QT100A
CHARGE-TRANSFER QTOUCH™ IC
The QT100A charge-transfer (‘QT’) touch sensor is a self-contained digital IC
capable of detecting near-proximity or touch. It will project a touch or proximity
field through any dielectric like glass, plastic, stone, ceramic, and even most
kinds of wood. It can also turn small metal-bearing objects into intrinsic
sensors, making them responsive to proximity or touch. This capability,
coupled with its ability to self-calibrate, can lead to entirely new product
concepts.
It is designed specifically for human interfaces, like control panels, appliances,
toys, lighting controls, or anywhere a mechanical switch or button may be
found.
OUT
1
VSS
2
SNSK
3
SYNC
5
VDD
4
SNS
6-WSON Package
This device can also project a proximity field to severa centimeters with the
proper electrode and circuit design.
These devices are intended to replace the QT100. Two package types are
offered, one of which can directly replace the QT100’s SOT-23-6 package but
with a reduced thickness.
QT100A
6
SNSK
1
10 OUT
SNS
2
9 N/C
N/C
3
N/C
4
7
VSS
5
6 VDD
AT A GLANCE
QT100A
8 SYNC
N/C
10-MSOP Package
Number of keys:
One
Technology:
Patented spread-spectrum charge-transfer.
Key outline sizes:
6mm x 6mm or larger (panel thickness dependent); widely different sizes and shapes possible
Electrode design:
Solid or ring electrode shapes
PCB Layers required:
One
Electrode materials:
Etched copper, silver, carbon, Indium Tin Oxide (ITO)
Electrode Substrates:
PCB, FPCB, plastic films, glass
Panel materials:
Plastic, glass, composites, painted surfaces (low particle density metallic paints possible)
Panel thickness:
Up to 50mm glass, 20mm plastic (electrode size and Cs dependent)
Key sensitivity:
Settable via capacitor (Cs)
Interface:
Digital output, active high
Moisture tolerance:
Good
Power:
2V ~ 5.5V; 6.5uA @ 2.0V typical
Package:
10 pin MSOP and 6 pin WSON; both are RoHS compliant
Signal processing:
Self-calibration, auto drift compensation, noise filtering
Applications:
Control panels, consumer appliances, toys, lighting controls, mechanical switch or button
Patents:
QTouch™ (patented Charge-transfer method)
Copyright © 2008 QRG Ltd.
QT100A_1R7.01_0308
Table 1.1 Pinlist
Name
SNSK
SNS
N/C
N/C
VSS
VDD
N/C
SYNC
N/C
OUT
Pin
Pin
10MSOP 6WSON
1
3
2
4
3
4
5
2
6
5
7
-†
8
6‡
9
10
1
Pin Type
I
I/O
O
Pwr
Type
Function
Notes
If Unused
I/O
I/O
O
O
Pwr
Pwr
I
I
O
O
Sense pin
Sense pin
Unused
Unused
Ground
Power
Unused
Sync & Mode Input
Unused
Output state
To Cs + Key
To Cs
Leave open - do not connect
Leave open - do not connect
Always connect to Sync
Leave open - do not connect
-
Ground
+2.0 ~ +5.5V
CMOS input only
CMOS I/O
CMOS push-pull output
Power / ground
Notes
†
In 6-WSON package this pin is internally connected to SYNC pin
‡
Pin is either Sync or Slow/Fast Mode depending on how strapped; see Section 2.1.
2
Copyright © 2008 QRG Ltd.
QT100A_1R7.01_0308
Figure 1.1 Basic Circuit Configuration
1 Overview
2 ~ 5V
1.1 Introduction
The QT100A is a digital burst mode charge-transfer (QT)
sensor designed specifically for touch controls; it includes all
hardware and signal processing functions necessary to
provide stable sensing under a wide variety of changing
conditions. Only a single low cost, noncritical capacitor is
required for operation.
SENSE
ELECTRODE
QT100A, 6-WSON Pkg.
