Self-sensing touch panel

• US 10,031,620 B2
• Filed: 06/30/2016
• Issued: 07/24/2018
• Est. Priority Date: 06/30/2016
• Status: Active Grant

• Current Assignee: STMicroelectronics Asia Pacific Pte Limited (STMicroelectronics NV)
• Original Assignee: STMicroelectronics Asia Pacific Pte Limited (STMicroelectronics NV)
• Inventors: Gicquel, Hugo, Ng, EngJye
• Primary Examiner(s): Moon, Patrick
• Application Number:
  US15/199,792
• Publication Number:
  US 20180004349A1
• Time in Patent Office: 754 Days
• Field of Search: None
• CPC Class Codes: G06F 2203/04101 2.5D-digitiser, i.e. digitiser detecting the X/Y position of the input means, finger or stylus, also when it does not touch, but is proximate to the digitiser's interaction surface and also measures the distance of the input means within a short range in the Z direction, possibly with a separate measurement setup
• CPC Class Codes: G06F 2203/04104 Multi-touch detection in digitiser, i.e. details about the simultaneous detection of a plurality of touching locations, e.g. multiple fingers or pen and finger
• CPC Class Codes: G06F 3/0412 Digitisers structurally integrated in a display
• CPC Class Codes: G06F 3/04182 Filtering of noise external to the device and not generated by digitiser components
• CPC Class Codes: G06F 3/044 by capacitive means

Abstract

A method of differential self-capacitance measurement is used to enhance a signal-to-noise ratio of sense lines in a touch panel display, thereby improving touch sensor accuracy. The differential self-capacitance measurement is implemented for a touch panel using charge sharing between adjacent sense lines of the touch panel matrix. Sequential differential self-capacitance measurements can be compared with one another by computing the difference |C_S1−C_S2|−|C_S2−C_S1| to sense a change caused by an intervening event. By scanning the entire touch panel matrix, events can be tracked across the touch panel.

Claims

1. A method comprising:

discharging a first sense line capacitor of a touch panel;

charging a second sense line capacitor of the touch panel;

coupling the first and second sense line capacitors to one another to engage charge sharing;

determining a differential self-capacitance;

comparing a common voltage of the first and second sense line capacitors with a reference voltage;

amplifying signals of the sense line capacitors;

repeating the steps of discharging, charging, coupling, determining, comparing, and amplifying, for a plurality of first and second sense line capacitors of the touch panel, the plurality depending on a size of the touch panel; and

determining, from changes in the differential self-capacitance measurements, a location on the touch panel where a finger is adjacent to the touch panel.

2. The method of claim 1, further comprising

discharging the first sense line capacitor;

charging the second sense line capacitor;

coupling the first and second sense line capacitors to one another to engage charge sharing;

determining a differential capacitance measurement to compare capacitances of the first and second sense line capacitors; and

amplifying output signals of the sense line capacitors.

3. The method of claim 1 wherein the amplifying entails use of an operational amplifier.

4. The method of claim 1 wherein the discharging is accomplished by grounding the second sense line capacitor.

5. The method of claim 1 wherein charging the sense line capacitor entails coupling the sense line capacitor to a power supply in the range of 3.3V-12 V.

6. A circuit for use with a touch panel, the circuit comprising:

a power supply node;

a ground node;

an output buffer having a first input, a second input and an output;

a first sense line capacitor coupled to the power supply node via a first switch configured to be turned on during a first time period to charge the first sense line capacitor;

a second sense line capacitor coupled to the ground node via a second switch configured to be turned on during the first time period to discharge the second sense line capacitor;

a first charge sharing switch in line between the first and second sense line capacitors and configured to be turned on during a second time period, after the first time period, to engage the first and second sense line capacitors in charge sharing; and

a second charge sharing switch coupled between the first sense line capacitor and the first input of the output buffer and configured to be turned on during a third first time period, after the second time period, to couple to the first sense line capacitor to the output buffer, the output buffer being configured to receive a reference voltage at the second input, compare a common voltage sensed at the first sense line capacitor with the reference voltage, and output a voltage representative of a difference between the common voltage and the reference voltage to determine a location on the touch panel where a finger is adjacent to the touch panel.

7. The circuit of claim 6 wherein the output buffer includes an operational amplifier having two input terminals.

8. The circuit of claim 7 wherein the output buffer further comprises a reference capacitor in a feedback loop of the operational amplifier.

9. The circuit of claim 8, further comprising

a first panel capacitor coupled in parallel with the first sense line capacitor;

and

a second panel capacitor coupled in parallel with the second sense line capacitor.

10. The circuit of claim 9, further comprising a panel capacitance cancellation device coupled between the sense line capacitors and the second charge sharing switch.

11. The circuit of claim 10, wherein the output buffer is further equipped with an input switch that optionally couples the two input terminals of the operational amplifier.

12. The circuit of claim 11, further comprising a bypass switch configured to optionally remove the reference capacitor from the feedback loop.

13. The circuit of claim 10, wherein the two input terminals of the operational amplifier are coupled to one another and the output buffer further comprises an output switch in the feedback loop.

14. The circuit of claim 11 wherein one of the terminals of the operational amplifier is a ground terminal.

15. The circuit of claim 10 wherein the touch panel is a component of a mobile electronic device.

16. A method of operating a touch panel, comprising:

charging a first capacitor of the touch panel to a supply voltage;

measuring electric charge on the first capacitor;

connecting the first capacitor to a second capacitor of the touch panel until charge on both capacitors reaches an equilibrium value;

determining a first differential capacitance value from the measured charge and the supply voltage;

after the charge on both capacitors reaches the equilibrium value, discharging the first capacitor and charging the second capacitor to the supply voltage at the same time;

coupling the first and second capacitors to one another to engage charge sharing;

measuring electric charge on the second capacitor;

determining a second differential capacitance value from the measured charge and the supply voltage; and

comparing the first and second differential capacitance values to detect a touch event associated with the touch panel.

17. The method of claim 16 wherein the supply voltage is less than or equal to 5.0 V.

18. The method of claim 17 wherein the touch panel is in a wake-up mode.

19. The method of claim 16 wherein the supply voltage is 12 V and the touch panel is in a normal mode.