Atmel QT113B
1-Channel QTouch
®
Touch Sensor IC
DATASHEET
Features
Number of Keys:
One
Configurable as either a single key or a proximity sensor
Economy:
Less expensive than many mechanical switches
Only one external part required – a low-cost capacitor
Signal processing:
Consensus filter for noise immunity
Sensitivity easily adjusted
100% autocal for life – no adjustments required
10 s, 60 s, infinite auto-recal timeouts (strap options)
Toggle mode for on/off control (strap option)
Digital output, active high
Increased moisture tolerance based on hardware design and firmware tuning
2.5 V to 5 V, 600 µA single supply operation
8-pin SOIC
Light switches, appliance control, access systems, elevator buttons, proximity
sensor applications, security systems, pointing devices, consumer devices,
mechanical switch or button
Interface:
Moisture tolerance:
Power:
Package:
Applications:
Patents:
QTouch
®
(patented charge-transfer method)
HeartBeat (monitors health of device)
9525D–AT42–05/2013
9525D–AT42–05/2013
1.
1.1
Pinout and Schematic
Pinout Configuration
VDD
OUT
OPT1
OPT2
1
2
3
4
8
VSS
SNS2
SNS1
GAIN
QT113B
7
6
5
1.2
Pinout Descriptions
Pin Listing
Name
VDD
Type
P
0
0
0
P
I
I
I
Comments
Supply
Output
Option selection 1
Option selection 2
Gain control
Sense 1
Sense 2
Ground
If Unused, Connect To...
–
–
See
Table 3-1 on page 10
See
Table 3-1 on page 10
See
Table 2-1 on page 7
–
–
–
Table 1-1.
Pin
1
2
3
4
5
6
7
8
OUT
OPT1
OPT2
GAIN
SNS1
SNS2
VSS
I
OD
Input only
Open drain output
O
P
Output only, push-pull
Ground or power
I/O
Input/output
QT113B [DATASHEET]
9525D–AT42–05/2013
2
1.3
Schematic
Figure 1-1. Basic Circuit
+2.5 to 5 V
SENSING
ELECTRODE
1
2
3
4
OUTPUT=DC
TIMEOUT=10 Secs
TOGGLE=OFF
GAIN=HIGH
Vdd
OUT
SNS2
7
5
6
C
s
R
SERIES
OPT1
GAIN
10 nF
C
x
OPT2
VSS
SNS1
8
QT113B [DATASHEET]
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2.
2.1
Overview
Introduction
The QT113B (QT113B) charge-transfer (QT
™
) touch sensor is a self-contained digital IC capable of detecting near-
proximity or touch. It will project a proximity sense field through air, and any dielectric like glass, plastic, stone,
ceramic, and 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 continuously 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; it may also be used for some material sensing and control applications
provided that the presence duration of objects does not exceed the recalibration timeout interval.
Power consumption is only 600 mA in most applications. In most cases the power supply need only be minimally
regulated, for example by Zener diodes or an inexpensive 3-terminal regulator. The QT113B requires only a
common inexpensive capacitor in order to function.
The QT113B RISC core employs signal processing techniques pioneered by Atmel. These are specifically designed
to make the device survive real-world challenges, such as
stuck sensor
conditions and signal drift.
The option-selectable toggle mode permits on/off touch control, for example for light switch replacement. The Atmel-
pioneered HeartBeat signal is also included, allowing a microcontroller to monitor the health of the QT113B
continuously, if desired. By using the charge transfer principle, the IC delivers a level of performance clearly superior
to older technologies in a highly cost-effective package.
The QT113B is a drop-in replacement for the QT113. The only circuit change required might be the use of a smaller
value C
S
capacitor. A reduction by a factor of 2 is often required, but some experimentation is necessary to ascertain
the correct value of C
S
.
Figure 1-1 on page 3
shows a basic circuit using the device.
2.2
Basic Operation
The QT113B 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 three consecutive confirmations of a detection before the output is activated.
The QT switches and charge measurement hardware functions are all internal to the QT113B (Figure
1-1 on page
3).
A 14-bit single-slope switched capacitor ADC includes both the required QT charge and transfer switches in a
configuration that provides direct ADC conversion. The ADC is designed to dynamically optimize the QT burst length
according to the rate of charge buildup on C
S
, which in turn depends on the values of C
S
, C
X
, and Vdd. Vdd is used
as the charge reference voltage. Larger values of C
X
cause the charge transferred into C
S
to rise more rapidly,
reducing available resolution; as a minimum resolution is required for proper operation, this can result in dramatically
reduced apparent gain. Conversely, larger values of C
S
reduce the rise of differential voltage across it, increasing
available resolution by permitting longer QT bursts. The value of C
S
can thus be increased to allow larger values of
C
X
to be tolerated (m TFigure
5-1, Figure 5-2,
and
Figure 5-3 on page 19).
QT113B [DATASHEET]
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Figure 2-1. Internal Switching and Timing
Result
ELECTRODE
Start
Burst
Con-
troller
Done
Single-
slope
14-bit
Switched
Capacitor
ADC
SNS2
C
S
SNS1
C
X
Charge
Amp
The IC is responsive to both C
X
and C
S
, and changes in C
S
can result in substantial changes in sensor gain.
Option pins allow the selection or alteration of several special features and sensitivity.
2.3
Electrode Drive
The internal ADC treats C
S
as a floating transfer capacitor; as a result, the sense electrode can in theory be
connected to either SNS1 or SNS2 with no performance difference. However the electrode should only be connected
to pin SNS2 for optimum noise immunity.
In all cases the rule C
S
» C
X
must be observed for proper operation; a typical load capacitance (C
X
) ranges from 10 –
20 pF while C
S
is usually around 10 – 50 nF.
Increasing amounts of C
X
destroy gain; therefore it is important to limit the amount of stray capacitance on both SNS
terminals, for example by minimizing trace lengths and widths and keeping these traces away from power or ground
traces or copper pours.
The traces and any components associated with SNS1 and SNS2 will become touch sensitive and should be treated
with caution to limit the touch area to the desired location.
A series resistor, R
series
, should be placed inline with the SNS2 pin to the electrode to suppress ESD and EMC
effects.
2.4
2.4.1
Electrode design
Electrode Geometry and Size
There is no restriction on the shape of the electrode; in most cases common sense and a little experimentation can
result in a good electrode design. The QT113B will operate equally well with long, thin electrodes as with round or
square ones; even random shapes are acceptable. The electrode can also be a 3-dimensional surface or object.
Sensitivity is related to electrode surface area, orientation with respect to the object being sensed, object
composition, and the ground coupling quality of both the sensor circuit and the sensed object.
If a relatively large electrode surface is desired, and if tests show that the electrode has more capacitance than the
QT113B can tolerate, the electrode can be made into a sparse mesh (Figure
2-2)
having lower C
X
than a solid plane.
Sensitivity may even remain the same, as the sensor will be operating in a lower region of the gain curves.
2.4.2
Kirchoff’s Current Law
Like all capacitance sensors, the QT113B relies on Kirchoff’s Current Law (Figure
2-2)
to detect the change in
capacitance of the electrode. This law as applied to capacitive sensing requires that the sensor field current must
complete a loop, returning back to its source in order for capacitance to be sensed. Although most designers relate
to Kirchoff’s law with regard to hardwired circuits, it applies equally to capacitive field flows. By implication it requires
QT113B [DATASHEET]
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