Capacitance voltage conversion circuit, input apparatus using the same, electronic instrument, and capacitance voltage conversion method

[0001] The present application is a continuation application of U.S. patent application Ser. No. 13/367,904, filed on Feb. 7, 2012, the entire contents of which are incorporated herein by reference and priority to which is hereby claimed. Application Ser. No. 13/367,904 claims the benefit of priority under 35 U.S.C. §119(a) and 35 U.S.C. §365(b) from Japanese Patent Applications Nos. 2011-025019, filed on Feb. 8, 2011, and 2011-182223, filed on Aug. 24, 2011, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a capacitance voltage conversion circuit which is used to measure capacitance.

[0003] Recent electronic instrument such as computers, portable terminals, and Personal Digital Assistants (PDAs) generally include an input device for manipulating the electronic instrument by touching the input device with a finger or approaching a finger to the input device. As such an input device, joysticks, touch pads, etc. have been known.

[0004] Further, as a sensor, capacitance sensors using capacitance have been known. The capacitance sensors may include a sensor electrode. When a user approaches or touches the sensor electrode, capacitance generated by the sensor electrode is changed. By converting the capacitance change into an electric signal with a capacitance voltage conversion circuit, whether a user touches the sensor electrode or not is determined.

[0005] Touch panels may be configured with a plurality of sensor electrodes. Touch panels having an X-Y matrix type include row sensor electrodes that are disposed in each row of the matrix, and column sensor electrodes that are disposed in each column of the matrix. By detecting the capacitance change of each of a plurality of sensor electrodes, coordinates touched by a user are specified.

[0006] A conventional capacitance detection circuit generally detects the capacitances of a plurality of sensor electrodes in a time division method. For example, an X-Y matrix type touch panel sequentially detects the capacitances of the column sensor electrodes, and sequentially detects the capacitances of the row sensor electrodes. In such a method, since the timings of capacitance detection for the respective sensor electrodes are different, the sensor electrodes may be respectively affected by different noises.

[0007] The present disclosure provides some embodiments of a capacitance voltage conversion circuit that can simultaneously detect the capacitances of a plurality of sensor electrodes.

[0008] According to one embodiment of the present disclosure, provided is a capacitance voltage conversion circuit, which converts a capacitance of each of a plurality of sensor capacitors into a voltage, including: a plurality of capacitance current conversion circuits disposed in respective correspondence with the sensor capacitors, each of the capacitance current conversion circuits configured to generate a detection current corresponding to the capacitance of the corresponding sensor capacitor; a current average circuit configured to average the detection currents, which are generated by the capacitance current conversion circuits, to generate an average current; and a plurality of current voltage conversion circuits disposed in respective correspondence with the sensor capacitors, each of the current voltage conversion circuits configured to convert a difference current between the corresponding detection current and the average current into a voltage.

[0009] In the present embodiment, each of the detection currents may correspond to a capacitance of a corresponding sensor capacitor, and the average current may correspond to an average capacitance of the capacitances of the sensor capacitors. Therefore, a voltage that is generated by the current voltage conversion circuit of each channel may indicate a difference between the capacitance of the sensor capacitor of each channel and an average capacitance of the capacitances of the sensor capacitors of all channels. According to such a configuration, the capacitances of the respective sensor capacitors can be detected simultaneously.

[0010] According to another embodiment of the present disclosure, provided is a capacitance voltage conversion circuit, which converts a capacitance of each of a plurality of sensor capacitors into a voltage, including: a plurality of reset switches disposed in respective correspondence with the sensor capacitors, each of the reset switches configured to initialize an electric potential of a corresponding sensor capacitor; a plurality of integral capacitors disposed in respective correspondence with the sensor capacitors, an electric potential of one end of each integral capacitor being fixed; an initialization circuit configured to initialize respective voltages of the integral capacitors; a plurality of current mirror circuits disposed in respective correspondence with the sensor capacitors, each of the current mirror circuits configured to supply a current, which flows in a second transistor of an output side of each of the current mirror circuits, to a corresponding integral capacitor in a first direction, a first transistor of an input side of each of the current mirror circuits being connected to a corresponding sensor capacitor; and a current average circuit configured to generate an average current of a plurality of detection currents which respectively flow in the sensor capacitors, and supply the average current to the integral capacitors in a second direction.

[0011] According to the present embodiment, in each channel, an integral capacitor may be charged (or discharged) by a detection current of a corresponding channel, and the capacitances of the respective sensor capacitors can be simultaneously detected by discharging (or charging) a corresponding capacitor by the average current.

