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On the design of projected capacitive touch screen

In Electronic Infomation Category: O | on February 07,2012

Abstract: projected capacitive touch screen with superior operability and MAX8578EUB datasheet and reliability, and MAX8578EUB price and gradually become the mainstream of mobile phones and MAX8578EUB suppliers and other electronic products. Articles from the principles, structures, etc., briefly projected capacitive touch screen design point.

1 works

projected capacitive touch screen is a glass surface with one or more layers ITO (Indium Tin Oxides, transparent conductive films, nano-indium tin oxide, has good electrical conductivity and transparency) Production X-axis and Y-axis electrode matrix, when the fingers touch, finger and ITO surface to form a coupling capacitor, causing a slight change in current through the X-axis and Y-axis scanning electrode matrix, detect changes in capacitance touch point, to calculate where the finger position.

projected capacitive touch screen can be divided into self and mutual capacitive touch screen capacitive touch screen.

capacitive touch screen from the X-axis and Y axis to form the electrode capacitance matrix, respectively, when the finger capacitive touch screen, finger capacitance and capacitor electrode stack, so that the electrode capacitance increases.

detection, since the capacitive touch screen respectively, followed by detection of X-axis and Y-axis electrode matrix, according to the touch before and after the change in capacitance, respectively, to determine the X-axis and Y axis, drawn capacitive screen touch-point coordinates.

single touch point in the X axis and Y axis coordinates are unique. When there are two touch points and the two touch points not in the same X-axis or the same Y-axis, the X-axis and Y axis in the formation of four coordinates, but only two coordinates are real, the other two is called " ghost points ", so self-capacitive touch screen can not achieve true multi-touch.

mutual capacitance touch-screen X-axis and Y-axis electrode capacitance matrix formed at the intersection, that is, X and Y axes, respectively, constitute a capacitor electrode polarization, when the finger capacitive touch screen, with touch points vicinity The two electrodes have capacitive coupling, resulting in capacitance between the two electrodes changes.

detection, Y-axis electrode matrix time-sequence signal, X-axis electrode matrix, while the received signal, resulting in all the X-axis and Y-axis intersection of the electrode capacitance matrix, based on capacitance change, calculate the coordinates of each touch point, can now support up to 10 finger touch.

2 design

common structure projected capacitive touch screen, there are three.

Glass + Glass structure (single-sided ITO), the first layer is called Glass Lens (cover), the second layer of Glass as SeNSor, two layers of glass through the OCA (Optical Clear Adhesive, optical transparent plastic, with good adhesion and permeability) are bonded together, ITO layer coated on the surface of the Sensor, which Y-Sensor connected through the metal bridge. Shown in Figure 2.

Glass + Glass structure (two-sided ITO), the only difference with the single place ITO, ITO layers were deposited on Sensor on / under the surface. Shown in Figure 3.

Glass + Film structure, and the other two structures is different is that the second layer of the Film (film), ITO layer coated on Film, according to the different IC manufacturing process and capacitive screen works different, can be a layer of film or multilayer structure. Shown in Figure 4.

Table 1 shows the three structures in terms of the comparison process.

Table 1 Glass + Glass structure (single-sided ITO), Glass + Glass structure (two-sided ITO), Glass + Film structure of the process compared

3 sensor alignment graphics and design

sensing layer of ITO Sensor graphics that graphic, usually diamond + diamond-shaped design, each IC according to another feature, as well as hexagonal + diamond, snowflake and other designs. Common diamond + diamond-shaped glass structure design shown in Figure 5.

Sensor layer of ITO traces mainly took part in panel alignment and cross-trace two (Figure 6). Y-Sensor and graphic required between the ground separated, X-Sensor line and the Y-Sensor also required between the ground separated.

4 drive circuit design

Figure 7 shows the most basic of projected capacitive touch screen driver circuit, using the IIC serial interface.

Figure 7 basic projected capacitive touch screen driver circuit

Table 2 projected capacitive touch screen start circuit analysis

5 FPC design

5.1 General Design

(1) Sensor line width is generally 0.075mm, Sensor Line Spacing is generally 0.075mm, through-hole diameter / inner diameter generally 0.4mm/0.2mm.

(2) X-Sensor line and the Y-Sensor line can not overlap; If parallel, the middle line of separation with GND, GND line width for the Sensor line width of 2 times; If you can not avoid cross- alignment, the vertical as possible, reducing the cross-area, lower parasitic capacitance.

(3) IC external components as close to the IC.

(4) FPC positive / negative paved ground copper grid, reducing the GND line resistance, shielded from external interference.

5.2 anti-ESD Design

(1) increase the VDD pin varistors to improve the anti-ESD voltage.

(2) to connect the data cable and the host IC Shop grid grounded copper foil to protect the IIC signals to prevent interference with the string into the ESD board.

Figure 9 FPC design

5 production processes

production process shown in Figure 10.

6 Conclusion

As technology evolves, projected capacitive touch screen applications has become increasingly diverse, from handheld devices to home appliances, touch technology is almost ubiquitous, projected capacitive touch screen human-computer interaction will inevitably to a new era.

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