Datasheet

14
9596A–AT42–10/10
AT42QT1070
LED terminals which are multiplexed or switched into a floating state, and which are within, or
physically very near, a key (even if on another nearby PCB) should be bypassed to either Vss or
Vdd with at least a 10 nF capacitor. This is 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.
3.3 PCB Cleanliness
Modern no-clean flux is generally compatible with capacitive sensing circuits.
If a PCB is reworked in any way, clean it thoroughly to remove all traces of the flux residue
around the capacitive sensor components. Dry it thoroughly before any further testing is
conducted.
3.4 Power Supply
See Section6.2 on page24 for the power supply range. If the power supply fluctuates slowly
with temperature, the device tracks and compensates for these changes automatically with only
minor changes in sensitivity. If the supply voltage drifts or shifts quickly, the drift compensation
mechanism is not able to keep up, causing sensitivity anomalies or false detections.
The usual power supply considerations with QT parts apply to the device. The power should be
clean and come from a separate regulator if possible. However, this device is designed to
minimize the effects of unstable power, and except in extreme conditions should not require a
separate Low Dropout (LDO) regulator.
It is assumed that a larger bypass capacitor (like 1 µF) is somewhere else in the power circuit;
for example, near the regulator.
To assist with transient regulator stability problems, the QT1070 waits 500 µs any time it wakes
up from a sleep state (in LP modes) before acquiring, to allow Vdd to fully stabilize.
CAUTION: If a PCB is reworked in any way, it is almost guaranteed that the behavior
of the no-clean flux will change. This can mean that the flux changes from an inert
material to one that can absorb moisture and dramatically affect capacitive
measurements due to additional leakage currents. If so, the circuit can become
erratic and exhibit poor environmental stability.
CAUTION: A regulator IC shared with other logic can result in erratic operation and is
not advised.
A single ceramic 0.1 µF bypass capacitor, with short traces, should be placed very
close to the power pins of the IC. Failure to do so can result in device oscillation,
high current consumption and erratic operation.