[0012] According to another embodiment of the present disclosure, provided is a capacitance voltage conversion circuit, which converts a capacitance of each of a plurality of sensor capacitors into a voltage, including: a plurality of integral capacitors disposed in respective correspondence with the sensor capacitors, an electric potential of one end of each of the integral capacitors being fixed; a plurality of reset switches disposed in respective correspondence with the sensor capacitors, each of the reset switches being connected to a corresponding sensor capacitor in parallel; a plurality of sensing switches disposed in respective correspondence with the sensor capacitors, one end of each of the sensing switches being connected to a corresponding sensor capacitor; a plurality of first transistors, being MOSFETs, disposed in respective correspondence with the sensor capacitors, each of the first transistors being disposed on a path of a corresponding sensing switch; a plurality of second transistors, being MOSFETs, disposed in respective correspondence with the sensor capacitors, a gate of each of the second transistors being connected to a gate of a corresponding first transistor, and a drain of each of the second transistors being connected to a corresponding integral capacitor; a plurality of fifth transistors, being MOSFETs, disposed in respective correspondence with the sensor capacitors, a gate of each of the fifth transistors being connected to a gate of a corresponding first transistor; a plurality of third transistors disposed on a path of a corresponding fifth transistor, in respective correspondence with the sensor capacitors; and a plurality of fourth transistors disposed in respective correspondence with the sensor capacitors, each of the fourth transistors being connected to a corresponding third transistor to form a current mirror circuit therewith, and a drain of each of the fourth transistors being connected to a corresponding integral capacitor.

[0013] According to the present embodiment, the capacitances of the respective sensor capacitors can be detected simultaneously.

[0014] According to another embodiment of the present disclosure, provided is an input device. The input device may include: a plurality of sensor capacitors; and a capacitance voltage conversion circuit configured to convert a capacitance of each of a plurality of sensor capacitors into a voltage. The sensor capacitors may be disposed in a matrix type.

[0015] Moreover, an arbitrary combination of the above elements or a change of an implementation of the present disclosure in methods and devices are effective.

[0025] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The same or equal elements, members, and processing illustrated in each of the drawings are indicated by the same reference numerals, and a repetitive description is not provided. Moreover, the embodiments do not limit the present disclosure but exemplify the present disclosure, and all features described in aspects or combination thereof may or may not be essential to the present disclosure.

[0026] In the specification, ┌a member A being connected to a member B┘ includes the case where the member A and the member B are physically and directly connected to each other, and the case where the member A and the member B are indirectly connected to each other through another member that does not affect an electrical connection.

[0027] Likewise, “a member C being prepared between the member A and the member B” includes the case where the member A and the member C are directly connected to each other or the member B and the member C are directly connected to each other, and the case where the member A and the member C are indirectly connected to each other through another member that does not affect an electrical connection or the member B and the member C are indirectly connected to each other through another member that does not affect an electrical connection.

[0028] FIG. 1 is a diagram illustrating a configuration of an electronic instrument 1 including an input device 2 according to an embodiment. The electronic instrument 1 includes a Digital Signal Processor (DSP) 6 and a Liquid Crystal Display (LCD) 7, in addition to the input device 2. The input device 2 includes a touch panel 3 and a control IC 4. The touch panel 3 includes a plurality of sensor capacitors C_S1 to C_Sn that are disposed regularly. The sensor capacitors C_S1 to C_Sn are substantially disposed in a matrix type. The control IC 4 is connected to the sensor capacitors C_S1 to C_Sn, detects the capacitances of the sensor capacitors C_S1 to C_Sn, and outputs data, indicating each of the capacitances, to the DSP 6.

[0029] When a user of the electronic instrument 1 touches the touch panel 3 with a finger 5, a pen or the like, the capacitance of the sensor capacitor C_S corresponding to touched coordinates is changed. The DSP 6 detects the coordinates touched by the user, on the basis of the capacitances of a plurality of sensor capacitors C_S. For example, the touch panel 3 may be disposed at a surface of the LCD 7, or disposed at another position of the LCD 7.

[0030] The above description has been made on the entire configuration of the electronic instrument 1. Next, the input device 2 will be described in detail.

[0031] FIG. 2 is a block diagram illustrating a configuration of the control IC 4 according to the embodiment. The control IC 4 includes a Capacitance/Voltage conversion circuit (C/V conversion circuit) 100, a multiplexer 40, and an Analog/Digital converter (A/D converter) 50, which are integrated on one semiconductor substrate. A portion of the function of the DSP 6 may also be built in the control IC 4.

[0032] The C/V conversion circuit 100 converts the capacitance of each of the sensor capacitors C_S1 to C_Sn into a detection voltage corresponding to the capacitance. As described below, the detection voltages of the respective sensor capacitors C_S are simultaneously generated and held. A plurality of buffers BUF₁ to BUF_n respectively receive detection voltages V_S1 to V_Sn, and output the received detection voltages to the multiplexer 40. The multiplexer 40 sequentially selects the detection voltages V_S1 to V_Sn in a time division method. The A/D converter 50 sequentially converts the respective detection voltages V_S selected by the multiplexer 40 into digital values D_OUT.

[0033] The C/V conversion circuit 100 converts the capacitances of the sensor capacitors C_S1 to C_Sn into detection voltages V_S1 to V_Sn, respectively. The C/V conversion circuit 100 includes a plurality of Capacitance/Current conversion circuits (C/I conversion circuits) 10₁ to 10_n, a current average circuit 20, and a plurality of Current/Voltage conversion circuits (I/V conversion circuits) 30₁ to 30_n.