*Cb
VDD
SNSK
OUT
SNS
3
4
Rs
Cs
Cx
SYNC
This device is designed to replace the QT100.
VSS
1.1.1 WSON package differences with QT100
A 6-pin WSON package version is available which uses the
same basic PCB footprint as the QT100’s SOT-23-6
package. The 6-WSON package has a reduced package
thickness.
*Note: Bypass capacitor Cb should be tightly wired
between Vdd and Vss
The traces and any components associated with SNS and
SNSK will become touch sensitive and should be treated with
caution to limit the touch area to the desired location.
In converting an existing design from the QT100 to the
QT100A, it should be noted that the 6-WSON package has a
bottom ‘center pad’ which must be either grounded
(connected to Vss) or isolated. This center pad is wide
enough that it can possibly touch the copper pads of the
QT100’s SOT-23-6 footprint, potentially creating a short
circuit between two or more pads. In order to prevent this the
solder mask layer of the PCB under the device should be
widened slightly, or, the copper PCB pads narrowed slightly
to avoid contact with the center pad. The pinout and pad
placement however are the same as for the QT100.
A series resistor, Rs, should be placed in line with SNSK to
the electrode to suppress ESD and EMC effects.
1.4 Sensitivity
1.4.1 Introduction
The sensitivity on the QT100A is a function of things like the
value of Cs, electrode size and capacitance, electrode shape
and orientation, the composition and aspect of the object to
be sensed, the thickness and composition of any overlaying
panel material, and the degree of ground coupling of both
sensor and object.
An application note is available to assist in this conversion
process.
A 10-pin MSOP package version is also available.
1.4.2 Increasing Sensitivity
1.1.2 Electrical differences with QT100
In some cases it may be desirable to increase sensitivity; for
example, when using the sensor with very thick panels
having a low dielectric constant. Sensitivity can often be
increased by using a larger electrode or reducing panel
thickness. Increasing electrode size can have diminishing
returns, as high values of Cx will reduce sensor gain.
The QT100A is behaviorally identical to the QT100, and no
change in the value of Cs is required to achieve the same
sensitivity as the QT100.
1.2 Basic Operation
Figure 1.1 shows a basic circuit using the device.
The value of Cs also has a dramatic effect on sensitivity, and
this can be increased in value with the trade-off of slower
response time and more power. Increasing the electrode's
surface area will not substantially increase touch sensitivity if
its diameter is already much larger in surface area than the
object being detected. Panel material can also be changed to
one having a higher dielectric constant, which will better help
to propagate the field.
The QT100A employs bursts of charge-transfer cycles to
acquire its signal. Burst mode permits power consumption in
the microamp range, dramatically reduces RF emissions,
lowers susceptibility to EMI, and yet permits excellent
response time. Internally the signals are digitally processed
to reject impulse noise, using a 'consensus' filter which
requires four consecutive confirmations of a detection before
the output is activated.
Ground planes around and under the electrode and its SNSK
trace will cause high Cx loading and destroy gain. The
possible signal-to-noise ratio benefits of ground area are
more than negated by the decreased gain from the circuit,
and so ground areas around electrodes are discouraged.
Metal areas near the electrode will reduce the field strength
and increase Cx loading and should be avoided, if possible.
Keep ground away from the electrodes and traces.
The QT switches and charge measurement hardware
functions are all internal to the QT100A.
1.3 Electrode Drive
For optimum noise immunity, the electrode should only be
connected to SNSK.
In all cases the rule Cs>>Cx must be observed for proper
operation; a typical load capacitance (Cx) ranges from
2-20pF while Cs is usually from 2-50nF.
1.4.3 Decreasing Sensitivity
In some cases the QT100A may be too sensitive. In this case
gain can be easily lowered by decreasing Cs.
Increasing amounts of Cx destroy gain, therefore it is
important to limit the amount of stray capacitance on both
SNS terminals. This can be done, for example, by minimizing
trace lengths and widths and keeping these traces away from
power or ground traces or copper pours.
