CAPACITANCE VOLTAGE CONVERSION CIRCUIT, INPUT APPARATUS USING THE SAME, ELECTRONIC INSTRUMENT, AND CAPACITANCE VOLTAGE CONVERSION METHOD

• US 20120200306A1
• Filed: 02/07/2012
• Published: 08/09/2012
• Est. Priority Date: 02/08/2011
• Status: Active Grant

• Current Assignee: ROHM Co., Ltd.
• Original Assignee: ROHM Co., Ltd.
• Inventors: IWAMOTO, Yukihiro, SHIMADA, Yuji
• Granted Patent: US 9,261,545 B2
• Time in Patent Office: Days
• US Class Current: 324/658
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: G06F 3/0416 Control or interface arrangements specially adapted for digitisers
• CPC Class Codes: G06F 3/0447 Position sensing using the local deformation of sensor cells
• CPC Class Codes: G06F 3/047 using sets of wires, e.g. crossed wires
• CPC Class Codes: H03K 17/962 Capacitive touch switches

Abstract

A capacitance voltage conversion circuit, which converts respective capacitances of a plurality of sensor capacitors into voltages, includes: a plurality of capacitance current conversion circuits disposed in respective correspondence with the sensor capacitors, each capacitance current conversion circuit configured to generate a detection current corresponding to a capacitance of a corresponding sensor capacitor; a current average circuit configured to average a plurality of detection currents, which are respectively 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 current voltage conversion circuit configured to convert a difference current between a corresponding detection current and the average current into a voltage.

Claims

1. A capacitance voltage conversion circuit which converts a capacitance of each of a plurality of sensor capacitors into a voltage, the capacitance voltage conversion circuit comprising:

a plurality of capacitance current conversion circuits disposed in respective correspondence with the sensor capacitors, each of the capacitance current conversion circuit configured to generate a detection current corresponding to a capacitance of a 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 circuit configured to convert a difference current between a corresponding detection current and the average current into a voltage.

2. The capacitance voltage conversion circuit of claim 1, wherein each capacitance current conversion circuit comprises:

a reset switch configured to initialize an electric charge of a corresponding sensor capacitor;

a sensing switch and a first transistor disposed sequentially and serially between the corresponding sensor capacitor and a fixed voltage terminal, the first transistor being a MOSFET; and

a second transistor connected to the first transistor to form a current mirror circuit therewith, and

wherein the capacitance current conversion circuit outputs a current, flowing in the second transistor, as the detection current.

3. The capacitance voltage conversion circuit of claim 1, wherein each current average circuit comprises:

a plurality of third transistors disposed on a path of the corresponding detection current, in respective correspondence to the sensor capacitors, respective control terminals of the third transistors being connected to each other in common; and

a plurality of fourth transistors disposed in respective correspondence with the sensor capacitors, each fourth transistor connected to a corresponding third transistor to form a current mirror circuit therewith,

wherein the current average circuit outputs a current, flowing in each of the fourth transistors, as the average current.

4. The capacitance voltage conversion circuit of claim 3, wherein the current average circuit further comprises a plurality of fifth transistors disposed in respective correspondence with the sensor capacitors, each of the fifth transistors connected to a corresponding first transistor to form a current mirror circuit therewith, and each of the third transistors being disposed on a path of a corresponding fifth transistor.

5. The capacitance voltage conversion circuit of claim 1, wherein each of the current voltage conversion circuits comprises an integral capacitor configured to be charged or discharged by the difference current, and an electric potential of one end of the integral capacitor being fixed.

6. The capacitance voltage conversion circuit of claim 5, wherein the current voltage conversion circuit comprises an initialization switch, one end of the initialization switch being connected to the integral capacitor, and a reference voltage being applied to the other end of the initialization switch.

7. The capacitance voltage conversion circuit of claim 1, wherein in a standby mode, whether a user touches at least one of the sensor capacitors is determined on the basis of the average current.

8. The capacitance voltage conversion circuit of claim 1, further comprising:

a reference capacitor;

a second capacitance current conversion circuit configured similarly to the capacitance current conversion circuit, and configured to generate a reference current corresponding to a capacitance of the reference capacitor; and

a second current voltage conversion circuit configured similarly to the current voltage conversion circuit, and configured to convert a difference current between the reference current and the average current into a voltage,

wherein the current average circuit averages the reference current and the detection currents which are respectively generated by the plurality of capacitance current conversion circuits, thereby generating the average current.

9. The capacitance voltage conversion circuit of claim 8, wherein in a standby mode, whether a user touches at least one of the sensor capacitors is determined on the basis of an output voltage of the second current voltage conversion circuit.

10. The capacitance voltage conversion circuit of claim 9, further comprising:

a multiplexer configured to receive the respective output voltages of the plurality of current voltage conversion circuits and the output voltage of the second current voltage conversion circuit, and to select one of the received output voltages; and

an A/D converter configured to convert the voltage, selected by the multiplexer, into a digital value,

wherein in the standby mode, the multiplexer selects the output voltage of the second current voltage conversion circuit.

11. The capacitance voltage conversion circuit of claim 10, wherein in the standby mode, the plurality of current voltage conversion circuits are stopped.

12. The capacitance voltage conversion circuit of claim 8, wherein the reference capacitor is a variable capacitor.

26. The capacitance voltage conversion circuit of claim 1, further comprising a plurality of offset cancel circuits disposed in respective correspondence with the sensor capacitors, and configured to supply a predetermined current to a node connected to the corresponding sensor capacitor for a predetermined duration.

27. The capacitance voltage conversion circuit of claim 26, wherein each offset cancel circuit comprises:

a current source configured to generate a predetermined reference current;

a current mirror circuit configured to copy the reference current and supply the copied reference current to the node; and

a cancel switch disposed on a path of an output current of the current mirror circuit, and configured to be turned on for the predetermined duration.