[0034] The C/I conversion circuits 10₁ to 10_n are disposed in respective correspondence with the sensor capacitors C_S1 to C_Sn. The C/I conversion circuit 10_i (where 1≦i≦n) generates a detection current I_Si that corresponds to the capacitance of the corresponding sensor capacitor C_Si, and outputs the detection current I_Si to the corresponding I/V conversion circuit 30_i, and the current average circuit 20.

[0035] The current average circuit 20 averages a plurality of the detection currents I_S1 to I_Sn that are respectively generated by the C/I conversion circuits 10₁ to 10_n. An averaged detection current I_AVE (hereinafter referred to as an average current) is supplied to the I/V conversion circuits 30₁ to 30_n:
I_AVE=Σ_i=1:nI_Si/n  (1)

[0036] The I/V conversion circuits 30₁ to 30_n are disposed in respective correspondence with the sensor capacitors C_S1 to C_Sn. The I/V conversion circuit 30_i, converts a difference current I_DIFFi (=I_Si−I_AVE) between the corresponding detection current I_Si and the average current I_AVE into a voltage, and outputs the converted voltage as the detection voltage V_Si.

[0037] FIG. 3 is a circuit diagram illustrating a detailed configuration example of the control IC 4. In FIG. 3, only portions respectively corresponding to the sensor capacitors C_S1 and C_S2 are illustrated.

[0038] The C/I conversion circuit 10_i includes a reset switch SW1, a sensing switch SW2, a first transistor M1, and a second transistor M2.

[0039] The reset switch SW1 is disposed for initializing the electric charge of a corresponding sensor capacitor C_Si. For example, the reset switch SW1 may be disposed in parallel to the sensor capacitor C_Si. When the reset switch SW1 is turned on, the electric charge of the sensor capacitor C_Si is discharged and thus initialized. That is, a potential difference between both ends of the sensor capacitor C_Si becomes zero. For example, the reset switch SW1 may include an N-channel MOSFET, and when a reset signal RST applied to a gate of the N-channel MOSFET is asserted (high level), the reset switch SW1 is turned on.

[0040] The sensing switch SW2 and the first transistor M1 are disposed sequentially and serially between the sensor capacitor C_Si and a fixed voltage terminal (herein, a power source terminal). The sensing switch SW2 is a P-channel MOSFET, and when a sensing signal EVALB inputted to a gate of the P-channel MOSFET is asserted (low level), the sensing switch SW2 is turned on.

[0041] The first transistor M1 is a P-channel MOSFET. Specifically, a drain of the first transistor M1 is connected to the sensor capacitor C_Si through the sensing switch SW2. A source of the first transistor M1 is connected to the power source terminal. In addition, an electric connection is formed between a gate and the drain of the first transistor M1. A charging current I_CHGi, which corresponds to the capacitance of a corresponding sensor capacitor C_Si, flows in the first transistor M1.

[0042] The second transistor M2 is a P-channel MOSFET of the same type as the first transistor M1, and is connected to the first transistor M1 to form a current mirror circuit therewith. Specifically, a gate of the second transistor M2 is connected to a gate of the first transistor M1, and a source of the second transistor M2 is connected to the power source terminal. A detection current I_Si, which corresponds to the capacitance of a corresponding sensor capacitor C_Si, flows in the second transistor M2. When a current ratio (size ratio) of the first and second transistors M1 and M2 is K1, the detection current I_Si is expressed as Equation (2):
I_Si=I_CHGi×K₁  (2)

[0043] The current mirror circuit configured with the first and second transistors M1 and M2 supplies the detection current I_Si, flowing in the second transistor M2, to a corresponding integral capacitor C_INTi in a first direction (charging direction in FIG. 3).

[0044] The current average circuit 20 includes a plurality of third transistors M3, a plurality of fourth transistors M4, and a plurality of fifth transistors M5.

[0045] The fifth transistors M5 are disposed in respective correspondence with the sensor capacitors C_Si. Each of the fifth transistors M5 is a MOSFET of the same type as the first transistor M1, and connected to a corresponding first transistor M1 to form a current mirror circuit therewith, thereby generating a current I_Si′ which corresponds to a corresponding detection current I_Si.

[0046] The third transistors M3 are disposed on a path of the current I_Si′ in respective correspondence to the sensor capacitors C_Si. Control terminals (gates) of the respective third transistors M3 are connected to each other in common Specifically, a source of each of the third transistors M3 is grounded, and a drain of each of the third transistors M3 is connected to a drain of a corresponding fifth transistor M5.

[0047] The fourth transistors M4 are disposed in respective correspondence with the sensor capacitors C_Si. Each of the fourth transistors M4 is connected to a corresponding third transistor M3 to form a current mirror circuit therewith. The size of each fourth transistor M4 is the same as that of each third transistor M3, in all channels. A current flowing in each fourth transistor M4 is the averaged detection current I_AVE, and is supplied to a corresponding I/V conversion circuit 30. Specifically, the current average circuits 20 respectively supply a plurality of average currents I_AVE to a plurality of integral capacitors C_INTi in a second direction (discharging direction in FIG. 3).

