at42qt100a ATMEL Corporation, at42qt100a Datasheet - Page 4

• Manufacturer Part Number: at42qt100a
• Description: Qtouch? Charge Transfer Ic
• Manufacturer: ATMEL Corporation

2.1.1
2.1.2
2.2
2.3
4
Basic Operation
Electrode Drive
AT42QT100A
WSON Package Differences with QT100
Electrical Differences With QT100
A 6-pin WSON package version is available which uses the same basic PCB footprint as the
QT100’s SOT-23-6 package. The 6-WSON package has a reduced package thickness.
In converting an existing design from the QT100 to the QT100A, it should be noted that the 6-
WSON package has a bottom “center pad” which must be either grounded (connected to Vss) or
isolated. This center pad is wide enough that it can possibly touch the copper pads of the
QT100’s SOT-23-6 footprint, potentially creating a short circuit between two or more pads. In
order to prevent this, the solder mask layer of the PCB should be widened slightly, or, the copper
PCB pads narrowed slightly in size.
Refer to the following application note for assistance with this conversion process:
A 10-pin MSOP package version is also available to special order with 50K MOQ.
In order to compensate for the fact that the QT100A silicon is 30 percent more sensitive to
capacitive changes than the QT100, the detection threshold has been increased by 30 percent
in the QT100A to compensate. The net effect is that the QT100A is behaviorally identical to the
QT100 (apart from no recalibration timeout), and no change in the value of Cs is required to
achieve the same sensitivity as the QT100.
Figure 1-1 on page 3
The QT100A 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 four consecutive
confirmations of a detection before the output is activated.
The QT switches and charge measurement hardware functions are all internal to the QT100A.
For optimum noise immunity, the electrode should only be connected to SNSK.
In all cases the rule Cs>>Cx must be observed for proper operation; a typical load capacitance
(Cx) ranges from 5-20 pF while Cs is usually about 2-50 nF.
Increasing amounts of Cx destroy gain, therefore it is important to limit the amount of stray
capacitance on both SNS terminals. This can be done, 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 SNS and SNSK will become touch sensitive
and should be treated with caution to limit the touch area to the desired location.
A series resistor, Rs, should be placed in line with SNSK to the electrode to suppress ESD and
EMC effects.
• QTAN0027 – PCB Design Considerations when Changing from 6-pin SOT23 to 6-pin WSON
Atmel® Products
shows a basic circuit using the 6-pin WSON package.
9531A–AT42–02/09