AT42QT1012_10 ATMEL [ATMEL Corporation], AT42QT1012_10 Datasheet - Page 20

• Manufacturer Part Number: AT42QT1012_10
• Description: One-channel Toggle-mode Touch Sensor IC with Power Management Functions
• Manufacturer: ATMEL [ATMEL Corporation]

4. Circuit Guidelines
4.1
4.2
4.3
4.4
20
More Information
Sample Capacitor
Rs Resistor
Power Supply and PCB Layout
AT42QT1012
Refer to Application Note QTAN0002, Secrets of a Successful QTouch™ Design and the Touch
Sensors Design Guide (both downloadable from the Atmel
construction and design methods.
Cs is the charge sensing sample capacitor. The required Cs value depends on the thickness of
the panel and its dielectric constant. Thicker panels require larger values of Cs. Typical values
are 2.2 nF to 50 nF depending on the sensitivity required; larger values of Cs demand higher
stability and better dielectric to ensure reliable sensing.
The Cs capacitor should be a stable type, such as X7R ceramic or PPS film. For more consistent
sensing from unit to unit, 5 percent tolerance capacitors are recommended. X7R ceramic types
can be obtained in 5 percent tolerance at little or no extra cost. In applications where high
sensitivity (long burst length) is required the use of PPS capacitors is recommended.
For battery powered operation a higher value sample capacitor may be required.
Series resistor Rs is in line with the electrode connection and should be used to limit ESD
currents and to suppress radio frequency interference (RFI). It should be approximately
4.7 k to 33 k.
Although this resistor may be omitted, the device may become susceptible to external noise or
RFI. See Application Note QTAN0002, Secrets of a Successful QTouch™ Design, for details of
how to select these resistors.
See
If the power supply is shared with another electronic system, care should be taken to ensure that
the supply is free of digital spikes, sags, and surges which can adversely affect the QT1012. The
QT1012 will track slow changes in Vdd, but it can be badly affected by rapid voltage fluctuations.
It is highly recommended that a separate voltage regulator be used just for the QT1012 to isolate
it from power supply shifts caused by other components.
If desired, the supply can be regulated using a Low Dropout (LDO) regulator, although such
regulators often have poor transient line and load stability. See Application Note QTAN0002,
Secrets of a Successful QTouch™ Design, for further information on power supply
considerations.
Parts placement: The chip should be placed to minimize the SNSK trace length to reduce low
frequency pickup, and to reduce stray Cx which degrades gain. The Cs and Rs resistors (see
Figure 1-1 on page
trace between Rs and the SNSK pin is very short, thereby reducing the antenna-like ability of
this trace to pick up high frequency signals and feed them directly into the chip. A ground plane
can be used under the chip and the associated discrete components, but the trace from the Rs
resistor and the electrode should not run near ground, to reduce loading.
For best EMC performance the circuit should be made entirely with SMT components.
Section 5.2 on page 22
3) should be placed as close to the body of the chip as possible so that the
for the power supply range.
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website), for more information on
9543D–AT42–08/10