qt100a Quantum Research Group, qt100a Datasheet - Page 6

• Manufacturer Part Number: qt100a
• Description: Charge-transfer Qtouch? Ic
• Manufacturer: Quantum Research Group

2.8 Spread Spectrum
The QT100A modulates its internal oscillator by ±7.5percent
during the measurement burst. This spreads the generated
noise over a wider band reducing emission levels. This also
reduces susceptibility since there is no longer a single
fundamental burst frequency.
2.9 Output Features
2.9.1 Output
The output of the QT100A is active-high upon detection. The
output will remain active-high for the duration of the
detection, or until the Max On-duration expires, whichever
occurs first. If a Max On-duration timeout occurs first, the
sensor performs a full recalibration and the output becomes
inactive (low) until the next detection.
2.9.2 HeartBeat™ Output
The QT100A output has a HeartBeat™ ‘health’ indicator
superimposed on it in all modes. This operates by taking the
output pin into a three-state mode for 15µs once before every
QT burst. This output state can be used to determine that the
sensor is operating properly, or it can be ignored, using one
of several simple methods.
The HeartBeat indicator can be sampled by using a pull-up
resistor on the OUT pin, and feeding the resulting
positive-going pulse into a counter, flip flop, one-shot, or
other circuit. The pulses will only be visible when the chip is
not detecting a touch.
If the sensor is wired to a microcontroller as shown in
Figure2.7, the microcontroller can reconfigure the load
resistor to either Vss or Vdd depending on the output state of
the QT100A, so that the pulses are evident in either state.
Electromechanical devices like relays will usually ignore the
short Heartbeat pulse. The pulse also has too low a duty
cycle to visibly affect LEDs. It can be filtered completely if
desired, by adding an RC filter to the output, or if interfacing
directly and only to a high-impedance CMOS input, by doing
nothing or at most adding a small noncritical capacitor from
OUT to Vss.
2.9.3 Output Drive
The OUT pin is active high and can sink or source up to
2mA. When a large value of Cs (>20nF) is used the OUT
current should be limited to <1mA to prevent gain-shifting
side effects, which happen when the load current creates
voltage drops on the die and bonding wires; these small
shifts can materially influence the signal level to cause
detection instability.
3 Circuit Guidelines
Refer to Application Note AN-KD02, downloadable from the
Quantum website for more information on construction and
design methods.
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6
3.1 Sample Capacitor
Cs is the charge sensing sample capacitor. The required Cs
value depends on the thickness of the panel and its dielectric
constant. Thicker panels require larger values of Cs. Typical
values are 2nF to 50nF depending on the sensitivity required;
larger values of Cs demand higher stability and better
dielectric to ensure reliable sensing.
The Cs capacitor should be a stable type, such as X7R
ceramic or PPS film. For more consistent sensing from unit
to unit, 5percent tolerance capacitors are recommended.
X7R ceramic types can be obtained in 5percent tolerance at
little or no extra cost. In applications where high sensitivity
(long burst length) is required the use of PPS capacitors is
recommended.
3.2 Power Supply, PCB Layout
The power supply can range between 2.0V and 5.5V. At 3V
current drain averages less than 500µA in Fast mode.
If the power 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 adversely affect
the QT100A. The QT100A will track slow changes in Vdd,
but it can be badly affected by rapid voltage fluctuations. It is
highly recommended that a separate voltage regulator be
used just for the QT100A to isolate it from power supply
shifts caused by other components.
If desired, the supply can be regulated using a Low Dropout
(LDO) regulator, although such regulators often have poor
transient line and load stability. See Application Note
AN-KD02 for further information on power supply
considerations.
Parts placement: The chip should be placed to minimize the
SNSK trace length to reduce low frequency pickup, and to
reduce stray Cx which degrades gain. The Cs and Rs
resistors (see Figure 1.1) should be placed as close to the
body of the chip as possible so that the trace between Rs
and the SNSK pin is very short, thereby reducing the
antenna-like ability of this trace to pick up high frequency
signals and feed them directly into the chip. A ground plane
can be used under the chip and the associated discretes, but
the trace from the Rs resistor and the electrode should not
run near ground to reduce loading.
For best EMC performance the circuit should be made
entirely with SMT components.
Electrode trace routing: Keep the electrode trace (and the
electrode itself) away from other signal, power, and ground
traces including over or next to ground planes. Adjacent
switching signals can induce noise onto the sensing signal;
any adjacent trace or ground plane next to, or under, the
electrode trace will cause an increase in Cx load and
desensitize the device.
Important Note: for proper operation a 100nF (0.1µF)
ceramic bypass capacitor must be used directly between
V
V
bypass capacitor should be placed very close to the Vss
and Vdd pins.
DD
DD
and V
transients; for example, during an ESD event. The
SS
, to prevent latch-up if there are substantial
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C opyright © 2008 QRG Ltd.
QT100A_1R7.01_0308