SNOA961A February   2017  – February 2023 LDC2112 , LDC2114 , LDC3114 , LDC3114-Q1

 

  1.   Inductive Touch System Design Guide for HMI Button Applications
  2. 1Mechanical Design
    1. 1.1 Theory of Operation
    2. 1.2 Button Construction
    3. 1.3 Mechanical Deflection
    4. 1.4 Mechanical Factors that Affect Sensitivity
      1. 1.4.1 Target Material Selection
        1. 1.4.1.1 Material Stiffness
        2. 1.4.1.2 Material Conductivity
      2. 1.4.2 Button Geometry
      3. 1.4.3 Spacing Between Target and Sensor
    5. 1.5 Layer Stacks of Touch Buttons
      1. 1.5.1 Conductive Surface
      2. 1.5.2 Non-Conductive Surface
    6. 1.6 Sensor Mounting Reference
    7. 1.7 Sensor Mounting Techniques
      1. 1.7.1 Adhesive-Based
      2. 1.7.2 Spring-Based
      3. 1.7.3 Slot-Based
    8. 1.8 Mechanical Isolation
  3. 2Sensor Design
    1. 2.1 Overview
      1. 2.1.1 Sensor Electrical Parameters
      2. 2.1.2 Sensor Frequency
      3. 2.1.3 Sensor RP and RS
      4. 2.1.4 Sensor Inductance
      5. 2.1.5 Sensor Capacitance
      6. 2.1.6 Sensor Quality Factor
    2. 2.2 Inductive Touch
    3. 2.3 LDC211x/LDC3114 Design Boundary Conditions
    4. 2.4 Sensor Physical Construction
      1. 2.4.1 Sensor Physical Size
      2. 2.4.2 Sensor Capacitor Position
      3. 2.4.3 Shielding INn traces
      4. 2.4.4 Shielding Capacitance
      5. 2.4.5 CCOM Sizing
      6. 2.4.6 Multi-Layer Design
        1. 2.4.6.1 Sensor Parasitic Capacitance
      7. 2.4.7 Sensor Spacers
      8. 2.4.8 Sensor Stiffener
      9. 2.4.9 Racetrack Inductor Shape
    5. 2.5 Example Sensor
  4. 3Summary
  5. 4Revision History

2.4.8 Sensor Stiffener

If a flex PCB is used for the sensor, the sensor must be supported by a stiffener. If a flex sensor is not supported, then it may deform under any movement, leading to false detection events. The support should be a uniform surface which has minimal warping across temperature, humidity, and acceleration. The supporting structure, which is often called a stiffener for LDC applications, should not be conductive; otherwise the sensor Q and RP may be reduced below the minimum levels the LDC211x/LDC3114 can support. Use of FR4 backing is a common technique for flex PCBs and is suitable for LDC sensor use. For a thinner sensor, it is acceptable to use an epoxy based stiffener.

The stiffener should be a non-conductive material, otherwise the sensor RP may be too low for the LDC211x/LDC3114 to drive; for this reason SuS and Al stiffeners should be avoided.

If multiple sensors are constructed on a single flex PCB, the stiffener should be separate for each sensor section; otherwise significantly more mechanical crosstalk can occur.

GUID-03CBCC02-4C81-4B57-8A4C-CCA35F68E29B-low.pngFigure 2-12 Separate Stiffener for Each Sensor

For some applications, the stiffener can be a component already present in the system—for example, a glass surface, or with sensors manufactured on a rigid material such as FR4.

Normal PCBs made of FR4 or other rigid materials do not require a dedicated stiffener.