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QT60160 Datasheet(PDF) 6 Page - List of Unclassifed Manufacturers |
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QT60160 Datasheet(HTML) 6 Page - List of Unclassifed Manufacturers |
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6 / 26 page  One way to determine X line settling time is to monitor the fields using a patch of metal foil or a small coin over the key (Figure 2.5). Only one key along a particular X line needs to be observed, as each of the keys along that X line will be identical. The 500ns dwell time should exceed the observed 95 percent settling of the X-pulse by 25 percent or more. In almost all cases, Ry should be set equal to Rx, which will ensure that the charge on the Y line is fully captured into the Cs capacitor. 2.8 Key Design Circuits can be constructed out of a variety of materials including conventional FR-4, Flexible Printed Circuit Boards (FPCB), silver silk-screened on PET plastic film, and even inexpensive punched single-sided CEM-1 and FR-2. The actual internal pattern style is not as important as the need to achieve regular X and Y widths and spacings of sufficient size to cover the desired graphical key area or a little bit more; ~3mm oversize is acceptable in most cases, since the key’s electric fields drop off near the edges anyway. The overall key size can range from 6mm x 6mm up to 100mm x 100mm but these are not hard limits. The keys can be any shape including round, rectangular, square, etc. The internal pattern can be interdigitated as shown in Figure 2.6. For small, dense keypads, electrodes such as shown in the lower half of Figure 2.6 can be used. Where the panels are thin (usually mobile phones have panels under 2mm thick) the electrode density can be quite high. For better surface moisture suppression, the outer perimeter of X should be as wide as possible, and there should be no ground planes near the keys. The variable ‘T’ in this drawing represents the total thickness of all materials that the keys must penetrate. 2.9 PCB Layout, Construction 2.9.1 Overview It is best to place the chip near the touch keys on the same PCB so as to reduce X and Y trace lengths, thereby reducing the chances for EMC problems. Long conn ection traces act as RF antennae. The Y (receive) lines are much more susceptible to noise pickup than the X (drive) lines. Even more importantly, all signal related discrete parts (resistors and capacitors) should be very close to the body of the chip. Wiring between the chip and the various resistors and capacitors should be as short and direct as possible to suppress noise pickup. Ground planes and traces should NOT be used around the keys and the Y lines from the keys. Ground areas, traces, and other adjacent signal conductors that act as AC ground (such as Vdd and LED drive lines etc.) will absorb the received key signals and reduce signal-to-noise ratio (SNR) and thus will be counterproductive. Ground planes around keys will also make water film effects worse. Ground planes, if used, should be placed under or around the QT chip itself and the associated resistors and capacitors in the circuit, under or around the power supply, and back to a connector, but nowhere else. 2.9.2 LED Traces and Other Switching Signals Digital switching signals near the Y lines will induce transients into the acquired signals, deteriorating the SNR perfomance of the device. Such signals should be routed away from the Y lines, or the design should be such that these lines are not switched during the course of signal acquisition (bursts). 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. 2.9.3 PCB Cleanliness 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 ‘no 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. 2.10 Power Supply Considerations The power supply can range from +1.8V to +5V nominal. The device can tolerate ±5mV/s short-term power supply fluctuations. If the 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 these devices use 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. Caution: A regulator IC shared with other logic can result in erratic operation and is not advised. A regulator can be shared among two or more QT devices on one board. One such regulator known to work well with QT chips is the S-817 series from Seiko Instruments (Seiko Instruments - www.sii-ic.com). lQ 6 QT60240-ISG R8.06/0906 |
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