[0048] Each of the I/V conversion circuits 30 includes an integral capacitor C_INTi and an initialization switch SW3_i. One end of the integral capacitor C_INTi is grounded, and an electric potential thereof is fixed. A corresponding detection current I_Si is supplied to the integral capacitor C_INTi in the first direction (charging direction), and a corresponding average current I_AVE is supplied to the integral capacitor C_INTi in the second direction (discharging direction). As a result, the integral capacitor C_INTi is charged or discharged by the difference current I_DIFFi(=I_Si−I_AVE).

[0049] The initialization switch SW3_i functions as an initialization circuit that initializes the voltage of the integral capacitor C_INTi before detection. One end of the initialization switch SW3_i is connected to the integral capacitor C_INTi and a reference voltage V_CM is applied to the other end of the initialization switch SW3_i by a buffer (voltage follower) 52. The initialization switch SW3_i may be a transfer gate, or may be another switch. When an initialization signal VCM_SW is asserted, the initialization switch SW3_i is turned on. The reference voltage V_CM, for example, may be a voltage in the vicinity of a central point between a source voltage Vdd and a ground voltage Vss.

[0050] In FIG. 3, the multiplexer 40 of FIG. 2 is illustrated as switches SW4₁ to SW4_n of respective channels. In addition, in FIG. 3, the A/D converter 50 of FIG. 2 is divided into two A/D converters ADC1 and ADC2. The detection voltages V_S1, V_S3, . . . of respective odd channels are allocated to the A/D converter ADC1, and the detection voltages V_S2, V_S4, . . . of respective even channels are allocated to the A/D converter ADC2. The outputs of the respective switches SW4₁, SW4₃, . . . of the odd channels are connected to each other in common, and connected to an input of the A/D converter ADC1. The outputs of the respective switches SW4₂, SW4₄, . . . of the even channels are connected to each other in common, and connected to an input of the A/D converter ADC2. Further, the detection voltage V_Si for all channels may be converted into a digital value by a single A/D converter.

[0051] The above description has been made on the detailed configuration of the control IC 4. Next, an operation of the control IC 4 will now be described. FIG. 4 is a waveform diagram showing the operation of the control IC 4 according to the embodiment.

[0052] First, the buffer 52 is turned on, and thus, the reference voltage V_CM becomes a certain level. In addition, the initialization signal VCM_SW for all channels is asserted, and thus, the plurality of initialization switches SW3₁ to SW3_n are turned on (at time t0). Therefore, the voltage levels of the integral capacitors C_INT1 to C_INTn of the respective channels are initialized to the reference voltage V_CM. When the initialization of the integral capacitor C_INTi is completed, the reference voltage V_CM becomes 0 V, and the initialization signal VCM_SW is negated, whereupon the initialization switches SW3₁ to SW3_n are turned off.

[0053] Subsequently, the reset signal RST is asserted, and the plurality of reset switches SW1₁ to SW1_n are turned on. Accordingly, the electric charges of the sensor capacitors C_S1 to C_Sn become zero and thus are initialized (at time t1). Next, the reset signal RST is negated, and the reset switches SW1₁ to SW1_n are turned off.

[0054] Then, the sensing signal EVALB is asserted (low level), and the plurality of sensing switches SW2₁ to SW2_n are turned on.

[0055] An ith channel is noted. When the sensing switch SW2_i is turned on, the charging current I_CHGi flows through the first transistor M1 and the sensing switch SW2 that correspond to the sensor capacitor C_Si, thereby boosting the electric potential of the sensor capacitor C_Si. Then, when the electric potential V_Xi rises to Vdd−Vth, the first transistor M1 is turned off, and charging is stopped. Vth is a threshold voltage between a gate and a source of the first transistor M1. An amount of electric charge, which is supplied to the sensor capacitor C_Si by the charging, is expressed as Equation (3) below and depends on the capacitance of the sensor capacitor C_Si.
Q_Si=C_Si×(Vdd−Vth)  (3)

[0056] That is, the C/I conversion circuit 10_i supplies the current I_CHGi to a corresponding sensor capacitor C_S until the electric potential of the sensor capacitor C_Si reaches a certain level Vdd−Vth.

[0057] The C/I conversion circuit 10_i, copies the charging current I_CHGi to generate the detection current I_Si corresponding to a capacitance, and charges the integral capacitor C_INT with the detection current I_Si. Since I_Si=K1×I_CHGi, an amount of electric charge Q_INTi supplied to the integral capacitor C_INTi is expressed as Equation (4) below:
Q_INTiQ_Si×K1  (4)

[0058] The current average circuit 20 discharges the integral capacitor C_INTi due to an average current I_AVE of the detection currents I_S1 to I_Sn of the respective channels. An amount of electric charge Q_INTAVE, which is discharged from the integral capacitor C_INTi by the current average circuit 20, is expressed as Equation (5) below:
Q_INTAVE=Q_SAVE×K1  (5)
where Q_SAVE is an average value, ΣQ_Si/n, of the amounts of electric charges that are respectively supplied to the sensor capacitors C_S1 to C_Sn of the respective channels, and expressed as Equation (6) below:
Q_SAVE=ΣQ_Si/n=ΣC_Si/n×(Vdd−Vth)  (6)

[0059] When the capacitance of the sensor capacitor C_Si is greater than an average value C_SAVE of the capacitances of the sensor capacitors C_S1 to C_Sn of the respective channels, since I_Si>I_AVE, the integral capacitor C_INTi is charged, and the detection voltage V_Si becomes higher by ΔV_i than the reference voltage V_CM that is an initial value.

