AT42QT2161-MMU Atmel, AT42QT2161-MMU Datasheet - Page 14

• Manufacturer Part Number: AT42QT2161-MMU
• Description: IC TOUCH SENSOR 16KEY QFN M-MOD
• Manufacturer: Atmel
• Type: Capacitiver

Specifications of AT42QT2161-MMU

• Touch Panel Interface: 8, 2-Wire
• Number Of Inputs/keys: 16 Key, Slider
• Resolution (bits): 10 b
• Data Interface: I²C, Serial
• Voltage Reference: Internal
• Voltage - Supply: 1.8 V ~ 5.5 V
• Current - Supply: 1.14mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-VFQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

3.10.3
3.10.4
3.11
14
Power Supply Considerations
AT42QT2161
Tracks
PCB Cleanliness
LED terminals which are multiplexed or switched into a floating state and which are within or
physically very near a key structure (even if on another nearby PCB) should be bypassed to
either Vss or Vdd with at least a 10nF capacitor to suppress capacitive coupling effects which
can induce false signal shifts. The bypass capacitor does not need to be next to the LED, in fact
it can be quite distant. The bypass capacitor is noncritical and can be of any type.
LED terminals which are constantly connected to Vss or Vdd do not need further bypassing.
The central pad on the underside of the chip should be connected to ground. Do not run any
tracks underneath the body of the chip, only ground.
Figure 3-7.
Modern no-clean flux is generally compatible with capacitive sensing circuits.
If a PCB is reworked in any way, clean it thoroughly to remove all traces of the flux residue
around the capacitive sensor components. Dry it thoroughly before any further testing is
conducted.
See
power supply fluctuates slowly with temperature, the device will track and compensate for these
changes automatically with only minor changes in sensitivity. If the supply voltage drifts or shifts
quickly, the drift compensation mechanism will not be able to keep up, causing sensitivity
anomalies or false detections.
As the device uses the power supply itself as an analog reference, the power should be very
clean and come from a separate regulator. A standard inexpensive Low Dropout (LDO) type
regulator should be used that is not also used to power other loads such as LEDs, relays, or
other high current devices. Load shifts on the output of the LDO can cause Vdd to fluctuate
enough to cause false detection or sensitivity shifts.
Section 9.2 on page 44
CAUTION:
of the no-clean flux will change. This can mean that the flux changes from an inert
material to one that can absorb moisture and dramatically affect capacitive
measurements due to additional leakage currents. If so, the circuit can become
erratic and exhibit poor environmental stability.
Position of Tracks
If a PCB is reworked in any way, it is almost guaranteed that the behavior
Example of g
for the Vdd range and short-term power supply fluctuations. If the
ood
tracking
Example of bad tracking
9614A–AT42–08/10