CAPACITANCE MEASURING CIRCUIT FOR TOUCH SENSOR

• US 20110187389A1
• Filed: 09/10/2008
• Published: 08/04/2011
• Est. Priority Date: 09/19/2007
• Status: Abandoned Application

• Current Assignee: Pointchips Co., Ltd.
• Original Assignee: Pointchips Co., Ltd.
• Inventors: Na, Do Yeun, Choi, In Jun, Han, Sang Hyun
• Application Number:
  US12/679,046
• Publication Number:
  US 20110187389A1
• Time in Patent Office: Days
• US Class Current: 324/679
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 2217/960705 Safety of capacitive touch and proximity switches, e.g. increasing reliability, fail-safe
• CPC Class Codes: H03K 2217/960715 Rc-timing; e.g. measurement of variation of charge time or discharge time of the sensor

Abstract

Disclosed herein is a capacitance measuring circuit for a touch sensor. The capacitance measuring circuit includes a reference voltage generation unit for generating a first reference voltage and a second reference voltage, a MUX unit for selecting one from among electrode voltages, a voltage comparator for comparing a voltage generated by the reference voltage generation unit with the electrode voltage, a charging/discharging circuit unit for performing charging of the input electrode voltage from the first reference voltage to the second reference voltage or performing discharging of the input electrode voltage from the second reference voltage to the first reference voltage, a timer unit for receiving an external control signal, measuring charging time and discharging time of the charging/discharging circuit unit, measuring entire charging time and entire discharging time, and outputting corresponding output signals, and a control unit for receiving an output signal of the voltage comparator and the external control signal, and controlling the charging/discharging circuit unit and the timer unit.

Claims

1. A capacitance measuring circuit for a touch sensor, comprising:

a reference voltage generation unit for generating a first reference voltage and a second reference voltage;

a MUX unit for selecting one from among electrode voltages input through a plurality of electrodes;

a voltage comparator for comparing a voltage generated by the reference voltage generation unit with the electrode voltage input from an electrode;

a charging/discharging circuit unit for performing charging of the input electrode voltage from the first reference voltage to the second reference voltage or performing discharging of the input electrode voltage from the second reference voltage to the first reference voltage;

a timer unit for receiving an external control signal, measuring charging time and discharging time of the charging/discharging circuit unit, measuring entire charging time and entire discharging time, and outputting corresponding output signals; and

a control unit for receiving an output signal of the voltage comparator and the external control signal and controlling the charging/discharging circuit unit and the timer unit.

2. The capacitance measuring circuit as set forth in claim 1, wherein the reference voltage generation unit includes three resistors connected in series, and provides the first reference voltage and the second reference voltage as linear voltages.

3. The capacitance measuring circuit as set forth in claim 1, wherein the voltage comparator comprises:

a second comparator for comparing the first reference voltage provided by the reference voltage generation unit with the electrode voltage generated in the electrode; and

a first comparator for comparing the second reference voltage provided by the reference voltage generation unit with an electrode voltage generated in the electrode.

4. The capacitance measuring circuit as set forth in claim 1, wherein the charging/discharging circuit unit comprises:

a current source for increasing the electrode voltage to the second reference voltage; and

a switch unit for selecting one from among charging and discharging of the electrode voltage.

5. The capacitance measuring circuit as set forth in claim 4, wherein the current source comprises:

a resistor (R) having one terminal connected to a power source voltage (VCC) and a remaining terminal connected to a drain terminal of an NMOS transistor (n0);

the NMOS transistor (n0) having a source terminal connected to a ground terminal and the drain terminal connected to the remaining terminal of the resistor (R);

an NMOS transistor (n1) having a source terminal connected to the ground terminal and a drain terminal connected to a drain terminal of a PMOS transistor (p0);

a PMOS transistor (n2) having a source terminal connected to the ground terminal and a drain terminal connected to the switch unit;

the PMOS transistor (p0) having a source terminal connected to a power source voltage (VCC) and the drain terminal connected to the drain terminal of the NMOS transistor (n1); and

a PMOS transistor (p1) having a source terminal connected to a power source voltage (VCC) and a drain terminal connected to the switch unit;

wherein a gate terminal and drain terminal of the NMOS transistor (n0) and a gate terminal of the NMOS transistor (n1) are commonly connected to a gate terminal of the NMOS transistor (n2); and

wherein the drain terminal and gate terminal of the PMOS transistor (p0) are commonly connected to a gate terminal of the PMOS transistor p1.

6. The capacitance measuring circuit as set forth in claim 4, wherein the switch unit comprises:

a first switch for selecting the charging of the electrode; and

a second switch for selecting the discharging of the electrode.

7. The capacitance measuring circuit as set forth in claim 4 or 6, wherein the switch unit comprises:

a first switch comprising a first inverter having an input terminal connected to an output terminal of the second comparator, and a PMOS transistor (p2) having a source terminal connected to a drain terminal of an NMOS transistor (n3), a drain terminal connected to a source terminal of the NMOS transistor (n3) and a gate terminal connected to an output terminal of the first inverter, wherein the output terminal of the first inverter and a gate terminal of the NMOS transistor (n3) are commonly connected to each other, and the source terminal of the PMOS transistor (p2) and the drain terminal of the NMOS transistor (n3) are connected to the current source; and

a second switch comprising a second inverter having an input terminal connected to an output terminal of the first comparator, and a PMOS transistor (p3) having a source terminal connected to a drain terminal of an NMOS transistor (n4), a drain terminal connected to a source terminal of the NMOS transistor (n4), and a gate terminal connected to an output terminal of the second inverter, wherein the output terminal of the first inverter and a gate terminal of the NMOS transistor (n4) are commonly connected to each other, and the source terminal of the PMOS transistor (p3) and the drain terminal of the NMOS transistor (n4) are connected to the current source.

8. The capacitance measuring circuit as set forth in claim 1, wherein capacitance is measured through an accumulated difference between charging and discharging time for existing capacitance and charging and discharging time for varied capacitance by successively performing a charging and discharging cycle one or more times.

9. A capacitance measuring circuit for a touch sensor, comprising:

a reference voltage generation unit for generating a first reference voltage and a second reference voltage;

a voltage comparator for comparing an electrode voltage input from an electrode with voltage generated by the reference voltage generation unit; and

a charging/discharging circuit unit for performing charging of the input electrode voltage from the first reference voltage to the second reference voltage or performing discharging of the input electrode voltage from the second reference voltage to the first reference voltage;

wherein charging and discharging time and total charging and discharging time consumed through the charging/discharging circuit unit are measured, a charging and discharging cycle is performed two or more times, and capacitance is measured through an accumulated difference between charging and discharging time for existing capacitance and charging and discharging time for varied capacitance using corresponding charging and discharging time and total charging and discharging time.