at42qt2100 ATMEL Corporation, at42qt2100 Datasheet - Page 8

• Manufacturer Part Number: at42qt2100
• Description: Qtouch Touch Sensor Ic
• Manufacturer: ATMEL Corporation

3. Wiring and Parts
3.1
3.2
3.3
8
Cs Sample Capacitors
Rs Series Resistors
Power Supply
AT42QT2100
The Cs (CsA1 to CSA3 and CsB1 to CSB7) sample capacitors accumulate the charge from
the key electrodes and determine sensitivity. Higher values of Cs make the corresponding
sensing channel more sensitive. The values of Cs can differ for each channel, permitting
differences in sensitivity from key to key or to balance unequal sensitivities.
Unequal sensitivities can occur due to key size and placement differences and stray wiring
capacitances. More stray capacitance on a sense trace will desensitize the corresponding key;
increasing the Cs for that key will compensate for the loss of sensitivity.
The Cs capacitors can be virtually any plastic film or low to medium-K ceramic capacitor.
Acceptable capacitor types for most uses include PPS film, polypropylene film, and NP0 and
X7R ceramics. Lower grade ceramics than X7R are not advised; the X5R grade should be
avoided because it is less stable than X7R. Larger values of Cs require better quality to ensure
reliable sensing
The “normal” Cs range is 1 nF to 100 nF for the keys and 4.7 nF to 220 nF for each channel of
the slider or wheel for good performance and position detection. The actual value used
depends on the sensitivity required. A 3 nF to 5 nF capacitor is typical for a touch key with an
electrode diameter of 10 to 12 mm and a cover of 1 to 2 mm plastic, and approximately 10 nF
to 15 nF for slider or wheel electrodes.
Series Rs resistors (RsA1 to RSA3 and RsB1 to RSB7) are in-line with the electrode
connections and are used to limit electrostatic discharge (ESD) currents and to suppress radio
frequency interference (RFI). For most applications the Rs resistors will be in the range 4.7 k
to 33 k each. In a few applications with low loading on the sense keys the value may be up to
100 k.
Although these resistors may be omitted, the device may become susceptible to external
noise or RFI. For details of how to select these resistors refer to Application Note QTAN0079
Buttons, Sliders and Wheels Sensor Design Guide.
The power supply can range from 2.0 to 5.5 volts. If this fluctuates slowly with temperature, the
device will track and compensate for these changes automatically with only minor changes in
sensitivity. If the supply voltage drifts or shifts quickly, the drift compensation mechanism will
not be able to keep up, causing sensitivity anomalies or false detections. In this situation a
dedicated voltage regulator should be included in the circuit.
The QT2100 power supply should be locally regulated using a three-terminal device, to
between 2.0V and 5.5V. If the supply is shared with another electronic system, care should be
taken to ensure that the supply is free of digital spikes, sags, and surges, all of which can
cause adverse effects.
For proper operation a 0.1 µF, or greater, bypass capacitor must be used between Vdd and
Vss; the bypass capacitor should be routed with very short tracks to the QT2100's Vss and
Vdd pins.
9554CX–AT42–11/11