CN112805665A

CN112805665A - Capacitive sensing system, sensing circuit of capacitive touch screen and sensing method

Info

Publication number: CN112805665A
Application number: CN201980001700.3A
Authority: CN
Inventors: 林永福; 庄朝贵; 徐嘉骏; 徐建昌; 徐荣贵
Original assignee: Shenzhen Goodix Technology Co Ltd
Current assignee: Shenzhen Goodix Technology Co Ltd
Priority date: 2019-08-29
Filing date: 2019-08-29
Publication date: 2021-05-14
Also published as: WO2021035617A1

Abstract

Provided are a sensing circuit of a capacitive touch screen, a capacitive sensing system and a sensing method of the capacitive touch screen. The sensing circuit includes a first conversion circuit (222), a first current mirror circuit (225), and a second conversion circuit (226). The first conversion circuit (222) is coupled to a sense node (110) of the capacitive touch screen for converting charge accumulated at the sense node into a first current according to a first input voltage. The first current mirror circuit (225) is coupled to the first conversion circuit (222) for generating a second current according to the first current, wherein the second current is a replica or a multiplication of the first current. The second conversion circuit (226) is coupled to the current mirror circuit (225) for generating an output voltage in response to at least the second current. The sensing circuit can substantially increase the contribution of capacitance value changes from the sensing node in the voltage variation range of the output voltage.

Description

Capacitive sensing system, sensing circuit of capacitive touch screen and sensing method

Technical Field

The present disclosure relates to capacitive sensing technologies, and in particular, to a sensing circuit and a sensing method for a capacitive touch screen, and a capacitive sensing system related thereto.

Background

When a touch object (such as a finger or a stylus pen) operates the capacitive touch screen, a sensing circuit of the capacitive touch screen can detect a capacitance value change of a capacitive node caused by the touch object and accordingly generate an output voltage as a detection result. In order to improve the accuracy of the operation, the variation range of the output voltage of the sensing circuit is usually increased as much as possible to increase the amount of signals generated due to the variation of the capacitance. However, a large portion of the output voltage of the sensing circuit is the contribution from the input voltage, and the contribution from the capacitance value change is only a small portion of the output voltage. Therefore, when the amplitude of the input voltage is increased to increase the variation range of the output voltage, a large part of the resultant variation range of the output voltage is still the contribution from the input voltage. Further, the variation range of the output voltage larger than the variation range of the power supply voltage cannot be obtained by arbitrarily increasing the amplitude of the input voltage, subject to the variation range of the power supply voltage. In the case where a large part of the variation range of the output voltage is the contribution from the input voltage, and the variation range of the output voltage is limited by the variation range of the power supply voltage, the contribution from the capacitance value variation in the variation range of the output voltage cannot be further increased.

Therefore, there is a need for an innovative capacitive sensing scheme that can increase the range of voltage output variation due to capacitance variation of the capacitive node.

Disclosure of Invention

An object of the present disclosure is to provide a sensing circuit and a sensing method for a capacitive touch screen, and a capacitive sensing system thereof, to solve the above problems.

An embodiment of the present disclosure provides a sensing circuit of a capacitive touch screen. The sensing circuit comprises a first conversion circuit, a first current mirror circuit and a second conversion circuit. The first conversion circuit is coupled to a sensing node of the capacitive touch screen and used for converting the charge accumulated by the sensing node into a first current according to a first input voltage. The first current mirror circuit is coupled to the first conversion circuit for generating a second current according to the first current, wherein the second current is a replica or a multiplication of the first current. The second conversion circuit is coupled to the current mirror circuit for generating an output voltage in response to at least the second current.

An embodiment of the present disclosure provides a capacitive sensing system. The capacitive sensing system includes a capacitive touch screen and a sensing circuit. The capacitive touch screen has sensing nodes that generate changes in capacitance values in response to touch events on the capacitive touch screen. The sensing circuit is used for sensing the change of the capacitance value. The sensing circuit comprises a first conversion circuit, a first current mirror circuit and a second conversion circuit. The first conversion circuit is coupled to the sensing node and is used for converting the charges accumulated by the sensing node into a first current according to a first input voltage. The first current mirror circuit is coupled to the first conversion circuit for generating a second current according to the first current, wherein the second current is a replica or a multiplication of the first current. The second conversion circuit is coupled to the current mirror circuit for generating an output voltage in response to at least the second current.

