QT1081 Datasheet, PDF (11/20 Pages) Quantum Research Group – 8-KEY QTOUCH SENSOR IC

3 Design Notes
3.1 Oscillator Frequency
The QT1081’s internal oscillator runs from an external resistor
network connected to the OSC and SS pins, as shown in
Figures 1.1 and 1.2, to achieve spread-spectrum operation. If
spread-spectrum mode is not required, the OSC pin should be
connected to Vdd with an 18KΩ one percent resistor.
Under different Vdd voltage conditions the resistor network (or
the solitary 18KΩ resistor) might require minor adjustment to
obtain the specified burst center frequency. The network
should be adjusted slightly so that the positive pulses on any
key are approximately 2.67µs wide in the ‘solitary 18KΩ
resistor’ mode, or 2.87µs wide at the beginning of a burst with
the recommended spread-spectrum circuit (see next section).
In practice, the pulse width has little effect on circuit
performance if it varies in the range of 2µs to 3.3µs. The only
effects seen will be proportional variations in Max On-Duration
and non-LP mode response times.
3.2 Spread-Spectrum Circuit
The QT1081 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 circuit is shown in both Figures 1.1 and 1.2.
The resistors Rb1 and Rb2 should be changed depending on
Vdd. As shown in Figures 1.1 and 1.2, two 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 an
18KΩ resistor from OSC to Vdd to drive the oscillator, and
connecting SS to Vss with a 100KΩ resistor.
The spread-spectrum RC network will need to be adjusted
according to the 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
The Css capacitor connected to the SS pin (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.
Use of the spread-spectrum facility has the following effect on
Idd:
• Full speed operation: Idd changes within ±10 percent.
• Idd increases by up to 15 percent.
In both cases the exact value depends on the precise circuit
component values and timing. Vdd variations can shift the
center frequency and spread slightly.
3.3 Cs Sample Capacitors - Sensitivity
The Cs sample capacitors accumulate the charge from the
key electrodes and determine sensitivity. Higher values of Cs
make the corresponding sensing channel more sensitive. The
values of Cs can differ for each channel, permitting
differences in sensitivity from key to key or to balance unequal
sensitivities. Unequal sensitivities can occur due to key size
and placement differences and stray wiring capacitances.
More stray capacitance on a sense trace will desensitize the
corresponding key; increasing the Cs for that key will
compensate for the loss of sensitivity.
The Cs capacitors can be virtually any plastic film or low to
medium-K ceramic capacitor. The normal Cs range is 1nF to
50nF depending on the sensitivity required; larger values of
Cs require better quality to ensure reliable sensing. In certain
circumstances the normal Cs range may be exceeded, hence
the different values in Section 4.2. Acceptable capacitor types
for most uses include PPS film, polypropylene film, and NP0
and X5R / X7R ceramics. Lower grades than X5R or X7R are
not recommended.
The required values of Cs can be noticeably affected by the
presence and connection of the option resistors (see
Section 2.2). Cs values should be adjusted for optimal
sensitivity after the option resistors are connected.
3.4 Power Supply
The power supply can range from 2.8 to 5.0 volts. 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 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 QT1081.
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.
3.5 PCB Layout and Construction
Refer to Quantum application note AN-KD02 for information
related to layout and construction matters.
lQ
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QT1081_1R0.04_0307