An out-of-spec oscillator can induce timing problems such as
large variations in Max On-Duration times and response
times as well as the serial port baud rate range.
•
•
More stray capacitance on an electrode or sense
trace will decrease sensitivity on the corresponding
key; Cs will have to be increased to compensate.
Effect on serial communications: The oscillator frequency
has no nominal effect on serial communications since the
baud rate is set by an auto-sensing mechanism. However, if
the oscillator is too far outside the recommended settings,
the possible range of serial communications will shrink. For
example, if the oscillator is too slow, the upper baud rate will
be reduced.
An option resistor pulling low will increase sensitivity
on the corresponding key; Cs will have to be reduced
to compensate.
The Cs capacitors can be virtually any plastic film or low to
medium-K ceramic capacitor. Acceptable capacitor types for
most uses include PPS film, polypropylene film, and NP0 and
X5R / X7R ceramics. Lower grades than X5R / X7R are not
advised.
The oscillator frequency can be verified by measuring the
burst pulses at the start of a burst.
For most applications Cs will be in the range 680pF to 50nF;
larger values of Cs require better quality capacitors to ensure
reliable sensing. In a few applications sufficient sensitivity will
be achieved with Cs less than 680pF.
•
In spread-spectrum mode, the first pulses of a burst
should ideally be 2.87µs
•
In non spread-spectrum mode, the target value is
2.67µs
If very high sensitivity is required then the 50nF value may be
exceeded hence the 100nF maximum in Section 4.2,
page 13; in this case greater care should be taken over the
QT1103 circuit layout and interactions with neighboring
electronics.
If in doubt, make the pulses on the narrower side (i.e. a faster
oscillator) when using the higher baud rates, and conversely
on the wider side when using the lowest baud rates.
As the sensitivity of the keys, and hence the required values
of Cs, are affected by the presence and connection of the
option resistors (see Section 2.2, page 9), then final selection
of Cs values should take place after the options choice has
been finalized.
3.2 Spread-spectrum Circuit
The QT1103 offers the ability to spectrally spread its
frequency of operation to heavily reduce susceptibility to
external noise sources and to limit RF emissions. The SS pin
is used to modulate an external passive RC network that
modulates the OSC pin. OSC is the main oscillator current
input. The circuits and recommended values are shown in
Figures 1.1 and 1.2.
3.4 Rsns Resistors
Series resistors RSNS (RSNS0...RSNS9) are in line with the
electrode connections and should be used to limit
electrostatic discharge (ESD) currents and to suppress radio
frequency interference (RFI). For most applications RSNS will
be in the range 4.7kꢀ to 33kꢀ each. In a few applications
with low loading on the sense keys the value may be up to
100kꢀ.
The resistors Rb1 and Rb2 should be changed depending on
Vdd. As shown in Figures 1.1 and 1.2, three sets of values
are recommended for these resistors depending on Vdd. The
power curves in Section 4.6 also show the effect of these
resistors.
The circuit can be eliminated, if it is not desired, by using a
resistor from OSC to VDD to drive the oscillator, and
connecting SS to Vss with a 100kΩ resistor (see Section 3.1).
Although these resistors may be omitted, the device may
become susceptible to external noise or RFI. For details of
how to select these resistors see the Application Note
AN-KD02, downloadable from the Quantum website
http://www.qprox.com (go to the Support tab and click
Application Notes).
The spread-spectrum RC network might need to be modified
slightly with longer burst lengths. The sawtooth waveform
observed on SS should reach a crest height as follows:
•
•
Vdd >= 3.6V: 17 percent of Vdd
Vdd < 3.6V: 20 percent of Vdd
3.5 Power Supply
The power supply can range from 2.8V to 5.0V. If this
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.
The Css capacitor connected to SS (Figures 1.1 and 1.2)
should be adjusted so that the waveform approximates the
above amplitude, ±10 percent, during normal operation in the
target circuit. Where the bursts are of differing lengths, the
adjustment should be done for the longer burst. If this is
done, the circuit will give a spectral modulation of 12-15
percent. A typical value of Css is 100nF.
The power supply should be locally regulated using a
three-terminal device, to between 2.8V and 5.0V. 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 QT1103.
3.3 Cs Sample Capacitors - Sensitivity
The Cs sample capacitors accumulate the charge from the
key electrodes and hence determine sensitivity. The values
of Cs can differ for each channel, permitting differences in
sensitivity from key to key or to balance unequal sensitivities.
Higher values of Cs make the corresponding key more
sensitive.
For proper operation a 0.1µF 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 Vss
and Vdd pins.
Unequal sensitivities can occur due to key size and
placement differences, stray wiring capacitances, and option
resistor connection.
3.6 PCB Layout and Construction
Refer to Quantum application note AN-KD02 for information
related to layout and construction matters.
Lq
12
QT1103_3R0.03_0607
QUANTUM [ QUANTUM RESEARCH GROUP ]
