10
QT113B [DATASHEET]
9525D–AT42–05/2013
3.1.3 Max On-Duration
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 timer causes the sensor to perform a full recalibration (when not set to infinite). This is known as the Max
On-Duration feature.
After the Max On-Duration interval, the sensor will once again function normally to the best of its ability given
electrode conditions. There are two finite timeout durations available via strap option: 10 and 60 seconds (Table 2-1
on page 7).
3.1.4 Detection Integrator
It is desirable to suppress detections generated by electrical noise or from quick brushes with an object. To
accomplish this, the QT113B incorporates a detect integration 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 QT113B, the required count is 3.
The Detection Integrator can also be viewed as a 'consensus' filter, that requires three successive detections to
create an output.
3.1.5 Forced Sensor Recalibration
The QT113B has no recalibration pin; a forced recalibration is accomplished only when the device is powered up.
However, supply drain is low so it is a simple matter to treat the entire IC as a controllable load; simply driving the
QT113B Vdd pin directly from another logic gate or a microcontroller port (Figure 3-2 on page 11) 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. Note that many 8051-based micros have only a weak pull-up drive capability and will
require CMOS buffering. 74HC or 74AC series gates can directly power the QT113B, as can most other
microcontrollers.
Option strap configurations are read by the QT113B only on power-up. Configurations can only be changed by
powering the QT113B down and back up again; again, a microcontroller can directly alter most of the configurations
and cycle power to put them in effect.
3.1.6 Response Time
The QT113B response time is highly dependent on burst length, which in turn is dependent on C
S
and C
X
(see
Figures 5-1 and 5-2). With increasing C
S
, response time slows, while increasing levels of C
S
reduce response time.
Figure 5-3 on page 19 shows the typical effects of C
S
and C
X
on response time.
3.2 Output Features
The QT113B is designed for maximum flexibility and can accommodate most popular sensing requirements. These
are selectable using strap options on pins OPT1 and OPT2. All options are shown inTable 3-1.
Table 3-1. Output Mode Strap Options
Mode Tie Pin 3 to: Tie Pin 4 to:
Max On
Duration
DC Out Vdd Vdd 10 s
DC Out Vdd Gnd 60 s
Toggle Gnd Gnd 10 s
DC Out Gnd Vdd infinite
