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

2.4.6 Multi-Layer Design

The inductance of a sensor is a function of the area, and the number of windings, and target distance. With many inductive touch applications, the desired physical size of the buttons may be 3-mm in diameter or smaller. The low total inductance of smaller sensors may result in a sensor frequency which is outside the design space of the LDC. By using multiple layers of alternating rotation sensors, the total inductance, due to additional mutual inductance between layers, is significantly higher compared to a single layer design.

GUID-798C6A2E-E82B-4211-92F5-660E74FDA2A6-low.pngFigure 2-8 2-Layer Sensor Design

For most applications, 2-layer or 4-layer designs are sufficient. While a 4-layer sensor is more complex and expensive compared to a similar geometry 2-layer sensor, the LDC211x and LDC3114 can effectively drive a physically smaller 4-layer sensor, as shown in #GUID-1D47ED31-A7E1-4356-B584-99B68CC4AF6D/T4726003-68.

Use of a single layer sensor is generally not as effective, as the mutual coupling between layers in a multilayer sensor provides a significant increase in the sensor inductance. In addition, there needs to be a second routing to bring the sensor current out from the center of the sensor back to the LDC.

Table 2-1 Approximate Minimum Sensor Width vs Fabrication Restrictions
AVAILABLE SPACING DISTANCE BETWEEN TURNSNUMBER OF LAYERSMINIMUM VIA SIZEMINIMUM SENSOR WIDTH
4 mil (0.1016 mm)215 mil (0.4 mm)2.85 mm
4 mil (0.1016 mm)415 mil (0.4 mm)2.30 mm
3 mil (0.076 mm)215 mil (0.4 mm)2.05 mm
3 mil (0.076 mm)415 mil (0.4 mm)1.91 mm
2 mil (0.051 mm)215 mil (0.4 mm)1.65 mm
2 mil (0.051 mm)415 mil (0.4 mm)1.53 mm
2 mil (0.051 mm)412 mil (0.305 mm)1.38 mm

Minimum sensor width of a fixed 8-mm sensor length with a target distance of 0.2 mm. These sensors have not been evaluated for performance. These sensors assume a 1-mil (25 µm) dielectric thickness between layers.