The principle and application method of touch chip

With the rapid development of microelectronics technology, electronic products tend to be smaller, easier to operate, and more stylish and beautiful in appearance. Mechanical switch is far from meeting the requirements of current electronic products. The appearance of touch switch has broken the original design concept of mechanical switch, making switch operations more flexible and convenient, and switch panels more novel and unique. At present, touch switches have been widely used in electronic products. Now the high-end products such as mobile phones and household appliances we use basically all use touch switches. Touch switches have gradually been recognized by consumers, and their applications have become more and more extensive.

1. The principle of touch chip

The touch switch is realized by the touch chip. The principle of the touch chip is to change the input capacitance through the touch of the human finger. The input capacitance is compared with the built-in reference capacitance of the touch chip to output a differential signal, which is then amplified by the amplifier circuit to obtain the high and low level at the output end, thus realizing the simulation Signal control. The main MCU detects the level signal of the output port of the touch chip to determine whether there is a finger touched the switch to realize the touch switch function.

2. Features of touch chip

Compared with traditional mechanical switches, the touch chip mainly has the following characteristics:

1) Since it is not a mechanical switch, there is no mechanical damage to the touch switch, and the switch life is longer, which can generally reach more than 100,000 times.

2) There are no actual buttons on the operation panel, the appearance of the operation panel is better, and the appearance can be more novel.

3) The sensitivity of the touch switch can be adjusted freely according to the sensitivity capacitance.

4) Because water vapor has little effect on the input capacitance, the touch switches are better moisture-proof and waterproof.

3. Application of touch chip

When the finger touches the screen of the panel switch, the finger acts as an electrode, the panel acts as the intermediate medium of the capacitor, and the conductive medium under the panel is connected to the sensing pad on the PCB as the other electrode of the capacitor, thus forming an electrode. Input capacitance. The input capacitance is compared with the built-in reference capacitance of the touch chip to output the differential signal, and then amplified by the amplifier circuit to obtain the high and low level at the output terminal (OUTPUT). The main MCU determines by detecting the level signal of the touch chip output port (OUTPUT) Check whether the finger touches the switch to realize the touch switch function.

4. Precautions for touch chip application

4.1 Panel media requirements

1) The thickness of the panel is preferably less than or equal to 6mm, and the panel thickness must be uniform. Due to the limited charge on the finger, if the panel medium is too thick and the capacitance change is too small, the touch chip will not be able to recognize and the switch will not respond.

2) The panel material can be PMMA (organic glass), marble and other non-conductive materials, and the silk-screen on the panel must also be insulated, otherwise there will be confusion when operating the keys.

4.2 PCB board design requirements

  • 1) The wire width of the sensing pad should not be too wide in case parasitic capacitance will increase, and switch sensitivity will decrease.
  • 2) Increase the spacing between the wires of the sensing pads as much as possible to prevent mutual interference.
  • 3) In order to improve the ESD characteristics and better anti-interference, grounding is required around the sensing pad, and the distance between the sensing pad and the sensing pad should not be too close; if the surrounding interference is large, it is better to pave the ground on both sides.
  • 4) The surface of the induction pad can be exposed to copper or covered with oil. Try not to be on the same level as the trace of the sensing pad.
  • 5) The traces of the sensing pads should minimize vias, and the traces should be as short as possible.
  • 6) The area of ​​the sensing pad is mainly related to the thickness of the attached object, the material and the working voltage of the IC. The thicker the material, the lower the working voltage of the IC, the larger the area of ​​the sensing pad is required; otherwise, its sensitivity will be reduced.
  • 7) Try to avoid wiring under the sensing pad, especially for high-current circuits.

4.3 Requirements for conductive medium between panel and PCB induction pad

  • 1) The conductive medium must be completely conductive to conduct electricity.
  • 2) The panel and the conductive medium must be in close contact. If there is an air gap between the panel and the electrode, it will cause malfunction, because the dielectric constant between the air is unstable.

4.4 Button sensitivity can be adjusted by sensing pad and external capacitor

The sensitivity of the button is mainly related to the area of ​​the sensing pad, the thickness of the pad attachment, the panel material, the thickness of the panel and the working voltage of the IC. Its sensitivity is mainly adjusted by the sensing pad and external capacitance.

4.5 Touch chip operating frequency requirements

The operating frequency of the touch chip is shown in the above table. In order to prevent the touch chip from being interfered by other operating frequencies or interfere with the operating frequency of other devices, it can generally be adjusted appropriately according to the actual use environment.

5. Development trend of touch chip

There are two main types of touch control chips currently in use, one is resistive and the other is capacitive. Because of the higher reliability of the capacitive type, it is more widely used. Like HYI’s touch switches, they are capacitive switches with reliability and high sensitivity.

2-button touch switch of HYI
HYI 2-button Touch Switch

At present, the single-point touch type is the most common and most widely used method for products using touch chips.

With the continuous development of touch chips, various forms of touch switches have appeared in our daily household appliances. For example, the emergence of scroll wheels and slider buttons has enriched the applications of touch chips.

Early touch chips were relatively simple and could only handle switches. With the further development of technology, the touch chip is no longer a simple switch processing chip. It can now be used as a main MCU. In addition to processing touch switches, it can also handle AD sampling, LED control, communication, etc. Wait. In the past, to realize the touch switch function, there must be two chips, a touch chip, and a main MCU. Now only one touch chip is needed to solve the problem, which greatly reduces the cost of the product and enables more products to use touch Key technology.

After the touch chip is integrated into the MCU, the switch detection becomes more flexible. Matrix scanning or AD sampling can be used, which makes the number of keys that can be identified more and more, which may require several touch chips to complete before. Now a new touch chip can fully meet the requirements.

To sum up

The emergence of touch chip technology has caused tremendous changes in the development of switch. Switch operations have become more flexible and comfortable, and switch panels have become more stylish and beautiful. Touch switches are being adopted by more and more products with their unique advantages.

With the continuous development of touch technology, the reliability and practicability of touch chips will also be further improved, making the use of touch chips more widespread. At present, the integration of touch chips has become higher and higher. The touch chips can be used as the main MCU or touch switches. This makes the touch chips more cost-effective and lower in cost. This shows that the application of touch chips will become more and more widespread.

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