QT60240-ATG-SL924 Atmel, QT60240-ATG-SL924 Datasheet - Page 6

• Manufacturer Part Number: QT60240-ATG-SL924
• Description: Interface - Specialized Integrated Circuit
• Manufacturer: Atmel

Specifications of QT60240-ATG-SL924

• Product Category: Interface - Specialized
• Rohs: yes
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: MLF-32
• Minimum Operating Temperature: - 40 C

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.
lQ
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 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
6
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).
QT60240-ISG R8.06/0906