch is coupled to the other terminal of the second switch. The fourth switch and the internal capacitor are coupled in series between a second voltage and the ground voltage, wherein the second voltage is a negative value of the first voltage, and one terminal of the fifth switch is coupled to the other terminal of the second switch and the other terminal of the fifth switch is coupled between the fourth switch and the internal capacitor. One input terminal of the integrator is coupled to the other terminal of the third switch, the other input terminal of the integrator is coupled to a reference voltage, and the output terminal of the integrator outputs an output voltage. The capacitor is coupled between the input terminal and the output terminal of the integrator.

The charge compensation method includes steps of: (a) when the capacitive touch sensing circuit is operated in the first phase, the first switch and the fourth switch are conducted and the second switch, the third switch and the fifth switch are not conducted; (b) when the capacitive touch sensing circuit is operated in the second phase, the second switch and the fifth switch are conducted and the first switch, the third switch and the fourth switch are not conducted; (c) when the capacitive touch sensing circuit is operated in the third phase, the second switch and the fourth switch are conducted and the first switch, the third switch and the fifth switch are not conducted; and (d) when the capacitive touch sensing circuit is operated in the fourth phase, the second switch and the fifth switch are conducted and the first switch, the third switch and the fourth switch are not conducted.

Compared to the prior art, the capacitive touch sensing circuit and the charge compensation method thereof applied to the capacitive touch panel according to the present invention can utilize the charge compensation mechanism of repeatedly compensating the external capacitor to effectively offset the relatively large external capacitance by the relatively small capacitance inside the circuit. So that it can be applied to the capacitive sensing environment under heavy load, effectively reducing the compensation capacitance area required in the capacitive touch sensing circuit and indeed improving the internal capacitance sensing range.

The advantage and spirit of the invention may be understood by the following detailed descriptions together with the appended drawings.

FIG. 1 illustrates a schematic diagram of the capacitive touch sensing circuit in a preferred embodiment of the invention.

FIG. 2 illustrates a flowchart of the charge compensation method in another preferred embodiment of the invention.

FIG. 3 illustrates timing diagrams of the first control signal PH 1 , the second control signal PH 2 , the third control signal PH 3 , the fourth control signal PH(K), the fifth control signal PH(K+1), the voltage Vn″ between the first switch and the external capacitor and the output voltage Vout in an embodiment respectively.

A preferred embodiment of the invention is a capacitive touch sensing circuit applied to a capacitive touch panel. In this embodiment, the capacitive touch sensing circuit is configured to sense a capacitance change of an external capacitance to be detected when the capacitive touch panel is touched and to suppress external environmental noise. The structure of the capacitive touch panel can be designed in different types such as in-cell, on-cell or out-cell.

Please refer to FIG. 1 . FIG. 1 illustrates a schematic diagram of the capacitive touch sensing circuit in this embodiment.

As shown in FIG. 1 , the capacitive touch sensing circuit 1 is coupled to an external capacitor Cext to be detected and an analog-to-digital converter ADC. The capacitive touch sensing circuit 1 includes a first switch SW 1 , a second switch SW 2 , a third switch SW 3 , a fourth switch SW 4 , a fifth switch SW 5 , an internal capacitor Cin, an integrator INT and a capacitor Cf. The first switch SW 1 , the second switch SW 2 , the third switch SW 3 , the fourth switch SW 4 and the fifth switch SW 5 are respectively controlled by a first control signal PH 1 , a second control signal PH 2 , a third control signal PH 3 , and a fourth control signal PH (K) and a fifth control signal PH (K+1).

The first switch SW 1 and the external capacitor Cext are coupled in series between the first voltage V 1 and the ground voltage GND. One terminal of the second switch SW 2 is coupled between the first switch SW 1 and the external capacitor Cext. One terminal of the third switch SW 3 is coupled to the other terminal of the second switch SW 2 . The fourth switch SW 4 and the internal capacitor Cin are coupled in series between the second voltage (−V 1 ) and the ground voltage GND, and the second voltage (−V 1 ) is a negative value of the first voltage V 1 . One terminal of the fifth switch SW 5 is coupled to the other terminal of the second switch SW 2 and the other terminal of the fifth switch SW 5 is coupled between the fourth switch SW 4 and the internal capacitor Cin. The input terminal + of the integrator INT is coupled to the other terminal of the third switch SW 3 , the input terminal − of the integrator INT is coupled to the reference voltage VREF and the output terminal K of the integrator INT outputs an output voltage Vout to the analog-to-digital converter ADC. The capacitor Cf is coupled between the input terminal + and the output terminal K of the integrator INT.

