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 chosen dwell time should exceed the observed 95%
settling of the X-pulse by 25% or more.
In almost all case, Ry should be set equal to Rx, which will
ensure that the charge on the Y line is fully captured into the Cs
capacitor.
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 of any type,
to suppress capacitive coupling effects which can induce false
signal shifts. Led terminals which are constantly connected to
Vss or Vdd do not need further bypassing.
2.9 Key Design
Circuits can be constructed out of a variety of materials
including flex circuits, FR4, and even inexpensive single-sided
CEM-1.
The actual internal pattern style is not as important as is 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 10mm x 10mm 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 as simple as a single bar of Y within a solid perimeter of
X, or (preferably) interdigitated as shown in Figure 2-6.
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.
See Figure 2-6 and page 30 for examples of key layouts.
2.10.2 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.
2.11 Power Supply Considerations
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 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.
Ceramic 0.1uF bypass capacitors should be placed very close
and routed with short traces to all power pins of the IC. There
should be at least 3 such capacitors around the part.
2.10 PCB Layout, Construction
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 (R’s and
C’s) should be very close to the body of the chip. Wiring
between the chip and the various R’s and C’s 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 R’s and C’s in the circuit,
under or around the power supply, and back to a connector, but
nowhere else.
See page 30 for an example of a 1-sided PCB layout.
2.12 Startup / Calibration Times
The devices require initialization times as follows:
1. From very first powerup to ability to communicate:
2,083ms (one time event to initialize all of eeprom, or to
recover eeprom copy from FLASH in the event of
eeprom corruption)
2. From powerup to ability to communicate:
2,100 ms in the event the part is being forced to restore
the factory defaults.
3. From powerup to ability to communicate:
36 ms in the event the setups have been changed and
the part needs to backup the EEPROM to flash.
4. Normal cold start to ability to communicate:
3ms (Normal initialization from any reset)
5. Calibration time per key vs. burst spacings for 32 and 48
enabled keys (Table below):
2.10.1 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
lQ
6
QT60486-AS R8.01/0105
QUANTUM [ QUANTUM RESEARCH GROUP ]
