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

2.5 Example Sensor

For this example, a dual sensor design is presented. #T4726003-77 shows the sensors are 2.85 mm × 8 mm in size with eight turns. The traces are 0.25 oz-cu (9 µm) thick, are 75 µm wide and have a spacing of 50 µm. The sensor free-space inductance is approximately 1.3 µH, and has a 47-pF sensor capacitor. When mounted, the sensor inductance decreases due to interaction with the conductive target.

GUID-6CF80003-F00B-4CE2-AEE3-7DE5CCE14420-low.png Figure 2-13 Sensor Racetrack Routing

The parameters of this sensor were estimated using the Racetrack Inductor Designer tab on the LDC Calculations Tool. #T4726003-78 is an example of the tool entries used to design the sensor described here. Note that the tool provides estimates of the sensor parameters such as RS, RP, Q, L, and frequency, based on GUID-6AC968E3-0011-4198-B542-46F8F31E3EDE.html#X6785, GUID-9C1D6CFD-667C-4F3D-BDFE-DE70B8873788.html#X1756, and GUID-5C771CC9-1180-4B3C-97F1-5477D165F8C1.html#X3711.

GUID-BFE4B51D-9E64-4176-A711-BB85220F800D-low.png Figure 2-14 Racetrack Inductor Design Tab of the LDC Calculations Tool

The tool output includes estimates for both free space parameters (no target present) and their values when the sensor is mounted in the system with a target close by. As seen in Table 2-2, the sensor parameters are within the LDC211x/LDC3114 operating space when the sensor is mounted.

Table 2-2 Sensor Parameters
SENSOR PARAMETERS SENSOR IN FREE SPACE SENSOR MOUNTED LDC211x/LDC3114 OPERATING SPACE
Sensor Inductance 1.3 µH 0.76 µH
Sensor Capacitance 47 pF 47 pF
Sensor Frequency 19.4 MHz 26.7 MHz 1 MHz – 30 MHz (LDC211x)
5 MHz – 30 MHz (LDC3114)
Sensor RP 7.3 kΩ 1.4 kΩ 350 Ω ≤ RP ≤ 10 kΩ
Sensor Q 41 11 5 ≤ Q ≤ 30

The routing between the sensor and the connector is shielded by the top and bottom layers, which are driven by the COM signal. Regularly spaced vias are used to tie the top and bottom shields.

The bend in the shielded routing is used for strain relief.

GUID-619ECBEF-6BEB-4BE3-B59F-D0558B80E558-low.png Figure 2-15 Example Dual Sensor Design

The stiffeners and spacers are integrated into the sensors for this example. #T4726003-81 shows the arrangement of the spacer and stiffeners.

GUID-C4F18FAB-6A3B-49B7-A237-4889A7B2D27E-low.png Figure 2-16 Sensor Region Construction

Each sensor region has a dedicated stiffener and two spacers. The flex sensor region between the two sensors provides mechanical isolation between the two sensors.

Table 2-3 shows the sensor stack. The thickness of the stiffener can be varied based on mechanical considerations. In general, incorporating the spacer into the sensor manufacturing can usually provide a tighter tolerance for the spacer thickness than machining the spacer on the case.

Table 2-3 Sensor Stack
LAYER TYPE MATERIAL THICKNESS (mil) THICKNESS (mm) DIELECTRIC MATERIAL
Stiffener Dielectric Core 32 0.813 FR4
Top Overlay Overlay
Flex Top Coverlay Solder Mask/Coverlay Surface Material 0.4 0.010 Coverlay
Top Layer Signal Copper 0.46 0.012
Flex1 Dielectric Film 0.47 0.012 Polyimide
Signal Layer Signal Copper 0.46 0.012
Flex2 Dielectric Film 1 0.025 Polyimide
Bottom Layer Signal Copper 0.46 0.012
Flex Bottom Coverlay Solder Mask/Coverlay Surface Material 0.4 0.010 Coverlay (PI)
Bottom Solder 1 Solder Mask/Coverlay Surface Material 0.4 0.010 Solder Resist
Bottom Overlay Overlay
Spacer Dielectric Film 5 0.127 Polyimide
Total Thickness 41.05 1.043

The spacer and stiffener are only present for a portion of the sensor design, as shown in #T4726003-83. The spacer is only required on the ends of the button locations. The stiffener is required over the sensor and any connectors. The stiffener can be manufactured with a thinner material, if needed for a specific application.

GUID-52064F8A-1D28-4B43-8DD1-F8B2AC56A9EF-low.png Figure 2-17 Sensor Stack Across Regions