[0060] $\begin{array}{cc}\begin{array}{c}\Delta V_i=\left(Q_{\mathrm{INTi}}-Q_{\mathrm{INTAVE}}\right)/C_{\mathrm{INTi}}\\ =\left(Q_{\mathrm{Si}}-Q_{\mathrm{SAVE}}\right)\times K1/C_{\mathrm{INTi}}\\ =\left(C_{\mathrm{Si}}-\Sigma C_{\mathrm{Si}}/n\right)/C_{\mathrm{INTi}}\times K1\times \left(\mathrm{Vdd}-\mathrm{Vth}\right)\end{array}& \left(7\right)\end{array}$

[0061] However, when the capacitance of the sensor capacitor C_Si is less than the average value C_SAVE, namely, when Q_Si<Q_SAVE, since I_Si<I_AVE, the integral capacitor C_INTi is discharged, and the detection voltage V_Si becomes lower by ΔV_i than the reference voltage V_CM that is the initial value.

[0062] When the capacitance of the sensor capacitor C_Si is equal to the average value C_SAVE, namely, when Q_Si=Q_SAVE, since I_Si<I_AVE, the amount of electric charge of the integral capacitor C_INTi is not changed, and ΔV_i becomes zero.

[0063] The final detection voltage V_Si is expressed as Equation (8) below:

[0064] $\begin{array}{cc}\begin{array}{c}{V}_{\mathrm{Si}}=V_{\mathrm{CM}}+\Delta V_i\\ =V_{\mathrm{CM}}+\left(C_{\mathrm{Si}}-\Sigma C_{\mathrm{Si}}/n\right)/C_{\mathrm{INTi}}\times K1\times \left(\mathrm{Vdd}-\mathrm{Vth}\right)\end{array}& \left(8\right)\end{array}$

[0065] In this way, the capacitance changes of the sensor capacitors C_S1 to C_Sn of the respective channels are respectively converted into the detection voltages V_S1 to V_Sn, which are respectively held by the integral capacitors C_INTi to C_INTn.

[0066] Subsequently, by controlling the switches SW4₁ to SW4_n according to an appropriate sequence, the detection voltages V_S1 to V_Sn of the respective channels are respectively converted into digital values by the two A/D converters ADC1 and ADC2.

[0067] The above description has been made on the operation of the control IC 4.

[0068] According to the C/I conversion circuit 100, each of the capacitances of the sensor capacitors C_S1 to C_Sn may be detected as a voltage proportional to a difference with respect to the average value C_SAVE (ΣC_Si/n) of the capacitances. Herein, when the number of channels n is sufficiently large and the change of each capacitance is small, the average value C_SAVE may be regarded as constant, and thus, the detection voltage V_Si is substantially changed linearly according to the capacitance change of the sensor capacitor C_Si.

[0069] The C/I conversion circuit 100 may simultaneously detect the capacitance changes of the respective sensor capacitors C_S of the plurality of channels. Accordingly, even when noise affecting the touch panel 3 is being continuously changed with time, common mode noise can be cancelled, thus enhancing the tolerance to noise compared to a conventional method.

[0070] Moreover, as clearly seen in Equation (8), the sensitivity of the detection voltage V_Si corresponding to the capacitance change may be adjusted with the mirror ratio K1 of the C/I conversion circuit 10_i and current average circuit 20, and the capacitance of the integral capacitor C_INTi.

[0071] The above description has been made on the present disclosure, on the basis of the embodiments. The embodiments are exemplified, and it can be understood by those skilled in the art that various modification examples can be implemented by the combinations of the elements or processors of the embodiments, and are within the technical spirit and scope of the present disclosure. Hereinafter, the modification examples will be described.

[0072] FIG. 5 is a circuit diagram illustrating a configuration of the control IC 4 according to a first modification example. In FIG. 5, only a configuration of a first channel is illustrated. In an actual circuit, a protection diode or a protection pad (not shown) is directly connected to a terminal N_S to which a sensor capacitor C_S of the control IC 4 is connected, and a parasitic capacitor C_P is formed due to the protection diode or the protection pad.

[0073] Specifically, a charging current for the sensor capacitor C_Si and a charging current for the parasitic capacitor C_Pi flow in the first transistor M1. The charging current flowing in the parasitic capacitor C_Pi is supplied as expressed in Equation (9) below:
Q_Pi=C_Pi×(Vdd−Vth)  (9)
The electric charge is charged into the integral capacitor C_INTi. When the parasitic capacitances of parasitic capacitors C_P1 to C_Pn of the respective channels are equal, the influences of the parasitic capacitances are cancelled, but when the parasitic capacitances of the respective channels differ, the parasitic capacitances exert a harmful influence on the detection of the sensor capacitors C_S.

