As microelectronics technology soars, electronic products are trending towards miniaturization, simplified and convenient operation, and more stylish and dazzling aesthetics. Mechanical buttons are no longer sufficient to meet the requirements of modern electronics, and the introduction of touch buttons has shattered the traditional design concepts of mechanical buttons, making operation more flexible and convenient and the keypad more innovative and unique. Currently, touch buttons are gaining wider application in electronic products. The high-end products we use today, such as smartphones and home appliances, largely employ touch buttons. Touch buttons are also gradually being recognized by a broad consumer base and their application scope is expanding.
1. The principle of touch chip
The touch buttons are achieved through a touch chip. The principle of the touch chip is to cause a change in the input capacitance by the human finger's touch, compare the input capacitance with the built-in reference capacitance of the touch chip to output a differential signal, amplify it through the amplification circuit, and then obtain high and low levels at the output terminal, thereby controlling the analog signal. The main MCU detects the level signal at the output of the touch chip to determine whether a finger is touching the button, thereby realizing the touch button function.
2. Features of Touch Chip
Compared to traditional mechanical buttons, touch chips mainly feature the following characteristics:
1) Touch buttons are free from mechanical damage, as they are not mechanical switches, resulting in a longer lifespan of over 300,000 presses.
2) The control panel features no actual buttons, resulting in a more cohesive overall appearance and a more innovative design.
3) The sensitivity of the touch buttons can be freely adjusted with the sensitivity capacitor.
4) Due to the minimal impact of moisture on the input capacitor, the touch button has excellent moisture and water resistance.
3. Application of Touch Chip Technology
When the finger touches the panel's printed keys, the finger acts as an electrode, with the panel serving as the intermediate dielectric medium. The conductive medium beneath the panel connects to the inductive pad on the PCB as the other electrode of the capacitor. This forms an input capacitor. The input capacitor compares its output differential signal with the internal reference capacitor of the touch chip, which is then amplified by the amplification circuit to produce high and low levels at the output (OUTPUT). The main MCU detects the level signal from the touch chip's output (OUTPUT) to determine whether a finger is touching the key, thereby enabling the touch key function.
4. Important Considerations for Touch Chip Applications
4.1 Panel Medium Requirements
1) The panel thickness must be less than or equal to 6mm, and the uniformity of the panel thickness is required. Due to the limited charge on the fingers, if the medium of the panel is too thick, the capacitance change will be too small, the touch chip will not be able to recognize, and the keys will not respond.
2) Panel materials can be PMMA (acrylic glass), marble, and other non-conductive materials. The silk-screened patterns 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 wire width of the inductive pad connection must not be too wide; otherwise, parasitic capacitance will increase, leading to reduced key sensitivity.
2) Maximize the spacing between the inductive pads to prevent interference.
3) To enhance ESD characteristics and improve resistance to interference, it is required to cover the ground around the inductive pad, ensuring a sufficient distance from the pad. If there is significant interference around, a double-sided ground cover is necessary.
4) The surface of the inductive pad can be露出铜或盖滤油. Avoid placing the routing of the inductive pad on the same plane as much as possible.
5) Minimize through-hole usage in the inductive pad routing, and aim for the shortest routing possible.
6) The area of the inductive pad is primarily related to the thickness and material of the adherent, as well as the working voltage of the IC. Thicker materials and lower working voltages for the IC require a larger inductive pad area; otherwise, its sensitivity will decrease.
Avoid routing wires underneath the inductive pads, especially in high-current circuits.
4.3 Requirements for Conductive Medium Between 4.3 Panel and PCB Solder Pads
1) Conductive media must be fully conductive to serve the purpose of conductivity.
2) The panel must be in close contact with the conductive medium. If there is an air gap between the panel and the electrode, this can lead to malfunctions due to the unstable dielectric constant of air.
4.4 The sensitivity of the 4.4 key can be adjusted through the inductive pad and additional capacitors.
Key sensitivity is primarily related to the area of the sensing pad, the thickness of the pad adhesive, the panel material, the panel thickness, and the working voltage of the IC. The sensitivity is mainly adjusted through the sensing pad and additional capacitors.
4.5 Touch Chip Operating Frequency Requirements
Touch chip operating frequencies are as shown in the table above. To ensure that the touch chip is not interfered with by other operating frequencies or disrupts the operating frequencies of other devices, adjustments can generally be made according to the actual usage environment requirements.
5. Trends in Touch Chip Technology
Currently, there are mainly two types of touch chips in use: resistive and capacitive. Capacitive touch chips are more widely used due to their higher reliability.
Currently, single-point touch is the most common and widely used method for products utilizing touch chips.
As touch chip technology continues to evolve, various forms of touch buttons have emerged in our daily household appliances, with the introduction of scroll wheels and slider buttons further enriching the applications of touch chips.
Early touch chips were simple, capable only of handling button presses. With further technological advancements, touch chips have evolved beyond mere button processors and can now serve as a primary MCU. In addition to handling touch buttons, they can manage tasks such as AD sampling, LED control, and communication, among others. Previously, achieving touch button functionality required two chips: a touch chip and a primary MCU. Now, a single touch chip is sufficient to address the issue, significantly reducing product costs and enabling a wider range of products to incorporate touch button technology.
Due to the integration of touch chip into MCU, the detection of buttons has become more flexible, allowing for matrix scanning or AD sampling. This enables the recognition of a greater number of buttons, which may have required multiple touch chips in the past. Now, a single touch chip can fully meet the requirements.