An embodiment of the present disclosure provides a sensing method of a capacitive touch screen. The sensing method comprises the following steps: converting charges accumulated at a sensing node of the capacitive touch screen into a first current according to a first input voltage; receiving the first current with a first current mirror circuit to generate a second current, wherein the second current is a replica or a multiplication of the first current; and generating an output voltage in response to at least the second current.

Drawings

Fig. 1 is a functional block schematic diagram of an embodiment of a capacitive sensing system of the present disclosure.

FIG. 2 is a schematic diagram of one embodiment of the sensing circuit shown in FIG. 1.

Correction 15.10.2019 in accordance with rules 91 fig. 3 is a schematic diagram of one embodiment of a sensing circuit for a capacitive touch screen.

FIG. 4 is a schematic diagram of another embodiment of the sensing circuit shown in FIG. 1.

FIG. 5 is a flow chart of an embodiment of a sensing method of the disclosed capacitive touch screen.

Wherein the reference numerals are as follows:

100 capacitive sensing system

110 capacitive touch screen

120. 220, 320, 420 sensing circuit

122. 222 first conversion circuit

125. 225 first current mirror circuit

126. 226 second conversion circuit

130 processing circuit

132 filter circuit

134 analog-to-digital conversion circuit

136 signal processor

223 voltage buffer

224. 228 amplifier

229 resistor-capacitor network

442 third conversion circuit

445 second current mirror circuit

502. 504 and 506 steps

C _LSensing capacitor

NS sensing node

R _L、R _FResistance (RC)

C _FCapacitor with a capacitor element

C _AAuxiliary capacitor

BFI input terminal

TI11, TI21, TI31 first input terminal

Second input terminal of TI12, TI22 and TI32

BFO, TO1, TO2 and TO3 output terminals

TE touch event

Vin first input voltage

Second input voltage of VA

Iout first current

Ix second current

IA third current

Iy fourth current

Vout output voltage

Vf filtered voltage

Vd digital signal

Vref reference voltage

CS control signal

Detailed Description

Certain terms are used throughout the description and following claims to refer to particular components. As one skilled in the art will appreciate, manufacturers may refer to a component by different names. This specification and the preceding claims do not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms "include" and "comprise" are used in an open-ended fashion, and thus should be interpreted to mean "include, but not limited to. Furthermore, the term "coupled" is intended to encompass any direct or indirect electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.

Furthermore, the terms "same," "equal to," and "similar," as used in the specification and the appended claims, mean that the difference between the numerical values of each other is within 10%, 5%, 1%, or 0.5% of a given value or range, or within an acceptable standard error.

Fig. 1 is a functional block schematic diagram of an embodiment of a capacitive sensing system of the present disclosure. The capacitive sensing system 100 can be disposed within an electronic device. The electronic device may be embodied as a portable electronic device, such as a mobile phone, tablet computer, laptop computer, or other type of portable electronic device. In this embodiment, the capacitive sensing system 100 can include (but is not limited to) a capacitive touch screen 110, a sensing circuit 120, and a processing circuit 130.

The capacitive touch screen 110 can employ self-capacitance (self-capacitance) or mutual-capacitance (mutual-capacitance) sensing technology to detect a touch event TE on the capacitive touch screen 110. The capacitive touch screen 110 may have a plurality of sensing nodes (or may be referred to as capacitive nodes); not shown in FIG. 1). For example, but not limiting to the present disclosure, the capacitive touch screen 110 may have a plurality of driving electrodes disposed in a first direction and a plurality of sensing electrodes disposed in a second direction (not shown in fig. 1), wherein the plurality of driving electrodes are disposed in one electrode layer (not shown in fig. 1) and the plurality of sensing electrodes are disposed in another electrode layer (not shown in fig. 1). Each sensing node may be located at, but is not limited to, an intersection of a driving electrode and a sensing electrode in the capacitive touch screen 110. When a touch event TE occurs (such as a touch object touching or approaching the capacitive touch screen 110), one or more sensing nodes of the capacitive touch screen 110 can generate a capacitance value change in response to the touch event TE to reflect the operational behavior of the touch object.

