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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.
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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
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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
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List of figures
AN4310
List of figures
Figure 1.
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Model of dielectric absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
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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.
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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.
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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.
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Capacitor comparison
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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.
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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.
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Revision history
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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
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