In this embodiment, the capacitive touch sensing circuit 1 will be operated in the first phase, the second phase, the third phase and the fourth phase. The detail will be described as follows:

When the capacitive touch sensing circuit 1 is operated in the first phase, the first switch SW 1 and the fourth switch SW 4 are respectively controlled by the first control signal PH 1 and the fourth control signal PH(K) to be conducted, and the second switch SW 2 , the third switch SW 3 and the fifth switch SW 5 are respectively controlled by the second control signal PH 2 , the third control signal PH 3 and the fifth control signal PH(K+1) to be non-conducted. Since the first switch SW 1 is conducted, the first voltage V 1 can charge the external capacitor Cext, and the amount of charge stored in the external capacitor Cext is Cext*V 1 . Similarly, since the fourth switch SW 4 is conducted, the second voltage (−V 1 ) can charge the internal capacitor Cin, and the amount of charge stored in the internal capacitor Cin is Cin*(−V 1 ).

When the capacitive touch sensing circuit 1 is operated in the second phase, the second switch SW 2 and the fifth switch SW 5 are respectively controlled by the second control signal PH 2 and the fifth control signal PH(K+1) to be conducted, and the first switch SW 1 , the third switch SW 3 , and the fourth switch SW 4 are respectively controlled by the first signal PH 1 , the third control signal PH 3 and the fourth control signal PH(K) respectively to be non-conducted. Since the second switch SW 2 and the fifth switch SW 5 are both conducted, the external capacitor Cext storing the charge amount Cext*V 1 and the internal capacitor Cin storing the charge amount Cin*(−V 1 ) can be connected to each other, so that the voltage level of them becomes V 1 ′ and V 1 ′=(Cext−Cin)*V 1 /(Cext+Cin).

When the capacitive touch sensing circuit 1 is operated in the third phase, the second switch SW 2 and the fourth switch SW 4 are respectively controlled by the second control signal PH 2 and the fourth control signal PH(K) to be conducted, and the first switch SW 1 , the third switch SW 3 and the fifth switch SW 5 are respectively controlled by the first control signal PH 1 , the third control signal PH 3 and the fifth control signal PH(K+1) to be non-conducted. Since the fifth switch SW 5 is not conducted, the external capacitor Cext is not coupled to the internal capacitor Cin, and since the first switch SW 1 is not conducted, the first voltage V 1 does not charge the external capacitor Cext, so that the external capacitor Cext is maintained at the voltage level V 1 ′. In addition, since the fourth switch SW 4 is conducted, the second voltage (−V 1 ) can charge the internal capacitor Cin again, so the voltage level of the internal capacitor Cin becomes (−V 1 ).

When the capacitive touch sensing circuit 1 is operated in the fourth phase, the second switch SW 2 and the fifth switch SW 5 are respectively controlled by the second control signal PH 2 and the fifth control signal PH(K+1) to be conducted, and the first switch SW 1 , the third switch SW 3 and the fourth switch SW 4 are respectively controlled by the first control signal PH 1 , the third control signal PH 3 and the fourth control signal PH(K) to be non-conducted. Since the second switch SW 2 and the fifth switch SW 5 are both conducted, the external capacitor Cext storing the charge amount Cext*V 1 ′ and the internal capacitor Cin storing the charge amount Cin*(−V 1 ) can be connected to each other, and the voltage level of them becomes V 2 ′ and V 2 ′=(Cext*V 1 ′−Cin*V 1 )/(Cext+Cin).

Then, the capacitive touch sensing circuit 1 can be operated in the third phase and the fourth phase n times until the voltage Vn′ between the second switch SW 2 and the fifth switch SW 5 approaches the reference voltage VREF, and Vn′=[Cext*V(n−1)′−Cin*V 1 ]/(Cext+Cin). At this time, the capacitive touch sensing circuit 1 controls the third switch SW 3 to be conducted through the third control signal PH 3 and controls the first switch SW 1 , the second switch SW 2 , the fourth switch SW 4 and the fifth switch SW 5 to be non-conducted through the first control signal PH 1 , the second control signal PH 2 , the fourth control signal PH(K) and the fifth control signal PH(K+1) respectively. Since the positive input terminal + and the negative input terminal − of the integrator INF respectively receive the voltage Vn′ and the reference voltage VREF and the voltage Vn' approaches the reference voltage VREF, the voltage change amount of the integrator INF will approach 0. Therefore, after the output voltage Vout outputted by the integrator INF is converted into the digital signal by the analog-to-digital converter ADC, it will be located at the output midpoint which can cover the largest variation range.