The sensing circuit 120 is coupled to a plurality of sensing nodes of the capacitive touch screen 110, and is configured to sense a capacitance value of each sensing node according to a first input voltage Vin (such as an analog voltage signal or an ac voltage) to generate an output voltage Vout (such as an analog voltage signal or an ac voltage). In this embodiment, the sensing circuit 120 may include, but is not limited to, a first converting circuit 122, a first current mirror circuit 125, and a second converting circuit 126. The first conversion circuit 122 is configured to convert the charges accumulated at the sensing nodes into a first current Iout according to the first input voltage Vin. That is, the first conversion circuit 122 can perform a charge-to-current conversion operation (charge-to-current conversion) on the charges accumulated at each sensing node. The first current mirror circuit 125 is coupled to the first conversion circuit 122 for generating a second current Ix according to the first current Iout. The second current Ix may be considered a replica or a multiplication of the first current Iout, wherein the ratio between the second current Ix and the first current Iout is adjustable. For example, the first current mirror circuit 125 may have a programmable gain (programmable gain). In addition, the second conversion circuit 126 is coupled to the first current mirror circuit 125 for generating the output voltage Vout at least in response to the second current Ix.

Through the first conversion circuit 122, the first current mirror circuit 125 and the second conversion circuit 126, the sensing circuit 120 can first convert the charges accumulated at each sensing node in response to the first input voltage Vin into the first current Iout, so as to generate the output voltage Vout according to the copy (or multiplication) of the first current Iout. Since the sensing circuit 120 may perform the charge amplification operation according to the current conversion result (the first current Iout or the second current Ix) of the first input voltage Vin, rather than directly using the first input voltage Vin, the sensing circuit 120 may reduce a contribution from a voltage variation range (such as a peak-to-peak value) of the first input voltage Vin in a voltage variation range (such as a peak-to-peak value) of the output voltage Vout. Further description will be described later.

The processing circuit 130 is coupled to the sensing circuit 120 for performing signal processing on the output voltage Vout to detect the touch event TE. For example, but not limiting to the disclosure, the processing circuit 130 may include a filtering circuit 132, an analog-to-digital conversion circuit 134, and a signal processor 136. The filter circuit 132 may filter the output voltage Vout to generate a filtered voltage Vf. For example, the filter circuit 132 may be implemented by a low-pass filter, such as an anti-aliasing filter (anti-alias filter), to reduce noise components in the output voltage Vout. The analog-to-digital conversion circuit 134 may convert the filtered voltage Vf into a digital signal Vd for the signal processor 136 to perform signal processing.

To facilitate understanding of the capacitive sensing scheme of the present disclosure, the self-capacitance sensing operation involved in one sensing node in the capacitive touch screen 110 shown in fig. 1 is described below. However, one skilled in the art will appreciate that the capacitive sensing scheme of the present disclosure may be applied to self-capacitive sensing operations involving multiple sensing nodes, as well as to mutual-capacitive sensing operations involving one or more sensing nodes. FIG. 2 is a schematic diagram of one embodiment of the sensing circuit 120 shown in FIG. 1. The sensing circuit 220 may include, but is not limited to, a first conversion circuit 222, a first current mirror circuit 225, and a second conversion circuit 226. The first conversion circuit 222, the first current mirror circuit 225, and the second conversion circuit 226 may be respectively used to implement the first conversion circuit 122, the first current mirror circuit 125, and the second conversion circuit 126 shown in fig. 1.

The first conversion circuit 222 is coupled to a sensing node NS of the capacitive touch screen 110 for converting charges accumulated at the sensing node NS into a first current Iout according to a first input voltage Vin. The sensing node NS is coupled to a sensing capacitor C_LWhich may correspond on behalf of sensing node NSThe driving electrode and the sensing electrode form an equivalent capacitance therebetween. Further, the first input voltage Vin may be implemented by an alternating voltage, such as a sine wave voltage. For example, but not limiting to the disclosure, taking the peak-to-peak value of the power voltage of the sensing circuit 220 as 2.6 volts as an example, the first input voltage Vin may be implemented with an ac voltage having a peak-to-peak value of 2 volts.

In this embodiment, the first conversion circuit 222 may include, but is not limited to, a voltage buffer 223. The input terminal BFI of the voltage buffer 223 is used for receiving the first input voltage Vin. Since the output terminal BFO of the voltage buffer 223 is coupled to the sensing node NS, the voltage buffer 223 may convert the charges accumulated at the sensing node NS into the first current Iout according to the buffered version of the first input voltage Vin (i.e., the buffered voltage Vbf).

