The LDC3114-Q1 is the inductive sensing IC used to implement the touch buttons for this reference design. There are two of these devices implemented on the board, each running four buttons. The LDC3114-Q1 uses an inductive coil to determine the change in a metal target. The force on the metal target creates a deflection in the button case and this deformation is measured by the IC to determine a button press. Additionally, the LDC3114-Q1 includes a baseline tracking algorithm that monitors the button surface and can adapt to environmental factors such as temperature change or damage to the surface. This device also includes digital outputs that trigger when the button data crosses above the given threshold. Since the LDC3114-Q1 does not have a changeable I2C address, an I2C switch (PCA9543) is used to communicate to both devices. This allows for each device to continuously run the baseline algorithm that monitors the button surface.

Implementing an inductive touch button requires a metal target that is at a fixed distance from the sensor coil. Aside form the general design theory for inductive sensors, there are a few important design considerations to keep in mind when designing inductive touch buttons. For some button designs, the desired touch surface is not metal. In this case, a thin metal plane can be added behind a non-conductive surface, like plastic, that acts as the metal target for the design. The distance between the sensor coil and the metal target is an important factor for getting the desired button sensitivity for an application. The general guideline for this is for the distance to stay between 3%–20% of the coil diameter. This makes sure that the metal target does not touch the sensor coil when it deflects but keeps it close enough to obtain a high force sensitivity. If the metal target gets farther away from the sensor coil, then the sensitivity of the button has decreased and can require more force to get a proper button output. Since the target distance is an import part of the design, the coil diameter becomes a critical design parameter as well. Many times, this parameter is constrained by the space available for the sensor coils to be implemented on a PCB. From there, other factors like the number of turns, trace width, trace spacing, and number of layers all account for the overall coil design. The trace width and spacing can be limited by the PCB manufacturing process but can also be useful for changing the series resistance of the inductor coil or allowing for more turns to be included. Generally, it is recommended to use the number of turns to set the inner diameter of the inductive coil between 20-80% since the inner turns do not have as much impact on the magnetic field. However, in a button application, the target is close enough that the inner coils still provide additional benefit so increasing the number of turns to decrease the inner diameter can be considered for the design. For more information on button design, please read the Inductive Touch System Design Guide for HMI Button Applications.

For this reference design, the mechanical structure of the button is comprised of the 3D printed housing, the PCB, and metal tape to provide a target surface. PCB space for the button was not a concern for this design so an 8 mm diameter was chosen for the coil design. The LDC Calculator Tool spreadsheet was used to help determine the rest of the coil parameters. A trace width and spacing of 5 mils was used along with 8 turns per layer. This makes the coil inner diameter slightly smaller than 4 mm. Or in other words, the coil fill ratio (inner diameter divided by outer diameter) is roughly 50%. For most button designs, minimizing the coil fill ratio can provide additional sensitivity but requires the target surface to be very close to the coil for a beneficial effect. Otherwise, it is best to keep the ratio between 20% and 80% to maximize the Q factor of the design. Since this is a two layer board, the number of layers for the coil design was set to two. The capacitance for this sensor design was selected to be 220 pF which put the frequency at 8.396 MHz.

Figure 2-2 TIDA-060039 LDC Sensor Coil Layout

Based on the coil diameter, the target distance is determined to be 0.8 mm away from the coil. This puts the target well within the recommended 3%–20% range of the coil diameter which gives a high sensitivity in response to a force on the button surface. Going back to the calculator spreadsheet, the target distance can be input to double check the design choices. With the target at 0.8 mm, the sensor frequency is now 11.081 MHz with a Q factor 0f 26. The spreadsheet gives a warning if any of the final parameters are out of range for the device, but in this case, no warnings appear.

Since the button surface is 3D printed, it also includes the required spacer for the button design instead of having a separate spacer material. The standoff and button surfaces are both 1 mm thick to provide a surface that has some slight flexibility and to set our target height as desired. The metal tape is then placed inside the button surface between the standoffs so that it deflects in the desired area when a force is applied to the button surface. The thickness of the tape then puts the target at about 0.8 mm away from the sensor coil.

Figure 2-3 Inductive Touch Button Stackup

The material of the button surface has an impact on how much deflection occurs to the metal target. Materials that are more stiff or that absorb the force cause less deflection and therefore require more force for a button press to be detected. This also comes into play when considering the thickness of the button surface. The LDC Calculator Tool Spreadsheet has a tab for determining the deflection of a material if the Young's Modulus and Poisson Ratio are known for the material. Since this design is 3D printed using nylon 12, a deflection of around 20 μm is expected for a 2N force applied to the surface. This amount of deflection is plenty for this button design since the target is so close to the sensor to begin with.

Using metal tape or a small metal target on the inside of the button surface allows for non-metal materials to work in a button fabrication. Performance varies depending on the metal used for the target. Use a metal with a high conductivity to maximize the sensitivity of the button. Because of this, both copper and aluminum tape are strong options since these materials have a high conductivity and can easily be cut to fit within the spacers for the button design. An alternative approach to this is having a metal layer attach to the button surface and placing the spacers between the metal layer and PCB sensors such as in the Figure 2-4. See the Inductive Touch System Design Guide for HMI Button Applications application note for more information on button design.

Figure 2-4 Non-Conductive Touch Button Alternative Stackup Example