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

2.3 LDC211x/LDC3114 Design Boundary Conditions

The LDC2112 and LDC2114 are high resolution Inductance-to-Digital Converters (LDCs) which internal algorithms can detect inductance shifts corresponding to button presses on metal or other surfaces. These devices require that the attached sensors meet the following parameters:

  • 1 MHz ≤ fSENSOR ≤ 30 MHz
  • 350 Ω ≤ RP ≤ 10 kΩ
  • 5 ≤ Q ≤ 30

The LDC3114 and LDC3114-Q1 are similar devices but require the sensor frequency to be between 5 MHz and 30 MHz and offer slightly different functionality such as the ability to access the raw data before the internal algorithm if desired. If the sensor parameters are not within these specifications, the LDC may not be able to measure inductance shifts, and as a result will not indicate Inductive Touch events. #T4726003-55 show these restrictions, which is derived from GUID-6AC968E3-0011-4198-B542-46F8F31E3EDE.html#X6785.

GUID-20DE2181-3A33-4D4E-B614-B4457D277802-low.pngFigure 2-3 LDC2112/LDC2114 Operating Region

Use #GUID-46511A1B-6819-4475-B37F-FD600103DBCD to calculate the minimum sensor frequency on the top-right boundary:

Equation 4. GUID-271E5395-20B6-4E8D-875F-D02F879888E6-low.gif

Use #GUID-F26F3761-4BC7-4561-A20B-E9F10E6F6080 to calculate the maximum sensor frequency on the bottom-left boundary:

Equation 5. GUID-583178DB-2AC4-4697-B400-549182F760ED-low.gif

On the left, if the sensor capacitance is too small, then parasitic capacitance effects may degrade the sensor operation. While this boundary is shown at 10 pF, some systems may encounter issues even with larger sensor capacitances. In general, TI recommends to use a sensor capacitance larger than 22 pF.