For example, but the disclosure is not limited thereto, the voltage buffer 223 may be implemented by a voltage follower, such that the buffer voltage Vbf may be a replica of the first input voltage Vin. In this embodiment, the voltage follower can be implemented by an amplifier 224 having a first input TI11, a second input TI12, and the output TO 1. The first input terminal TI11 is used for receiving a first input voltage Vin. The second input TI12 and the output TO1 are connected TO each other and coupled TO the sensing node NS. In this embodiment, the equivalent resistance of the signal path (such as a signal line or a wire) between the sensing node NS and the second input terminal TI12 can be represented by the resistance R_LTo indicate. In addition, the output terminal TO1 is coupled TO the first current mirror circuit 224 for outputting the first current Iout.

The first current mirror circuit 225 is used for generating a second current Ix according to the first current Iout, which can be a replica or a multiplication of the first current Iout. It is noted that the first current Iout (which carries the charge accumulation information of the sense node NS) may flow through the output terminal TO1 TO the inside of the amplifier 224, rather than TO the outside of the amplifier 224 (such as from the output terminal BFO TO the first current mirror circuit 225). In order to enable the circuit located at the rear end of the first conversion circuit 222 to receive the information carried by the first current Iout, the sensing circuit 220 may utilize the first current mirror circuit 225 to generate the second current Ix flowing to the circuit located at the rear end of the first conversion circuit 222, so that the charge accumulation information of the sensing node NS carried by the first current Iout can be transmitted back for the rear end circuit (such as the second conversion circuit 226) to process.

The second conversion circuit 226 includes, but is not limited to, an amplifier 228 and a resistor-capacitor network 229. The amplifier 228 has a first input TI21, a second input TI22, and an output TO 2. The first input terminal TI21 is coupled to a reference voltage Vref, such as a common mode voltage or a ground voltage. The second input terminal TI22 is coupled to the first current mirror circuit 225 for receiving a replica or multiplication of the first current Iout, i.e., the second current Ix. The output terminal TO2 is used for outputting an output voltage Vout for a next stage circuit (such as the processing circuit 130 shown in fig. 1).

The resistor-capacitor network 229 is coupled between the second input TI22 and the output TO2 of the amplifier 228. In this embodiment, the RC network 229 may include (but is not limited to) a resistor R_FAnd a capacitor C_F. By applying a resistance R_FAnd a capacitor C_FArranged in parallel between the second input terminal TI22 and the output terminal TO2, the second converting circuit 226 can amplify the charge information accumulated at the sensing node NS carried in the second current Ix and generate the output voltage Vout accordingly.

In operation, when a touch object touches (or approaches) the capacitive touch screen 110, the capacitance at the sense node NS changes, i.e., the charge accumulated at the sense node NS changes. The first conversion circuit 222 may convert the charge accumulated at the sensing node NS into a first current Iout according to the first input voltage Vin. To simplify the circuit analysis, the first current Iout (function with real variable (time)) can be transformed into a function with complex variable (complex variable) s. For example, Laplace transform Iout(s) of the first current Iout is equal to a product of Laplace transform Vin(s) of the first input voltage Vin and a transfer function H1(s), and can be represented by the following equation (1):

where CL represents the capacitance of the sense node NS and RL represents the resistance R_LThe resistance value of (2).

The second conversion circuit 226 may generate the output voltage Vout in response to the second current Ix (a replica or multiplication of the first current Iout). In embodiments where the second current Ix is a replica of the first current Iout, the laplace transform Vout(s) of the output voltage Vout may be represented by the product of the laplace transform Iout(s) and the transfer function H2(s):

wherein CF represents a capacitance C_FRF represents the resistance R_FThe resistance value of (2).

