9

AT42QT2100 [DATASHEET]

9554E–AT42–01/13

3.

Wiring and Parts

3.1

Cs Sample Capacitors

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.

3.2

Rs Series Resistors

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.

3.3

Power Supply

The power supply can range from 2.0 V to 5.5 V. 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 2.0 V and 5.5 V. 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 VSS and VDD pins.

3.4

MLF Package Restrictions

The central pad on the underside of the MLF chip should be connected to ground. Do not run any tracks underneath

the body of the chip, only ground.

Figure 3-1 shows an example of good and bad tracking.