SNOA951 June   2016 LDC1312 , LDC1312-Q1 , LDC1314 , LDC1314-Q1 , LDC1612 , LDC1612-Q1 , LDC1614 , LDC1614-Q1

 

  1.   Inductive Sensing Touch-On-Metal Buttons Design Guide
    1.     Trademarks
    2. 1 ToM Basics
    3. 2 How Are Inductive Touch-On-Metal Buttons Implemented?
    4. 3 System Design Procedure
      1. 3.1 Mechanical System Design
        1. 3.1.1 Designing for Natural Button Force
          1. 3.1.1.1 Metal Composition
          2. 3.1.1.2 Metal Thickness
          3. 3.1.1.3 Mechanical Structure of the Button
        2. 3.1.2 Target Distance
        3. 3.1.3 Mechanical Isolation
        4. 3.1.4 Mounting Techniques
      2. 3.2 Sensor Design
        1. 3.2.1 PCB Design
        2. 3.2.2 Sensor Frequency Selection
        3. 3.2.3 Sensor Amplitude Selection
      3. 3.3 Other Considerations
        1. 3.3.1 Button Quantity and Multiplexing
        2. 3.3.2 Power Consumption
        3. 3.3.3 Software Algorithm
        4. 3.3.4 EMI Emissions Testing
      4. 3.4 Design Implementation
    5. 4 Results
    6. 5 Summary
    7. 6 Additional resources

3.1.1.3 Mechanical Structure of the Button

The structure and shape of the button will also determine how much deflection is achieved. A ToM design should consist of a flat sheet of metal with spacers between the metal and the PCB to allow for deflection. The width and position of the spacers act like fulcrums to the metal and can improve button deflection if narrow and placed far apart. The designer may also etch a cutout beneath the button to allow for controlled deflection. This would allow the PCB to be placed flush against the metal without the use of spacers. Additionally, the cutout will decrease the thickness of the metal localized to the button area only which will improve the deflection and thus the response of the button. Depending on the available assembly processes and cost constraints, one approach may be more desirable than the other.