3
Copyright © 2008 QRG Ltd.
QT100A_1R7.01_0308
2.1.2 Fast Mode (SYNC = 1)
Key
touch
Figure 2.1 Low Power Mode
~85ms
SNSK
QT100A
sleep
The QT100A runs in Fast mode if the SYNC pin is
permanently high. In this mode the QT100A runs at
maximum speed at the expense of increased current
consumption. Fast mode is useful when speed of response is
the prime design requirement. The delay between bursts in
Fast mode is approximately 1ms, as shown in Figure 2.2.
fast detect
integrator
sleep
sleep
2.1.3 Low Power Mode (SYNC = 0)
The QT100A runs in Low Power (LP) mode if the SYNC pin
is held low. In this mode it sleeps for approximately 85ms at
the end of each burst, saving power but slowing response.
On detecting a possible key touch, it temporarily switches to
Fast mode until either the key touch is confirmed or found to
be spurious (via the detect integration process). It then
returns to LP mode after the key touch is resolved as shown
in Figure 2.1.
SYNC
OUT
(SYNC held low)
Figure 2.2 Fast Mode Bursts
(SYNC held high)
2.1.4 Sync Mode
It is possible to synchronize the device to an external clock
source by placing an appropriate waveform on the SYNC pin.
Sync mode can synchronize multiple QT100A devices to
each other to prevent cross-interference, or it can be used to
enhance noise immunity from low frequency sources such as
50Hz or 60Hz mains signals.
SNSK
QT100A
~1ms
SYNC
The Sync pin is sampled at the end of each burst. If the
device is in Fast mode and the Sync pin is sampled high,
then the device continues to operate in Fast mode (Figure
2.2). If SYNC is sampled low, then the device goes to sleep.
From then on, it will operate in Sync mode (Figure 2.1).
Therefore, to guarantee entry into Sync mode the low period
of the Sync signal should be longer than the burst length
(Figure 2.3).
Figure 2.3 Sync Mode (triggered by negative edges on SYNC)
SNSK
QT100A
sleep
sleep
SYNC
sleep
Revert to
Fast Mode
However, once Sync mode has been entered, if the Sync
signal consists of a series of short pulses (>10µs) then a
burst will only occur on the falling edge of each pulse
(Figure2.4).
slow mode sleep period
SNSK
QT100A
sleep
sleep
sleep
In Sync mode, the device will sleep after each measurement
burst (just as in LP mode) but will be awakened by the falling
edge of the Sync signal, resulting in a new measurement
burst. If Sync remains unchanged for a period longer than
the LP mode sleep period (about 85ms), the device will
resume operation in either Fast or LP mode depending on
the level of the Sync pin (Figure 2.3).
Revert to
Slow Mode
slow mode sleep period
SYNC
There is no DI in Sync mode (each touch is a detection) but
the Max On-duration will depend on the time between Sync
pulses; see Sections 2.3 and 2.4. Recalibration timeout is a
fixed number of measurements so will vary with the Sync
period.
Figure 2.4 Sync Mode (Short Pulses)
SNSK
QT100A
>10us
>10us
>10us
2.2 Threshold
SYNC
The internal signal threshold level is fixed at 13 counts of
change with respect to the internal reference level, which in
turn adjusts itself slowly in accordance with the drift
compensation mechanism.
2 Operation Specifics
The QT100A employs a hysteresis dropout of 3 counts of the
delta between the reference and threshold levels.
2.1 Run Modes
2.3 Max On-duration
2.1.1 Introduction
If an object or material obstructs the sense pad the signal
may rise enough to create a detection, preventing further
operation. To prevent this, the sensor includes a timer which
monitors detections. If a detection exceeds the timer setting
the sensor performs a full recalibration. This is known as the
The QT100A has three running modes which depend on the
logic level applied to the SYNC pin.
