at42qt2160 ATMEL Corporation, at42qt2160 Datasheet - Page 11

• Manufacturer Part Number: at42qt2160
• Description: Qslide?, 16-key Qmatrix? Sensor Ic
• Manufacturer: ATMEL Corporation

4.10.3
4.10.4
4.11
9502A–AT42–07/08
Power Supply Considerations
Tracks
PCB Cleanliness
LED terminals which are multiplexed or switched into a floating state and which are within or
physically very near a key structure (even if on another nearby PCB) should be bypassed to
either Vss or Vdd with at least a 10nF capacitor to suppress capacitive coupling effects which
can induce false signal shifts. The bypass capacitor does not need to be next to the LED, in fact
it can be quite distant. The bypass capacitor is noncritical and can be of any type.
LED terminals which are constantly connected to Vss or Vdd do not need further bypassing.
The central pad on the underside of the chip should be connected to ground. Do not run any
tracks underneath the body of the chip, only ground.
Figure 4-7.
All capacitive sensors should be treated as highly sensitive circuits which can be influenced by
stray conductive leakage paths. QT devices have a basic resolution in the femtofarad range; in
this region, there is no such thing as “clean flux". Flux absorbs moisture and becomes
conductive between solder joints, causing signal drift and resultant false detections or transient
losses of sensitivity or instability. Conformal coatings will trap in existing amounts of moisture
which will then become highly temperature sensitive.
The designer should specify ultrasonic cleaning as part of the manufacturing process, and in
cases where a high level of humidity is anticipated, the use of conformal coatings after cleaning
to keep out moisture.
See
power supply 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.
As the device uses the power supply itself as an analog reference, the power should be very
clean and come from a separate regulator. A standard inexpensive Low Dropout (LDO) type
regulator should be used that is not also used to power other loads such as LEDs, relays, or
other high current devices. Load shifts on the output of the LDO can cause Vdd to fluctuate
enough to cause false detection or sensitivity shifts.
Section 10.2 on page 43
Position of Tracks
Example of g
for the Vdd range and short-term power supply fluctuations. If the
ood
tracking
Example of bad tracking
AT42QT2160
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