QST108 datasheet(25/38 Pages) STMICROELECTRONICS

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QST108

Design guidelines

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5.3

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 VDD and

VSS. The bypass capacitor should be routed with very short tracks to the device’s VDD and

VSS pins.

The PCB should, if possible, include a copper pour under and around the device, but not

extensively under the SNS lines.

5.4

ESD protection

In normal environmental conditions, only one series resistor is required for ESD

suppression. A 10 kOhm RS resistor in series with the sense trace is sufficient in most

cases. The dielectric panel (glass or plastic) usually provides a high degree of isolation to

prevent ESD discharge from reaching the circuit. RS should be placed close to the chip. If

the CX load is high, RS can prevent total charge and transfer and as a result gain can

deteriorate. If a reduction in RS increases gain noticeably, the lower value should be used.

Conversely, increasing the RS can result in added ESD and EMC benefits, provided that the

increase does not decrease sensitivity.

5.5

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-CX are anticipated (for

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 RS will make crosstalk problems worse; try to keep RS to 22 kOhm or less

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

Cx load, reduce interference effects, and increase signal gain. The one and only valid

reason to run ground near SNS traces is to provide crosstalk isolation between traces, and

then only on an as-needed basis.

5.6

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.

QST108 Datasheet(PDF) 25 Page - STMicroelectronics

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