4
Copyright © 2008 QRG Ltd.
QT100A_1R7.01_0308
Max On-duration feature and is set to ~80s (at 3V). This will
vary slightly with Cs and if Sync mode is used. As the
internal timebase for Max On-duration is determined by the
burst rate, the use of Sync can cause dramatic changes in
this parameter depending on the Sync pulse spacing.
Figure 2.5 Drift Compensation
S ignal
H ysteresis
Threshold
2.4 Detect Integrator
R eference
It is desirable to suppress detections generated by electrical
noise or from quick brushes with an object. To accomplish
this, the QT100A incorporates a ‘detect integration’ (DI)
counter that increments with each detection until a limit is
reached, after which the output is activated. If no detection is
sensed prior to the final count, the counter is reset
immediately to zero. In the QT100A, the required count is
four. In LP mode the device will switch to Fast mode
temporarily in order to resolve the detection more quickly;
after a touch is either confirmed or denied the device will
revert back to normal LP mode operation automatically.
Output
Once an object is sensed, the drift compensation mechanism
ceases since the signal is legitimately high, and therefore
should not cause the reference level to change.
The DI can also be viewed as a 'consensus' filter, that
requires four successive detections to create an output.
The QT100A's drift compensation is 'asymmetric'; the
reference level drift-compensates in one direction faster than
it does in the other. Specifically, it compensates faster for
decreasing signals than for increasing signals. Increasing
signals should not be compensated for quickly, since an
approaching finger could be compensated for partially or
entirely before even approaching the sense electrode.
However, an obstruction over the sense pad, for which the
sensor has already made full allowance, could suddenly be
removed leaving the sensor with an artificially elevated
reference level and thus become insensitive to touch. In this
latter case, the sensor will compensate for the object's
removal very quickly, usually in only a few seconds.
2.5 Forced Sensor Recalibration
The QT100A has no recalibration pin; a forced recalibration
is accomplished when the device is powered up or after the
recalibration timeout. However, supply drain is low so it is a
simple matter to treat the entire IC as a controllable load;
driving the QT100A's Vdd pin directly from another logic gate
or a microcontroller port will serve as both power and 'forced
recal'. The source resistance of most CMOS gates and
microcontrollers are low enough to provide direct power
without problem.
2.6 Drift Compensation
With large values of Cs and small values of Cx, drift
compensation will appear to operate more slowly than with
the converse. Note that the positive and negative drift
compensation rates are different.
Signal drift can occur because of changes in Cx and Cs over
time. It is crucial that drift be compensated for, otherwise
false detections, nondetections, and sensitivity shifts will
follow.
Drift compensation (Figure 2.5). is performed by making the
reference level track the raw signal at a slow rate, but only
while there is no detection in effect. The rate of adjustment
must be performed slowly, otherwise legitimate detections
could be ignored. The QT100A drift compensates using a
slew-rate limited change to the reference level; the threshold
and hysteresis values are slaved to this reference.
2.7 Response Time
The QT100A's response time is highly dependent on run
mode and burst length, which in turn is dependent on Cs and
Cx. With increasing Cs, response time slows, while
increasing levels of Cx reduce response time. The response
time will also be a lot slower in LP or Sync mode due to a
longer time between burst measurements.
Figure 2.6
Figure 2.7
Getting HeartBeat pulses with a pull-up resistor
Using a micro to obtain HeartBeat pulses in either output state
VDD
Ro
5
VDD
1
3
SNSK
OUT
R0
Microcontroller
HeartBeat™ Pulses
SNS
4
OUT
SNSK
SNS
Port_M.y
SYNC
1
Port_M.x
SYNC
3
4
6
6
VSS
2
5
Copyright © 2008 QRG Ltd.
QT100A_1R7.01_0308
2.8 Spread Spectrum
3.1 Sample Capacitor
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.
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.
2.9 Output Features
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.
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.
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.
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.
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.
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 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.
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.
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.
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