It should be noted that the number n of times that the capacitive touch sensing circuit 1 and repeatedly operated in the third phase and the fourth phase and the frequency thereof will be related to the resistance-capacitance value of the overall path in the circuit and the ratio of the internal capacitance value to the external capacitance value, not limited to a specific number of times or frequency.

In addition, in the above embodiment, although the time point at which the third switch SW 3 is conducted occurs after the end of the multiple charge compensations, in fact, the time point at which the third switch SW 3 is conducted can occur before the multiple charge compensations or during the multiple charge compensations. There are no specific restrictions.

Another preferred embodiment of the invention is a charge compensation method. In this embodiment, the charge compensation method is applied to a capacitive touch sensing circuit. The capacitive touch sensing circuit includes a first switch, a second switch, a third switch, a fourth switch, a fifth switch, an internal capacitor, an integrator and a capacitor. The first switch and an external capacitor are coupled in series between a first voltage and a ground voltage, one terminal of the second switch is coupled between the first switch and the external capacitor, and one terminal of the third switch is coupled to the other terminal of the second switch. The fourth switch and the internal capacitor are coupled in series between a second voltage and the ground voltage, wherein the second voltage is a negative value of the first voltage, and one terminal of the fifth switch is coupled to the other terminal of the second switch and the other terminal of the fifth switch is coupled between the fourth switch and the internal capacitor. One input terminal of the integrator is coupled to the other terminal of the third switch, the other input terminal of the integrator is coupled to a reference voltage, and the output terminal of the integrator outputs an output voltage. The capacitor is coupled between the input terminal and the output terminal of the integrator.

Please refer to FIG. 2 . FIG. 2 illustrates a flowchart of the charge compensation method in this embodiment.

As shown in FIG. 2 , the charge compensation method applied to the capacitive touch sensing circuit can include steps of:

step S 10 : when the capacitive touch sensing circuit is operated in the first phase, the first switch and the fourth switch are conducted and the second switch, the third switch and the fifth switch are not conducted;

step S 12 : when the capacitive touch sensing circuit is operated in the second phase, the second switch and the fifth switch are conducted and the first switch, the third switch and the fourth switch are not conducted;

step S 14 : when the capacitive touch sensing circuit is operated in the third phase, the second switch and the fourth switch are conducted and the first switch, the third switch and the fifth switch are not conducted;

step S 16 : when the capacitive touch sensing circuit is operated in the fourth phase, the second switch and the fifth switch are conducted and the first switch, the third switch and the fourth switch are not conducted;

step S 18 : determining whether a voltage between the second switch and the fifth switch approaches the reference voltage;

if the determination result of the step S 18 is no, then repeating the steps S 14 and S 16 ; and if the determination result of the step S 18 is yes, then step S 20 is performed to conduct the third switch and the first switch, the second switch, the fourth switch and the fifth switch are not conducted.

Please refer to FIG. 3 . FIG. 3 illustrates timing diagrams of the first control signal PH 1 , the second control signal PH 2 , the third control signal PH 3 , the fourth control signal PH(K), the fifth control signal PH(K+1), the voltage Vn″ between the first switch SW 1 and the external capacitor Cext and the output voltage Vout in an embodiment respectively.

As shown in FIG. 3 , at the first time t 1 , when the first control signal PH 1 is pulled up, the voltage Vn″ between the first switch SW 1 and the external capacitor Cext will be pulled to the first voltage V 1 (e.g., 3 volts). At the second time t 2 , when the second control signal PH 2 is pulled up, the voltage Vn between the first switch SW 1 and the external capacitor Cext will be stably pulled down to approach the reference voltage VREF (e.g., 1.5 volts) by repeatedly switching the fourth control signal PH(K) and the fifth control signal PH(K+1). Assuming that the voltage Vn″ drops to approach the reference voltage VREF at the third time t 3 , then the third control signal PH 3 is pulled up at the third time t 3 such that the voltage Vn″ is equal to the reference voltage VREF (e.g., 1.5 volts), and the resulting charge change will cause the voltage change of the output voltage Vout as shown in the circle in FIG. 3 .

Compared to the prior art, the capacitive touch sensing circuit and the charge compensation method thereof applied to the capacitive touch panel according to the present invention can utilize the charge compensation mechanism of repeatedly compensating the external capacitor to effectively offset the relatively large external capacitance by the relatively small capacitance inside the circuit. So that it can be applied to the capacitive sensing environment under heavy load, effectively reducing the compensation capacitance area required in the capacitive touch sensing circuit and indeed improving the internal capacitance sensing range.

With the example and explanations above, the features and spirits of the invention will be hopefully well described. Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.