By substituting equation (1) for equation (2), a transfer function H3(s) is obtained that reflects the mapping relationship between the laplace transform vout(s) and the laplace transform vin(s):

compared to a manner of directly performing the charge amplification operation using the first input voltage Vin, the sensing circuit 220 may perform the charge amplification operation according to a current conversion result of the first input voltage Vin, such as a copy (or multiplication) of the first current Iout, thereby greatly reducing a ratio of a voltage variation range (such as a peak-to-peak value) of the first input voltage Vin to a voltage variation range (such as a peak-to-peak value) of the output voltage Vout. For example, fig. 3 shows an embodiment in which a current signal generated by directly applying the first input voltage Vin to the sensing node NS is used for performing a charge amplification operation. In the sensing circuit 320 shown in fig. 3, the laplace transform Vout(s) of the output voltage Vout can be represented by the product of the laplace transform Vin(s) of Vin and the transfer function H4(s).

It should be noted that the charge accumulated on the sensing node NS varies with the voltage level of the first input voltage Vin, which causes the capacitance of the sensing node NS and the current level of the first current Iout to vary, thereby changing the voltage level of the output voltage Vout. Therefore, the product of the laplace transform Vin(s) and the transfer function H3(s) may correspond to the voltage variation range generated by the capacitance value variation of the sensing node NS in response to the first input voltage Vin, and the laplace transform Vin(s) corresponds to the voltage variation range of the first input voltage Vin.

In the case where the voltage variation range (peak-to-peak value) of the power voltage of the sensing circuit 320 is, but not limited to, 2.6 volts, and the voltage variation range (peak-to-peak value) of the output voltage Vout is set at, but not limited to, 2 volts to maintain the circuit operation, since the absolute value of the transfer function H3(s) is usually much smaller than 1, the 2 volt voltage variation range of the output voltage Vout is mostly a contribution from the voltage variation range (peak-to-peak value; corresponding to the laplace transform Vin (s)) of the first input voltage Vin. The voltage variation range corresponding to the change in capacitance value of the sensing node NS (corresponding to the product of the laplace transform vin(s) and the transfer function H3 (s)) occupies a relatively small proportion in this 2-volt voltage variation range. For example, the voltage variation range of the first input voltage Vin accounts for 80% of the voltage variation range of the output voltage Vout, and the corresponding voltage variation range in which the capacitance value of the sensing node NS varies accounts for only 20% of the voltage variation range of the output voltage Vout. Even if the voltage variation range corresponding to the capacitance value variation of the sensing node NS is increased by increasing the amplitude of the first input voltage Vin, the amplitude of the first input voltage Vin is increased by a limited amount due to the limitation of the voltage variation range of the power supply voltage, so that the contribution of the capacitance value variation from the sensing node NS in the voltage variation range of the output voltage Vout cannot be further increased.

In contrast, in the embodiment shown in FIG. 2, the voltage variation range (peak-to-peak) of the power supply voltage at the sensing circuit 220The value) is (but not limited to) 2.6 volts, and the voltage variation range (peak-to-peak value) of the output voltage Vout is set to (but not limited to) 2 volts to maintain the circuit operation, since the laplace transform Vout(s) is equal to the product of the laplace transform vin(s) and the transfer function H3(s), rather than the product of the transfer function H4(s) (the addition of 1 and the transfer function H3(s), the 2 volt voltage variation range of the output voltage Vout is the voltage variation range corresponding to the capacitance value variation of the sensing node NS. That is, the capacitive sensing scheme of the present disclosure can greatly reduce the ratio of the voltage variation range of the first input voltage Vin to the voltage variation range of the output voltage Vout, so that the contribution of the capacitance value variation from the sensing node NS in the voltage variation range of the output voltage Vout can be increased. Furthermore, in some embodiments, the resistance R may also be increased_FFurther increases the voltage variation range corresponding to the capacitance variation of the sensing node NS.

FIG. 4 is a schematic diagram of another embodiment of the sensing circuit 120 shown in FIG. 1. The structure of the sensing circuit 420 shown in fig. 4 is substantially the same as/similar to the sensing circuit 220 shown in fig. 2, and the difference between the two is that the sensing circuit 420 further includes a compensation circuit structure to more accurately obtain the capacitance value variation at the sensing node NS. For example, when a touch event TE occurs (such as a touch object touching/approaching the capacitive touch screen 110), the sensing node NS can generate a capacitance value change in response to the touch event TE, which can be represented by the sensing capacitance C_LParallel capacitance Δ C_LTo indicate. The sensing circuit 420 can adopt a compensation circuit structure to obtain the capacitance Δ C more accurately_LThe related information of (2).

