AN4310 Application note Sampling capacitor selection guide for MCU based touch sensing applications Introduction 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 STMTouch 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 Type Microcontrollers October 2015 Applicable products STM32F0 Series, STM32F3 Series, STM32L0 Series, STM32L1 Series, STM32L4 Series, STM8L Series, STM8AL Series. DocID024789 Rev 3 1/11 www.st.com Contents AN4310 Contents 1 Charge transfer acquisition principle overview . . . . . . . . . . . . . . . . . . . 5 2 Capacitor characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1 Dielectric absorption or soakage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2 Non-zero temperature coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 Dissipation factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Capacitor comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2/11 DocID024789 Rev 3 AN4310 List of tables List of tables Table 1. Table 2. Table 3. Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Characteristics of film SMD capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 DocID024789 Rev 3 3/11 3 List of figures AN4310 List of figures Figure 1. 4/11 Model of dielectric absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 DocID024789 Rev 3 AN4310 1 Charge transfer acquisition principle overview Charge transfer acquisition 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 CS capacitor is an integral part of the design, it is important to consider the nonideal 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. DocID024789 Rev 3 5/11 10 Capacitor characteristics 2 AN4310 Capacitor characteristics The most common short comings of capacitors are the following: • 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 Dielectric absorption (DA) or soakage can be detrimental to the operation and accuracy of capacitive sensors that rely on a stable reference capacitor. DA is caused by the charge that is soaked-up in the dielectric and remains there during the discharge period. The charge then trickles back out of the dielectric during the relaxation period and cause a voltage to appear on the CS capacitor. This phenomenon effectively creates a memory effect in the capacitor. The size of the offset voltage is dependent on the relaxation time between transfers and the discharge time of the CS capacitor. This phenomenon is illustrated in Figure 1. The residual charge bleeds back (IRESIDUAL) through the insulation resistor (IR) to cause a voltage offset on the CS capacitor. Figure 1. Model of dielectric absorption ,5 &6 ,5(6,'8$/ &5(6,'8$/ 069 This offset voltage influences the sensitivity of the system by reducing the number of transfers needed to reach the internal reference voltage threshold and may cause false proximity detections to occur. By choosing a capacitor with a low dielectric absorption factor, a higher sensitivity level can be selected, ensuring a more stable and reliable design with improved proximity detections. Refer to Table 2 for a comparison of dielectric absorption factors for the different types of capacitor dielectrics. 6/11 DocID024789 Rev 3 AN4310 2.2 Capacitor characteristics 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. DocID024789 Rev 3 7/11 10 Capacitor comparison 3 AN4310 Capacitor comparison Table 2 compares the most important characteristics that need to be reviewed when selecting a CS capacitor. Table 2. Characteristics of film SMD capacitors PET PEN PPS NPO X7R Tantalum Operating temperature (°C) −55 to 125 −55 to 125 −55 to 140 −55 to 125 −55 to 125 −55 to 125 ΔC/C with temperature (°C) ±5 ±5 ±1.5 ±1 ±1 ±10 1 kHz 0.8 0.8 0.2 0.1 2.5 8 10 kHz 1.5 1.5 0.25 0.1 - - 100 kHz 3.0 3.0 0.5 0.1 - - Dielectric absorption (%) 0.5 1 0.05 0.6 2.5 - ESR Low Low Very low Low Moderate to high High Reliability High High High High Moderate Low Dissipation factor (%) The PPS (polyphenylene sulfide) dielectric and the NPO ceramic capacitors performs excellently in all categories. The PET (metalized polyester) and the PEN (metalized polyphenylene naphthalate) capacitors also perform quite well and can be used in all touch sensing applications. Tantalum capacitors should be avoided as they have a very high dissipation factor and a high effective series resistance (ESR). X7R ceramic capacitors can be used in certain applications when a less sensitive level is required. 8/11 DocID024789 Rev 3 AN4310 4 Conclusion 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. DocID024789 Rev 3 9/11 10 Revision history 5 AN4310 Revision history Table 3. Document revision history 10/11 Date Revision Changes 15-Jul-2013 1 Initial release. 11-Jun-2014 2 Added support for STM32L0 Series and STM8AL Series. 20-Oct-2015 3 Updated Table 1 Added support for STM32L4 Series DocID024789 Rev 3 AN4310 IMPORTANT NOTICE – PLEASE READ CAREFULLY STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, enhancements, modifications, and improvements to ST products and/or to this document at any time without notice. Purchasers should obtain the latest relevant information on ST products before placing orders. ST products are sold pursuant to ST’s terms and conditions of sale in place at the time of order acknowledgement. Purchasers are solely responsible for the choice, selection, and use of ST products and ST assumes no liability for application assistance or the design of Purchasers’ products. No license, express or implied, to any intellectual property right is granted by ST herein. Resale of ST products with provisions different from the information set forth herein shall void any warranty granted by ST for such product. ST and the ST logo are trademarks of ST. All other product or service names are the property of their respective owners. Information in this document supersedes and replaces information previously supplied in any prior versions of this document. © 2015 STMicroelectronics – All rights reserved DocID024789 Rev 3 11/11 11