28. The capacitance voltage conversion circuit of claim 1, wherein the capacitance voltage conversion circuit is integrated on one semiconductor integrated circuit.

29. An input apparatus, comprising:

a plurality of sensor capacitors; and

the capacitance voltage conversion circuit of claim 1 configured to convert a capacitance of each of the sensor capacitors into a voltage.

30. The input apparatus of claim 29, wherein the plurality of sensor capacitors are substantially disposed in a matrix type.

31. An electronic instrument, comprising the input apparatus of claim 29.

13. A capacitance voltage conversion circuit which converts a capacitance of each of a plurality of sensor capacitors into a voltage, the capacitance voltage conversion circuit comprising:

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 the 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.

14. The capacitance voltage conversion circuit of claim 13, wherein the current average circuit comprises:

a plurality of fifth transistors disposed in respective correspondence with the sensor capacitors, each fifth transistor connected to the first transistor of a corresponding current mirror circuit to form a current mirror circuit therewith;

a plurality of third transistors disposed on a path of a corresponding fifth transistor; and

a plurality of fourth transistors connected to a corresponding third transistor to form a current mirror circuit therewith,

wherein the current average circuit supplies the average current, flowing in each of the fourth transistors, to a corresponding integral capacitor.

15. The capacitance voltage conversion circuit of claim 13, wherein the initialization circuit comprises a plurality of initialization switches, one end of each of the initialization switches being connected to a corresponding integral capacitor, and a reference voltage being applied to the other end of each of the initialization switches.

16. The capacitance voltage conversion circuit of claim 13, wherein in a standby mode, whether a user touches at least one of the sensor capacitors is determined on the basis of the average current.

17. The capacitance voltage conversion circuit of claim 13, further comprising:

a reference capacitor;

a second integral capacitor disposed in correspondence with the reference capacitor, an electric potential of one end of the second integral capacitor being fixed;

a second initialization circuit configured to initialize a voltage of the second integral capacitor; and

a second current mirror circuit configured to supply a current, which flows in a second transistor of an output side of the second current mirror circuit, to the second integral capacitor in a first direction, a first transistor of an input side of the second current mirror circuit being connected to the reference capacitor,

wherein the current average circuit averages the detection currents flowing in the respective sensor capacitors and a reference current flowing in the reference capacitor to generate the average current, and supplies the average current to the plurality of integral capacitors and the second integral capacitor in a second direction.

18. The capacitance voltage conversion circuit of claim 17, wherein in a standby mode, whether a user touches at least one of the sensor capacitors is determined on the basis of the voltage of the second integral capacitor.

19. The capacitance voltage conversion circuit of claim 18, further comprising:

a multiplexer configured to receive the respective voltages of the plurality of integral capacitors and the voltage of the second integral capacitor, and to select one of the received voltages; and

an A/D converter configured to convert the voltage, selected by the multiplexer, into a digital value,

wherein in the standby mode, the multiplexer selects only the voltage of the second integral capacitor.

20. The capacitance voltage conversion circuit of claim 17, wherein the reference capacitor is a variable capacitor.

21. A capacitance voltage conversion circuit which converts a capacitance of each of a plurality of sensor capacitors into a voltage, the capacitance voltage conversion circuit comprising:

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 the 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 the 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 the 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 the corresponding integral capacitor.

22. The capacitance voltage conversion circuit of claim 21, further comprising:

a reference capacitor;

a second integral capacitor disposed in correspondence with the reference capacitor, an electric potential of one end of the second integral capacitor being fixed;

a second reset switch connected to the reference capacitor in parallel;

a second sensing switch, one end of the second sensing switch being connected to the reference capacitor;

a sixth transistor, being a MOSFET, disposed on a path of the second sensing switch;

a seventh transistor, being a MOSFET, a gate of the seventh transistor being connected to a gate of the sixth transistor, and a drain of the seventh transistor being connected to the second integral capacitor;

a tenth transistor, being a MOSFET, a gate of the tenth transistor being connected to a gate of the sixth transistor;

an eighth transistor disposed on a path of the tenth transistor; and

a ninth transistor connected to the eighth transistor to form a current mirror circuit therewith, and a drain of the ninth transistor being connected to the second integral capacitor.

23. The capacitance voltage conversion circuit of claim 22, wherein in a standby mode, whether a user touches at least one of the sensor capacitors is determined on the basis of a voltage of the second integral capacitor.

24. The capacitance voltage conversion circuit of claim 23, further comprising:

a multiplexer configured to receive the respective voltages of the plurality of integral capacitors and the voltage of the second integral capacitor, and to select one of the received voltages; and

an A/D converter configured to convert the voltage, selected by the multiplexer, into a digital value,

wherein in the standby mode, the multiplexer selects the voltage of the second integral capacitor.

25. The capacitance voltage conversion circuit of claim 22, wherein the reference capacitor is a variable capacitor.

32. A method of converting a capacitance of each of a plurality of sensor capacitors into a voltage, the method comprising:

initializing an electric potential of each of the sensor capacitors;

supplying a first current until each of the voltages reaches a predetermined level, in correspondence with each of the sensor capacitors;

supplying a second current, which corresponds to a corresponding first current, to an integral capacitor, which is disposed in correspondence with each of the sensor capacitors, in a first direction, and supplying an average current of a plurality of second currents to each of the integral capacitors in a second direction; and

outputting a voltage, generated in each of the integral capacitors, as a detection voltage which corresponds to a corresponding capacitance.

33. The method of claim 32, wherein in a standby mode, whether a user touches at least one of the sensor capacitors is determined on the basis of the average current of the second currents.