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

Patent metadata

Field	Value
Publication number	US-9261545-B2
Application number	US-201213367904-A
Country	US
Kind code	B2
Filing date	Feb 7, 2012
Priority date	Feb 8, 2011
Publication date	Feb 16, 2016
Grant date	Feb 16, 2016

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.

First claim

What is claimed is:

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,
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.

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 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.

4. 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.

5. The capacitance voltage conversion circuit of claim 4 , 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.

6. 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.

7. 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;
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;
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.

8. The capacitance voltage conversion circuit of claim 7 , 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.

9. The capacitance voltage conversion circuit of claim 8 , 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.

10. The capacitance voltage conversion circuit of claim 9 , wherein in the standby mode, the plurality of current voltage conversion circuits are stopped.

11. The capacitance voltage conversion circuit of claim 7 , wherein the reference capacitor is a variable capacitor.

12. 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.

13. The capacitance voltage conversion circuit of claim 12 , 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.

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

15. 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.

16. The input apparatus of claim 15 , wherein the plurality of sensor capacitors are substantially disposed in a matrix type.

17. An electronic instrument, comprising the input apparatus of claim 15 .

18. A capacitance voltage conversion circuit which converts a cap

Assignees

• Iwamoto Yukihiro
• Shimada Yuji
• Rohm Co Ltd

Inventors

• Iwamoto Yukihiro
• Shimada Yuji

Classifications

• H03K17/962

  Capacitive touch switches · CPC title

• G01R27/2605 Primary

  Measuring capacitance (capacitive sensors G01D5/24) · CPC title

• G06F3/044

  by capacitive means · CPC title

• G01R19/003

  Measuring mean values of current or voltage during a given time interval · CPC title

• G01R1/206

  Switches for connection of measuring instruments or electric motors to measuring loads (switches in general H01H) · CPC title