QST104KT6 STMicroelectronics, QST104KT6 Datasheet - Page 27

• Manufacturer Part Number: QST104KT6
• Description: Capacitive Touch Sensor Device 4 Keys With Individual Key State Outputs Or I2c Interface
• Manufacturer: STMicroelectronics

QST104
5.3
5.4
5.5
5.6
Power supply
If the power supply fluctuates slowly with temperature, the QST device compensates
automatically for these changes with only minor changes in sensitivity. However, 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 power supply should be locally regulated, using a three-terminal regulator. 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 which can cause adverse effects. It is not
recommended to include a series inductor in the power supply to the QST device.
For proper operation, a 0.1 µF or greater bypass capacitor must be used between V
V
V
The PCB should, if possible, include a copper pour under and around the device, but not
extensively under the SNS lines.
ESD protection
In normal environmental conditions, only one series resistor is required for ESD
suppression. A 10 kOhm R
cases. The dielectric panel (glass or plastic) usually provides a high degree of isolation to
prevent ESD discharge from reaching the circuit. R
the C
deteriorate. If a reduction in R
Conversely, increasing the R
increase does not decrease sensitivity.
Crosstalk precautions
Adjacent sense traces might require intervening ground traces in order to reduce capacitive
cross bleed if high sensitivity is required or high values of delta-C
example, from direct human touch to an electrode connection). In normal touch applications
behind plastic panels, this is rarely a problem regardless of how the electrodes are wired.
Higher values of R
if possible. In general try to keep the QST device close to the electrodes and reduce the
adjacency of the sense wiring to ground planes and other signal traces; this will reduce the
C
reason to run ground near SNS traces is to provide crosstalk isolation between traces, and
then only on an as-needed basis.
PCB layout and construction
The PCB traces, wiring, and any components associated with or in contact with either SNS
pin will become touch sensitive and should be treated with caution to limit the touch area to
the desired location.
Multiple touch electrodes connected to any sensing channel can be used, for example, to
create control surfaces on both sides of an object.
SS
SS
x
load, reduce interference effects, and increase signal gain. The one and only valid
. The bypass capacitor should be routed with very short tracks to the device’s V
pins.
X
load is high, R
S
will make crosstalk problems worse; try to keep R
S
can prevent total charge and transfer and as a result gain can
S
S
resistor in series with the sense trace is sufficient in most
S
can result in added ESD and EMC benefits, provided that the
increases gain noticeably, the lower value should be used.
S
should be placed close to the chip. If
X
are anticipated (for
S
to 22 kOhm or less
Design guidelines
DD
DD
and
27/45
and