[0074] Therefore, the control IC 4a of FIG. 5 further includes an offset cancel circuit 60 for cancelling the parasitic capacitor C_Pi of each channel. The offset cancel circuit 60 supplies a predetermined reference current Iref to a node (line) N_S to which a corresponding sensor capacitor C_S is connected, for a predetermined duration.

[0075] Specifically, the offset cancel circuit 60 includes: a current source 62 that generates the predetermined reference current Iref; a current mirror circuit 64 that copies the reference current Iref and supplies the copied reference current Iref to the node N_S; and a cancel switch SW5 that is disposed on a path of a output current of the current mirror circuit 64 and is turned on for the predetermined duration. The cancel switch SW5 is turned on for a duration where a control signal PWMB[1] is asserted (low level).

[0076] An operation of the C/V conversion circuit 100a of FIG. 5 will now be described. FIG. 6 is a waveform diagram showing the operation of the control IC 4a of FIG. 5.

[0077] Before the sensing signal EVALB is asserted, in each channel, the control signal PWMB[i] is asserted (low level) for an arbitrary calibration duration T_i. The calibration durations T_i, of the respective channels may differ. In the calibration duration T_i, an electric charge expressed in the Equation below is given to the node N_S.
Q=Iref×T_i

[0078] When the amount of the electric charge is equal to C_Pi×(Vdd−Vth), the influence of the parasitic capacitor C_Pi may be cancelled. In this case, the calibration duration T_i is provided in the expressed Equation below.
T_i=C_Pi×(Vdd−Vth)/Iref

[0079] The calibration duration T_i may be common in all channels, and the value of a reference current Iref_i may be adjusted for each channel. Alternatively, both the calibration duration T_i and the reference current Iref_i may be adjusted.

[0080] In this way, by disposing the offset cancel circuit 60, the influence of the parasitic capacitor C_Pi can be cancelled, and the capacitance change of the sensor capacitor C_Si can be detected with high precision.

[0081] The electronic instrument 1 including the input device 2 is switchable between two states, namely, a standby state (slip state) and a normal state. The input device 2 is set to a standby mode in the standby state, and set to a normal mode in the normal state. In the normal mode, the capacitance change of each sensor capacitor C_S of the touch panel 3 is sensed. In the standby mode, in order to reduce the consumption power of the electronic instrument 1, whether a user touches the touch panel 3 is sensed. In the standby mode, when the user touches the touch panel 3, the input device 2 returns to the normal mode. In the present modification example, a description will be made on technology that detects whether the user touches the touch panel 3, at a low consumption power.

[0082] As described above, each detection current I_S corresponds to the capacitance of a corresponding sensor capacitor C_S, and the average current I_AVE corresponds to an average value of the capacitances of the plurality of sensor capacitors C_S. When a user touches any one position of the touch panel 3, the average value of the capacitances of the sensor capacitors C_S is changed. Such a principle is applied to an input device 2a according to the second modification example, and the input device 2a determines whether the user touches at least one of the sensor capacitors C_S, on the basis of the average current I_AVE.

[0083] FIG. 7 is a block diagram illustrating a configuration of the control IC 4a according to a second modification example. The control IC 4a includes a reference capacitor C_REF, a second C/I conversion circuit 11, a second I/V conversion circuit 31, and a buffer BUF_n+1, in addition to the elements of the control IC 4 of FIG. 2.

[0084] The second C/I conversion circuit 11 is configured similarly to the C/I conversion circuit 10, and generates a reference current I_REF corresponding to the capacitance of the reference capacitor C_REF. The second I/V conversion circuit 31 is configured similarly to the I/V conversion circuit 30, and converts a difference current I_DIFF(=I_REF−I_AVE) between the average current I_AVE (which is generated by a current average circuit 20a) and the reference current I_REF into a voltage V_Sn+1.

[0085] The current average circuit 20a averages the detection currents I_S generated by the plurality of capacitance current conversion circuit 10 and the reference current I_REF, thereby generating the average current I_AVE

[0086] In the present configuration, since the capacitance of the reference capacitor C_REF is fixed, the reference current I_REF is constant irrespective of whether the user touches the touch panel 3. Therefore, the difference current “I_REF−I_AVE” generated by the second I/V conversion circuit 31 has a value corresponding to the average current I_AVE. Accordingly, in the standby mode, the input device 2a may determine whether the user touches the touch panel 3, on the basis of the output voltage V_Sn+1 of the second I/V voltage conversion circuit 31.

[0087] In the normal mode, the multiplexer 40 cyclically selects a plurality of voltages V_S1 to V_Sn. The A/D converter 50 sequentially converts the voltages V_S1 to V_Sn into respective digital values. In the standby mode, the multiplexer 40 steadily selects the voltage V_Sn+1 from the second I/V conversion circuit 31. The A/D converter 50 converts only the voltage V_Sn+1 into a digital value. Thus, in the standby mode, the switching operation of the multiplexer 40 is not required, thus decreasing consumption power.