In this embodiment, the sensing circuit 420 further includes (but is not limited to) an auxiliary capacitor C_AA third conversion circuit 442 and a second current mirror circuit 445, wherein the auxiliary capacitor C_AThe third conversion circuit 442 and the second current mirror circuit 445 may be at least a part of the compensation circuit structure of the sensing circuit 420. For example, the first conversion circuit 222, the first current mirror circuit 225, the second conversion circuit 226, and the auxiliary capacitor C_AA third conversion circuit 442 andthe second current mirror circuitry 445 may be implemented on the same chip.

Auxiliary capacitance C_AMay be equal to the capacitance value that the sense node NS had before the touch event TE occurred, such as the sense capacitance C_LThe capacitance value of (2). The third converting circuit 442 is used for converting the auxiliary capacitor C according to a second input voltage VA_AThe accumulated charges are converted into a third current IA, wherein the second input voltage VA has a phase opposite to that of the first input voltage Vin. For example, in case that the first input voltage Vin is implemented by a sine wave voltage, the second input voltage VA may be a sine wave voltage 180 degrees out of phase with the first input voltage Vin, wherein the first input voltage Vin and the second input voltage VA may have the same amplitude.

The circuit structure of the third converting circuit 442 can be substantially the same as that of the first converting circuit 222 to simulate the situation where the first converting circuit 222 generates a current signal in response to the charge accumulated at the sensing node NS before the touch event TE occurs. In this embodiment, the third converting circuit 442 may include a resistor R_AAnd a voltage buffer 443. Resistance R_ACan be equal to the resistance R_LThe resistance value of (2). The voltage buffer 443 can be implemented by an amplifier 444 having a first input TI31, a second input TI32 and an output TO 3. The first input terminal TI31 is used for receiving the second input voltage VA. The second input TI32 and the output TO3 are connected TO each other via a resistor R_AIs coupled to the auxiliary capacitor C_A. In addition, the output terminal TO3 is used for outputting a third current IA.

The second current mirror circuit 445 is coupled to the third converting circuit 442 for generating a fourth current Iy, which is a replica or a multiple of the third current IA, according to the third current IA. The current gain of the second current mirror circuitry 445 may be the same/similar to the current gain of the first current mirror circuitry 225. For example, the difference between the current gain of the second current mirror circuitry 445 and the current gain of the first current mirror circuitry 225 may be within 10%, 5%, 1%, or 0.5% of the current gain of the first current mirror circuitry 225.

In operation, when a touch event TE occurs, in addition to the second current Ix carrying the capacitance value change to the second conversion circuit 226, a fourth current Iy carrying capacitance value information before the touch event TE occurs may also be transferred to the second conversion circuit 226. The second conversion circuit 226 may generate the output voltage Vout in response to the second current Ix and the fourth current Iy. It is noted that the second input voltage VA and the first input voltage Vin have the same amplitude but opposite phase, and the auxiliary capacitor C_AAnd a sensing capacitor C_LThe capacitance values can be the same, so the sensing capacitor C in the third current IA and the first current Iout_LThe current components generated in response to the first input voltage Vin are identical in amplitude but opposite in phase to each other. That is, the third current IA generated by the third converting circuit 442 can be used to eliminate/reduce the sense capacitance C before the touch event TE occurs_LIn response to the current generated by the first input voltage Vin. Thus, the fourth current Iy generated by the second current mirror circuitry 445 may be used to eliminate/reduce the sense capacitance C in the second current Ix_LIn response to the current component generated by the first input voltage Vin, the information carried by the output voltage Vout generated by the second conversion circuit 226 can be mostly derived from the capacitor Δ C_LThe capacitance value of (a) changes.

In some embodiments, the first current mirror circuit 225 may adaptively adjust the ratio between the second current Ix and the first current Iout, thereby improving the sensing quality. For example, when the ambient noise is large, the first current mirror circuit 225 may reduce the current gain according to a control signal CS, which may be generated by a back-end circuit (such as the signal processor 136 shown in fig. 1) through noise detection. In some embodiments, in the case that the first current mirror circuit 225 can adjust the ratio between the second current Ix and the first current Iout according to the control signal CS, the second current mirror circuit 445 can also adjust the ratio between the fourth current Iy and the third current IA according to the control signal CS.

