AT42QT2161-MMU Atmel, AT42QT2161-MMU Datasheet - Page 11

• 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

3.8
9614A–AT42–08/10
Key Design
Dwell time is the duration in which charge coupled from X to Y is captured
on page
off, causing the loss of captured charge (and hence loss of signal strength) from the keys.
The dwell time is a minimum of 250 ns. If the X pulses have not settled within 250 ns, key gain
will be reduced; if this happens, either the stray capacitance on the X line(s) should be reduced
(by a layout change, for example by reducing X line exposure to nearby ground planes or
traces), or, the Rx resistor needs to be reduced in value (or a combination of both approaches).
One way to determine X line settling time is to monitor the fields using a patch of metal foil or a
small coin over the key
observed, 250 ns dwell time should exceed the observed 95 percent settling of the X-pulse by
25 percent or more.
In almost all cases, Ry should be set equal to Rx, which will ensure that the charge on the Y line
is fully captured into the Cs capacitor.
Figure 3-5.
Circuits can be constructed out of a variety of materials including conventional FR-4, Flexible
Printed Circuit Boards (FPCB), silver silk-screened on PET plastic film, and even inexpensive
punched single-sided CEM-1 and FR-2.
The actual internal pattern style is not as important as the need to achieve regular X and Y
widths and spacings of sufficient size to cover the desired graphical key area or a little bit more;
~3 mm oversize is acceptable in most cases, since the key’s electric fields drop off near the
edges anyway. The overall key size can range from 6 mm x 6 mm up to 100 mm x 100 mm but
these are not hard limits. The keys can be any shape including round, rectangular, square, etc.
The internal pattern can be interdigitated as shown in
For small, dense keypads, electrodes such as shown in the lower half of
Where the panels are thin (under 2 mm thick) the electrode density can be quite high.
10). Increasing Rx values will cause the leading edge of the X pulses to increasingly roll
Probing X-Drive Waveforms With a Coin
(Figure
3-5). Only one key along a particular X line needs to be
Figure
3-6.
AT42QT2161
Figure 3-6
can be used.
(Figure 3-4
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