[0088] Moreover, since the A/D converter 50 merely converts the voltage V_Sn+1 of one channel, consumption power can be considerably reduced compared to a case in which an A/D conversion is performed on all channels. Further, the DSP 6 receiving the output data of the A/D converter 50 detects whether the user touches the touch panel 3 by processing only the voltage V_Sn+1 of one channel, thus considerably decreasing the number of operations of the DSP6.

[0089] In order to further decrease consumption power, in the standby mode, unnecessary circuit blocks such as I/V conversion circuits 30₁ to 30_n or buffers BUF₁ to BUF_n can be stopped.

[0090] FIG. 8 is a circuit diagram illustrating a detailed configuration example of the control IC 4a of FIG. 7.

[0091] The control IC 4a includes a second reset switch SW6, a second sensing switch SW7, and a second current mirror circuit.

[0092] The second reset switch SW6 is disposed in parallel to the reference capacitor C_REF, and initializes the electric charge of the reference capacitor C_REF when turned on. One end of the second sensing switch SW7 is connected to the reference capacitor C_REF. A sixth transistor M6 and a seventh transistor M7 configure the second current mirror circuit. The sixth transistor M6 is disposed on a path of the second sensing switch SW7, and is connected to the reference capacitor C_REF through the second sensing switch SW7. The second current mirror circuit supplies a current I_REF, which flows in the seventh transistor M7 of an output side of the second current mirror circuit, to a second integral capacitor C_INTn+1 in the first direction.

[0093] The second integral capacitor C_INTn+1 is disposed in correspondence with the reference capacitor C_REF, and the electric potential of one end of the second integral capacitor C_INTn+1 is fixed. An initialization switch SW3_n+1, which is a second initialization circuit, is disposed for initializing the voltage of the second integral capacitor C_INTn+1.

[0094] The current average circuit 20a averages a plurality of detection currents I_Si′ to I_Sn′ (which flow in the plurality of sensor capacitors C_S1 to C_Sn, respectively) and a reference current I_REF′ flowing in the reference capacitor C_REF, thereby generating the average current I_AVE. The current average circuit 20a supplies the average current I_AVE to the plurality of integral capacitors C_INT1 to C_INTn and the second integral capacitor C_INTn+1 in the second direction.

[0095] The current average circuit 20a includes an eighth transistor M8, a ninth transistor M9, and a tenth transistor M10, in addition to the elements of the current average circuit 20 of FIG. 3. A gate of the tenth transistor M10 is connected to a gate of the sixth transistor M6. The eighth transistor M8 is disposed on a path of the tenth transistor M10.

[0096] The ninth transistor M9 is connected to the eighth transistor M8 to form a current mirror circuit therewith, and a drain of the ninth transistor M9 is connected to the second integral capacitor C_INTn+1.

[0097] In the standby mode, whether a user touches the touch panel 3 is determined on the basis of the voltage V_Sn+1 of the second integral capacitor C_INTn+1.

[0098] The reference capacitor C_REF is built in the control IC 4a, and may be preferably configured as a variable capacitor. As described below, the voltage V_Sn+1 has a level corresponding to a difference between the capacitance of the reference capacitor C_REF and an average capacitance of the capacitances of the sensor capacitors C_S. Therefore, by varying the capacitance of the reference capacitor C_REF, the range of the voltage V_Sn+1 can be matched with a circuit of a next stage.

[0099] The above description has been made on the configuration of the control IC 4a. Next, an operation of the control IC 4a will be described. FIG. 9 is a waveform diagram showing an operation in the standby mode of the control IC 4a of FIG. 7.

[0100] In the standby mode, an operation before time t2 is as shown in the waveform diagram of FIG. 4.

[0101] First, the buffer 52 is turned on, and thus, the reference voltage V_CM becomes a certain level. In addition, the initialization signal VCM_SW for all channels is asserted, and thus, the plurality of initialization switch SW3₁ to SW3_n+1 is turned on (at time t0). Therefore, the voltage level of the integral capacitors C_INT1 to C_INTn+1 of the respective channels is initialized to the reference voltage V_CM. When the initialization of the integral capacitor C_INT is completed, the reference voltage V_CM becomes 0 V, and the initialization signal VCM_SW is negated, whereupon the initialization switches SW3₁ to SW3_n are turned off.

[0102] Subsequently, the reset signal RST is asserted, and the plurality of reset switches SW1₁ to SW1_n and SW6 are turned on. Accordingly, the electric charges of the sensor capacitors C_S1 to C_Sn become zero and thus are initialized (at time t1). Next, the reset signal RST is negated, and the reset switches SW1₁ to SW1_n and SW6 are turned off.

[0103] Then, the sensing signal EVALB is asserted (low level), and the plurality of sensing switches SW2₁ to SW2_n and SW7 are turned on. As a result, the C/I conversion circuit 10_i supplies the current I_CHGi to the corresponding sensor capacitor C_S, until the electric potential of the sensor capacitor C_Si reaches a certain level, Vdd−Vth, and the C/I conversion circuit 11 supplies a current I_CHGn+1 to the corresponding reference capacitor C_REF until the electric potential of the reference capacitor C_REF reaches the certain level, Vdd−Vth.