It is noted that the foregoing is illustrative and is not intended to limit the present disclosure. For example, the first converting circuit 222 may be implemented by using other charge-to-current converters. That is, any conversion circuit that can convert the charges accumulated at the sensing node NS according to the first input voltage Vin to generate the first current Iout is within the scope of the present disclosure.

The capacitive sensing scheme of the present disclosure can be briefly summarized as a flow chart shown in fig. 5. FIG. 5 is a flow chart of an embodiment of a sensing method of the disclosed capacitive touch screen. If the results are substantially the same, the steps do not have to be performed in the order shown in FIG. 5. For example, certain steps may be interposed therein. For convenience of description, the sensing method shown in fig. 5 will be described below in conjunction with the capacitive touch screen 110 and the sensing circuit 420 shown in fig. 4. However, it is feasible to apply the sensing method shown in fig. 5 to the sensing circuit 120 shown in fig. 1 and/or the sensing circuit 220 shown in fig. 2. The sensing method shown in fig. 5 can be briefly summarized as follows.

Step 502: the charge accumulated at a sensing node of the capacitive touch screen is converted into a first current according to a first input voltage. For example, the first conversion circuit 222 converts the charge accumulated at the sense node NS into a first current Iout.

Step 504: a first current mirror circuit is used for receiving the first current to generate a second current, wherein the second current is a copy or multiplication of the first current. For example, the first current Iout is received by the first current mirror circuit 225 to generate the second current Ix.

Step 506: an output voltage is generated in response to at least the second current. For example, the second conversion circuit 226 generates the output voltage Vout in response to at least the second current Ix. In some embodiments, the capacitance value of the sensing node NS varies in response to the first input voltage Vin to generate a voltage variation range equal to the voltage variation range of the output voltage Vout. That is, the change in the capacitance of the sensing node NS with the first input voltage Vin causes the charge accumulated on the sensing node NS to change,

in step 502, the first input voltage may be coupled to an input of a voltage buffer, and the sense node may be coupled to an output of the voltage buffer to convert the charge accumulated at the sense node into the first current. For example, the first input voltage Vin may be received from the input terminal BFI of the voltage buffer 223 in the first conversion circuit 222, and the sensing node NS may be coupled to the output terminal BFO of the voltage buffer 223 to convert the charge accumulated at the sensing node NS into the first current Iout.

In some embodiments, the capacitive sensing scheme of the present disclosure can also more accurately obtain the capacitance value change at the sensing node in response to the touch event through current compensation. For example, in step 506, the third converting circuit 442 may convert the auxiliary capacitor C according to the second input voltage VA_AThe accumulated charges are converted into a third current IA, wherein the second input voltage VA has a phase opposite to that of the first input voltage Vin, and an auxiliary capacitor C_AIs equal to the capacitance value that the sense node NS had before the touch event TE occurred (such as the sense capacitance C)_LThe capacitance value of). The second current mirror circuitry 445 receives the third current IA to generate a fourth current Iy. Next, the second converting circuit 226 can generate the output voltage Vout in response to the second current Ix and the fourth current Iy, and most of the information carried by the output voltage Vout can be derived from the capacitor Δ C_LThe capacitance value of (a) changes.

Since the details of each step in the sensing method shown in fig. 5 can be understood by those skilled in the art after reading the paragraphs related to fig. 1 to fig. 4, further description is omitted here for brevity.

Through the charge-current conversion operation, the capacitive sensing scheme of the present disclosure can greatly reduce the proportion of the input voltage variation range in the output voltage variation range. In addition, the capacitive sensing scheme of the present disclosure may further improve sensing quality by having a current mirror circuit with programmable gain, and/or a compensation circuit structure.

The above description is only an example of the present disclosure and is not intended to limit the present disclosure, and various modifications and changes may be made to the present disclosure by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present disclosure should be included in the protection scope of the present disclosure.

