With the rapid development of microelectronics technology, electronic products are tending towards miniaturization, simpler and more convenient operation, and more fashionable and beautiful appearance. Mechanical buttons are far from meeting the requirements of current electronic products. The emergence of touch buttons has broken the design concept of mechanical buttons, making button operations more flexible and convenient, and the button panel more innovative and unique. At present, touch buttons have been increasingly widely used in electronic products. Nowadays, high-end products such as mobile phones and household appliances that we use mostly use touch buttons. Touch buttons have gradually gained recognition from consumers and their application range is becoming increasingly widespread.

1. The principle of touch chips

Touch buttons are implemented through touch chips. The principle of a touch chip is to cause changes in the input capacitance through the touch of human fingers. The input capacitance is compared with the reference capacitance built into the touch chip to output a differential signal, which is then amplified by an amplification circuit to obtain high and low levels at the output end, thereby achieving control of the analog signal. The main MCU detects the level signal at the output port of the touch chip to determine whether there is finger contact with the button, achieving the touch button function.

2. Characteristics of touch chips

Compared with traditional mechanical buttons, touch chips mainly have the following characteristics:

1) Due to the fact that it is not a mechanical button, there is no mechanical damage to the touch button, and the lifespan of the button is longer, usually reaching over 300000 times.

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

3) The sensitivity of the touch button can be freely adjusted based on the sensitivity capacitance.

4) Due to the small impact of moisture on the input capacitance, the touch buttons have good moisture-proof and waterproof properties.

3. Application of touch chips

When the finger contacts the screen printing of the panel buttons, the finger acts as an electrode, and the panel acts as the intermediate medium of the capacitor. The conductive medium under the panel is connected to the induction pad on the PCB board as the other electrode of the capacitor, forming an input capacitor. The input capacitor is compared with the reference capacitor embedded in the touch chip to output a differential signal. After being amplified by the amplification circuit, the high and low levels are obtained at the output end (OUTPUT). The main MCU detects the level signal at the output port (OUTPUT) of the touch chip to determine whether there is finger contact with the press button, achieving the touch button function.

4. Precautions for touch chip application

4.1 Panel Media Requirements

1) The thickness of the panel should preferably be less than or equal to 6mm, and the panel thickness must be uniform. Due to the limited charge on the fingers, 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 buttons will not respond.

2) The panel material can use non conductive materials such as PMMA (organic glass) and marble, and the screen printing on the panel must also be insulated, otherwise there will be confusion when operating the buttons.

4.2 Design requirements for PCB boards

1) The width of the connecting line of the induction pad should not be too wide, as parasitic capacitance will increase and button sensitivity will decrease if it is too wide.

2) The spacing between the connecting wires of the induction pad should be widened as much as possible to prevent mutual interference.

3) In order to improve ESD characteristics and better anti-interference, the surrounding area of the induction pad should be paved and the distance between it and the induction pad should not be too close; If there is significant interference around, it is best to lay the ground on both sides.

4) The surface of the induction pad can be exposed to copper or covered with oil filter. Try not to route the wires on the same level as the induction pad.

5) The wiring of induction pads should minimize through holes as much as possible, and the shorter the wiring, the better.

6) The area of the induction bonding pad is mainly related to the thickness and material of the attachment, as well as the working voltage of the IC. The thicker the material and the lower the working voltage of the IC, the larger the area of the induction bonding pad is required; Otherwise, its sensitivity will decrease.

7) Try not to run wires under the induction pad, especially in high current circuits.

4.3 Requirements for conductive media between panels and PCB induction pads

1) The conductive medium must be completely conductive to conduct electricity.

2) The panel must be in close contact with the conductive medium. If there is an air gap between the panel and the electrode, it can cause abnormal operation because the dielectric constant between the air is unstable.

4.4 Button sensitivity can be adjusted through induction pads and external capacitors

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

4.5 Operating frequency requirements for touch chips

The working frequency of the touch chip is shown in the table above. In order to prevent the touch chip from being affected by other working frequencies or interfering with the working frequencies of other devices, it can generally be adjusted appropriately according to the actual usage environment requirements.

5. Development Trend of Touch Chip

There are currently two main types of touch chips used, one is resistive and the other is capacitive. Due to its higher reliability, capacitive type has been widely used.

At present, single touch is the most common and widely used method for products that use touch chips.

With the continuous development of touch chips, various forms of touch buttons have appeared in our daily household appliances, such as the emergence of scroll and slider buttons, which have enriched the application of touch chips.

Early touch chips were relatively simple and could only handle buttons. With the further development of technology, touch chips are no longer a simple button processing chip. They can now be fully used as a main MCU, not only to handle touch buttons, but also to handle AD sampling, LED control, communication, and so on. Previously, two chips were required to achieve touch button functionality, one touch chip and one main MCU. Now, only one touch chip is needed to solve the problem, greatly reducing product costs and allowing more products to use touch button technology.

Due to the integration of touch chips into MCU, button detection has become more flexible. Matrix scanning or AD sampling can be used, which increases the number of recognizable buttons. Previously, several touch chips were needed to complete the task, but now a new touch chip can fully meet the requirements.

summary

The emergence of touch chip technology has led to significant changes in the way buttons are pressed, making button operations more flexible and comfortable, and the button panel more fashionable and beautiful. Touch buttons are being adopted by more and more products due to their unique advantages.

With the continuous development of touch technology, the reliability and practicality of touch chips will also be further improved, making the use of touch chips more widespread. At present, the integration level of touch chips has become increasingly high. Touch chips can be used as both the main MCU and touch buttons, making them more cost-effective and cost-effective. It can be seen that the application of touch chips will become increasingly widespread.