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

3.3.2 Power Consumption

The full sample speed of the LDC is not required for HMI application which allows for duty cycling and power consumption savings. The LDC can be sampled at periodic intervals to conserve power consumption, while still providing real-time button detection.

The Inductive Sensing Design Calculator Tool spreadsheet (SLYC137) can calculate the average current consumption in duty-cycled applications. A typical application with two buttons and a sample rate of 10 samples per second may have an average current consumption of 225 µA (113 µA per button), as shown Table 3. In battery powered applications, the tradeoff between sample rate and resolution can be adjusted for further power savings.

Table 3. Average Current Consumption

Parameter Value Unit
fREFERENCE input to CLKIN 20 MHz
RCOUNT Setting (range:0x0003 to 0xFFFF) 1000 hex
Desired Sample Rate 10 sps
Number of channels of measurement 2
Sensor Frequency (open air) 8 MHz
Sensor Frequency (in presence of Aluminum at 0.55mm distance) 9.1 MHz
Sensor Q (max) 20
I2C Data rate 400 kbit/s
Sensor RP 10 kΩ
LDC Device LDC1612
Programmed settle count 64
Minimum Settle Count Register Setting 0x4 (registers 0x10:0x13)
single conversion time 3.28 ms
LDC Current consumption using Shutdown Mode 225.29 µA
LDC Current consumption using Sleep Mode 256.24 µA