Claims

A sensing circuit for a capacitive touch screen, comprising:

the first conversion circuit is coupled to a sensing node of the capacitive touch screen and used for converting the charge accumulated by the sensing node into a first current according to a first input voltage;

a first current mirror circuit coupled to the first conversion circuit for generating a second current according to the first current, wherein the second current is a replica or a multiplication of the first current; and

a second conversion circuit coupled to the first current mirror circuit for generating an output voltage in response to at least the second current.
The sensing circuit of claim 1, wherein the first conversion circuit comprises a voltage buffer; the input end of the voltage buffer is used for receiving the first input voltage; an output of the voltage buffer is coupled to the sensing node and the first current mirror circuit for outputting the first current.
The sensing circuit of claim 1, wherein the first conversion circuit comprises a voltage follower; a first input terminal of the voltage follower is used for receiving the first input voltage; a second input of the voltage follower is coupled to the sense node; the output end of the voltage follower is coupled to the first current mirror circuit for outputting the first current; the second input end of the voltage follower and the output end of the voltage follower are connected with each other.
The sensing circuit of any of claims 1-3, wherein a voltage variation range generated by a change in capacitance value of the sensing node in response to the first input voltage is equal to a voltage variation range of the output voltage.
The sensing circuit of any one of claims 1 to3, wherein the first current mirror circuit has a programmable gain.
The sensing circuit of any of claims 1-3, wherein the second conversion circuit comprises:

an amplifier having a first input coupled to a reference voltage, a second input coupled to the first current mirror circuit, and an output to output the output voltage; and

and the resistor-capacitor network is coupled between the second input end and the output end.
The sensing circuit of any of claims 1-3, wherein the sensing node generates a change in capacitance value in response to a touch event on the capacitive touch screen, the sensing circuit further comprising:

an auxiliary capacitance, wherein a capacitance value of the auxiliary capacitance is equal to a capacitance value that the sensing node had before the touch event occurred;

a third conversion circuit configured to convert the electric charge accumulated in the auxiliary capacitance into a third current according to a second input voltage having a phase opposite to that of the first input voltage; and

a second current mirror circuit coupled to the third conversion circuit for generating a fourth current according to the third current;

wherein the second conversion circuit is configured to generate the output voltage in response to the second current and the fourth current.
The sensing circuit of claim 7, wherein the second input voltage has the same amplitude as the first input voltage.
The sensing circuit of any of claims 1-3, wherein a relationship between the first current and the first input voltage is represented by:

where iout(s) is the laplace transform of the first current, vin(s) is the laplace transform of the first input voltage, CL is the capacitance value of the sensing node, and RL is the resistance value of the signal path between the sensing node and the first conversion circuit.
The sensing circuit of claim 9, wherein the second switching circuit comprises an amplifier, a resistor, and a capacitor; a first input of the amplifier is coupled to a reference voltage; the resistor and the capacitor are connected in parallel between the second input end of the amplifier and the output end of the amplifier; the output end of the amplifier is used for outputting the output voltage; the relationship between the output voltage and the first input voltage is represented by:

where VOUT(s) is the Laplace transform of the output voltage, RF is the resistance value of the resistor, and CF is the capacitance value of the capacitor.
A capacitive sensing system, comprising.

A capacitive touch screen having a sensing node that generates a change in capacitance value in response to a touch event on the capacitive touch screen; and

at least one sensing circuit according to any one of claims 1 to 10, for sensing a change in said capacitance value.
A sensing method of a capacitive touch screen is characterized by comprising the following steps:

converting charges accumulated at a sensing node of the capacitive touch screen into a first current according to a first input voltage;

receiving the first current with a first current mirror circuit to generate a second current, wherein the second current is a replica or a multiplication of the first current; and

an output voltage is generated in response to at least the second current.
The sensing method of claim 12, wherein converting the charge accumulated at the sensing node into the first current based on the first input voltage comprises:

coupling the first input voltage to an input of a voltage buffer; and

coupling the sense node to an output of the voltage buffer to convert charge accumulated at the sense node into the first current.
The sensing method of claim 12 or 13, wherein a voltage variation range generated by the capacitance value variation of the sensing node in response to the first input voltage is equal to a voltage variation range of the output voltage.
The sensing method of claim 12 or 13, wherein the sensing node generates a change in capacitance value in response to a touch event on the capacitive touch screen; the step of generating the output voltage in response to at least the second current comprises:

converting the charge accumulated by the auxiliary capacitance into a third current according to a second input voltage, wherein the second input voltage has a phase opposite to that of the first input voltage, and a capacitance value of the auxiliary capacitance is equal to a capacitance value that the sensing node has before the touch event occurs;

receiving the third current by a second current mirror circuit to generate a fourth current; and

generating the output voltage in response to the second current and the fourth current.
The sensing method of claim 15, wherein the second input voltage has the same amplitude as the first input voltage.