at42qt113b ATMEL Corporation, at42qt113b Datasheet - Page 12

• Manufacturer Part Number: at42qt113b
• Manufacturer: ATMEL Corporation

3.2.5
4. Wiring and Parts
4.1
12
Cs Sample Capacitors
AT42QT113B
Output Drive
Electromechanical devices like relays will usually ignore this short 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
time constant to filter 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 ground (see
Figure
Figure 3-5.
The QT113B‘s output is active low and can sink up to 5 mA of non-inductive current. If an
inductive load is used, such as a small relay, the load should be diode clamped to prevent
damage. When set to operate in a proximity mode (at high gain) the current should be limited to
1 mA to prevent gain-shifting side effects from occurring, which happens 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 as described below.
Care should be taken when the QT113B and the load are both powered from the same supply,
and the supply is minimally regulated. The QT113B derives its internal references from the
power supply, and sensitivity shifts can occur with changes in Vdd, as happens when loads are
switched on. This can induce detection “cycling”, whereby an object is detected, the load is
turned on, the supply sags, the detection is no longer sensed, the load is turned off, the supply
rises and the object is reacquired, ad infinitum. To prevent this occurrence, the output should
only be lightly loaded if the device is operated from an unregulated supply, for example,
batteries. Detection “stiction”, the opposite effect, can occur if a load is shed when OUT is active.
The output of the QT113B can directly drive a resistively limited LED. The LED should be
connected with its cathode to the output and its anode towards Vdd, so that it lights when the
sensor is active. If desired the LED can be connected from OUT to ground, and driven on when
the sensor is inactive.
Charge sampler Cs can be virtually any plastic film or medium-K ceramic capacitor. The
acceptable Cs range is from 10 nF to 500 nF depending on the sensitivity required; larger values
of Cs demand higher stability to ensure reliable sensing. Acceptable capacitor types include
PPS film, polypropylene film, NPO/C0G ceramic, and X7R ceramic.
3-5).
C M O S
Eliminating HB Pulses
MIC RO INP U T
G ATE OR
100pF
C
o
2
3
4
O UT
O PT1
O PT2
SN S2
GAIN
SN S1
7
5
6
9525B–AT42–09/10