AN4310 STMICROELECTRONICS | Alldatasheet

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Technical content

Sampling capacitor selection guide for MCU based touch sensing

applications

Capacitors feature some non-ideal characteristics that unfortunately limit their use in certain applications. The objective of this application note is to help designers in selecting the right sampling capacitor (CS) for their touch sensing applications by investigating the most important undesirable characteristics. Note: STMicroelectronics is providing free STMT ouch touch sensing firmware libraries which are available either as standalone packages (STM8L-TOUCH-LIB) or directly integrated into the corresponding STM32Cube package (STM32CubeL0, STM32CubeF0, …). Table 1. Applicable products STM8L series, STM8AL series.

Charge transfer acquisition principle overview AN4310

1 Charge transfer acquisi tion principle overview

STM32xx, STM8L and STM8AL touch sensing applications may use charge transfer acquisition principle(a) to sense changes in capacitance. The electrode capacitance (CX) is charged to a stable reference voltage (VDD for general purpose STM8/STM32 devices). The charge is then transferred to a known capacitor referred to as the sampling capacitor (CS). This sequence is repeated until the voltage on the CS capacitor reaches an internal reference voltage (VIH for general purpose STM8/STM32 devices). The number of transfers required to reach the threshold depends on the size of the electrode capacitance and represents its value. To ensure stable operation of the solution, the number of transfers needed to reach the threshold is adjusted by an infinite impulse response (IIR) filter which compensates for environmental changes such as temperature, power supply, moisture, and surrounding conductive objects. Since the C S capacitor is an integral part of the design, it is important to consider the non- ideal effects of capacitors. a. Charge transfer acquisition principle is supported by STMTouch touch sensing libraries. See Introduction for a list of supported microcontrollers and associated STMTouch libraries.

2 Capacitor characteristics

  • Series resistance
  • Series inductance
  • Parallel resistance (leakage current)
  • Non-zero temperature coefficient
  • Dielectric absorption (DA) or soakage
  • Dissipation Factor The three most important characteristics that need to be examined are non-zero temperature coefficient, dissipation factor and dielectric absorption (DA). The effect of these non-ideal characteristics on the operation of the system will be briefly examined in the following sections.

2.1 Dielectric absorption or soakage

capacitive sensors that rely on a stable reference capacitor. the insulation resistor (IR) to cause a voltage offset on the CS capacitor. Figure 1. Model of dielectric absorption proximity detections to occur. be selected, ensuring a more stable and reliable design with improved proximity detections.

Capacitor characteristics AN4310

2.2 Non-zero temperature coefficient

To ensure trouble free operation over the final application operating temperature range, it is important to select a capacitor featuring a stable temperature coefficient. Dielectrics like PET, PEN, PPS and NPO usually have higher temperature characteristics than normal ceramic capacitors and are thus recommended.

2.3 Dissipation factor

The dissipation factor is an indication of the energy loss, usually in the form of heat. Capacitors with a high dissipation factor generally cause self-heating which affects the capacitance. This change in capacitance in turn affects the number of charge transfers needed to reach the internal reference voltage threshold. This also emphasizes the need to choose a dielectric with a stable temperature coefficient. Please refer to Table 2 for a comparison of the dissipation factors for the various dielectrics.

3 Capacitor comparison

applications when a less sensitive level is required. Table 2. Characteristics of film SMD capacitors

4 Conclusion

As explained, the sampling capacitor characteristics play an important role in the correct and stable operation of a capacitive sensing application. Consequently, it is necessary to select it carefully. The recommendations for STMTouch touch sensing library-based applications are summarized below:

  • If the solution uses an MCU low power mode to reduce overall power consumption, PET, PEN, PPS or NPO capacitor types should be used.
  • If the solution uses linear or rotary touch sensors, PET, PEN, PPS or NPO capacitor types should be used.
  • If the solution uses only touchkey sensors, all capacitor types except tantalum can be used.

5 Revision history

Table 3. Document revision history 15-Jul-2013 1 Initial release. 11-Jun-2014 2 Added support for STM32L0 series and STM8AL series.