Datasheet

2. Overview of the AT42QT1012

2.1 Introduction

The AT42QT1012 (QT1012) is a single key device featuring a touch on/touch off (toggle) output with a

programmable auto switch-off capability.

The QT1012 is a digital burst mode charge-transfer sensor designed specifically for touch controls. It

includes all hardware and signal processing functions necessary to provide stable sensing under a wide

variety of changing conditions; only low cost, noncritical components are required for operation. With its

tiny low-cost packages, this device can suit almost any product needing a power switch or other toggle-

mode controlled function, especially power control of small appliances and battery-operated products.

A unique “green” feature of the QT1012 is the timeout function, which can turn off power after a time

delay.

Like all QTouch

devices, the QT1012 features automatic self-calibration, drift compensation, and spread-

spectrum burst modulation in order to provide for the most reliable touch sensing possible.

2.2 Basic Operation

Figure 1-1 and Figure 1-2 show basic circuits for the 6-pin and 8-pin devices.

The QT1012 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 QT1012.

2.3 Electrode Drive

Figure 2-1 shows the sense electrode connections (SNS, SNSK) for the QT1012.

For optimum noise immunity, the electrode should only be connected to the SNSK pin.

In all cases the sample capacitor Cs should be much larger than the load capacitance (Cx). Typical

values for Cx are 5 – 20 pF while Cs is usually 2.2 – 50 nF.

Note: Cx is not a physical discrete component on the PCB, it is the capacitance of the touch electrode

and wiring. It is show in Figure 2-1 to aid understanding of the equivalent circuit.

Increasing amounts of Cx decrease gain, therefore it is important to limit the amount of load 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.

To endure that the correct output mode is selected at power-up, the OUT trace should also be carefully

routed.

A series resistor, Rs, should be placed in line with SNSK to the electrode to suppress electrostatic

discharge (ESD) and electromagnetic compatibility (EMC) effects.

AT42QT1012

Datasheet

DS40001948A-page 7