[0104] An n+1st channel is noted. The current I_REF, which is supplied to the second integral capacitor C_INTn+1 in the first direction, has a constant level regardless of whether the user touches the touch panel 3. The current I_AVE, which is supplied to the second integral capacitor C_INTn+1 in the second direction, is changed according to whether the user touches the touch panel 3.

[0105] When the capacitance of the reference capacitor C_REF is equal to an average capacitance of the capacitances of the plurality of sensor capacitors C_S, the charging current I_REF of the second integral capacitor C_INTn+1 becomes equal to the discharging current I_AVE of the second integral capacitor C_INTn+1, and thus, the electric potential V_Sn+1 of the second integral capacitor C_INTn+1 becomes equal to the reference voltage V_CM. In addition, when the capacitance of the reference capacitor C_REF is greater than the average capacitance of the capacitances of the plurality of sensor capacitors C_S, since I_REF>I_AVE, the electric potential V_Sn+1 of the second integral capacitor C_INTn+1 becomes equal to the reference voltage V_CM.

[0106] However, when the capacitance of the reference capacitor C_REF is less than the average capacitance of the capacitances of the plurality of sensor capacitors C_S, since I_REF<I_AVE, the electric potential V_Sn+1 of the reference capacitor C_REF becomes lower than the reference voltage V_CM.

[0107] Herein, when the user touches the touch panel 3, the capacitance of at least one of the sensor capacitors C_S increases, and thus, the average capacitance of the capacitances of the sensor capacitors C_S increases. Accordingly, the electric potential V_Sn+1 of the reference capacitor C_REF has a level that varies according to whether the user touches the touch panel 3.

[0108] After the sensing signal EVALB is asserted, a switch AD_SWn+1 of a multiplexer 40a is turned on, and thus, the voltage V_Sn+1 is inputted to the A/D converter ADC2 and converted into a digital value. The DSP 6 determines whether the user touches the touch panel 3 on the basis of the digital value, and when detecting the touch, the DSP 6 performs the normal mode.

[0109] As shown in FIG. 9, in the standby mode, a plurality of switches AD_SW1 to AD_SWn are always turned off, and thus, the multiplexer 40a steadily selects the voltage V_Sn+1. Accordingly, the switching of the switch is not required. Further, an operation of each of the buffers BUF₁ to BUF_n is not required. Additionally, an A/D conversion may be performed only once in an n+1st channel. Moreover, data is transferred from the control IC 4 to the DSP 6 for one channel. Accordingly, the consumption current of the control IC 4 can be greatly reduced.

[0110] Further, the DSP 6 determines whether the user touches the touch panel 3 with data for one channel. The determination merely compares digital data, corresponding to the voltage V_Sn+1 from the DSP 6, with a threshold voltage, and thus is a very simple operation. Accordingly, the number of operations of the DSP 6 can be considerably reduced.

[0111] In the above embodiments, the touch panel 3 where the plurality of sensor capacitors C_S are substantially disposed in the matrix type has been described as an example, but the application of the C/V conversion circuit 100 is not limited thereto. For example, the C/V conversion circuit 100 may be applied even to an X-Y type of touch panel, in which case the capacitances of a plurality of row sensor electrodes and the capacitances of a plurality of column sensor electrodes may be detected simultaneously.

[0112] The C/V conversion circuit 100, which has been described in the above embodiments, may be switched in its upper and lower positions. In this case, it can be understood by those skilled in the art that the P-channel MOSFETs and the N-channel MOSFETs may be appropriately switched in their disposed positions. Herein, charging and discharging are changed therebetween, but their fundamental operations are not changed. Furthermore, some transistors may be respectively replaced by bipolar transistors instead of MOSFETs.

[0113] In the above embodiments, the capacitance voltage conversion circuit 100 has been described as being applied to the input device using the capacitance change, but the use of the capacitance voltage conversion circuit 100 is not limited thereto. For example, the capacitance voltage conversion circuit 100 may be applied to a microphone such as a capacitor type microphone, in which a capacitor is formed by a diaphragm electrode and a back plate electrode and the capacitance of the capacitor is changed by a sound pressure.

[0114] Moreover, since the capacitance voltage conversion circuit 100 amplifies and detects a very small capacitance change, the capacitance voltage conversion circuit 100 may be applied to various applications.

[0115] In the above embodiments, the capacitance voltage conversion circuit 100 has been described as being integrated on one semiconductor integrated circuit, but is not limited thereto. As another example, each circuit block may be configured with a chip component or a discrete element. The number of blocks to be integrated may be determined according to an applied semiconductor manufacture process, cost, characteristic, etc.

[0116] The input device according to the above embodiments may be applied to various electronic instruments, which include various input devices, such as personal computers, PDAs, digital cameras, remote controllers of CD players, etc., in addition to portable terminals that have been described in the above embodiments.

[0117] According to the embodiments of the present disclosure, the capacitance voltage conversion circuit can simultaneously detect the capacitances of the plurality of sensor electrodes.

[0118] While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the novel methods and apparatuses described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosures.