1 Overview

1.1 Introduction

The QT100 is a digital burst mode charge-transfer (QT)

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 a single low cost, noncritical capacitor is

required for operation.

Figure 1.1 shows a basic circuit using the device.

1.2 Basic Operation

The QT100 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 QT100.

1.3 Electrode Drive

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 5-20pF

while Cs is usually about 2-50nF.

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.

1.4 Sensitivity

1.4.1 Introduction

The sensitivity on the QT100 is a function of things like the

value of Cs, electrode size and capacitance, electrode shape

and orientation, the composition and aspect of the object to be

sensed, the thickness and composition of any overlaying

panel material, and the degree of ground coupling of both

sensor and object.

1.4.2 Increasing Sensitivity

In some cases it may be desirable to increase sensitivity; for

example, when using the sensor with very thick panels having

a low dielectric constant. Sensitivity can often be increased by

using a larger electrode or reducing panel thickness.

Increasing electrode size can have diminishing returns, as

high values of Cx will reduce sensor gain.

The value of Cs also has a dramatic effect on sensitivity, and

this can be increased in value with the trade-off of slower

response time and more power. Increasing the electrode's

surface area will not substantially increase touch sensitivity if

its diameter is already much larger in surface area than the

object being detected. Panel material can also be changed to

one having a higher dielectric constant, which will better help

to propagate the field.

Ground planes around and under the electrode and its SNSK

trace will cause high Cx loading and destroy gain. The

possible signal-to-noise ratio benefits of ground area are more

than negated by the decreased gain from the circuit, and so

ground areas around electrodes are discouraged. Metal areas

near the electrode will reduce the field strength and increase

Cx loading and should be avoided, if possible. Keep ground

away from the electrodes and traces.

1.4.3 Decreasing Sensitivity

In some cases the QT100 may be too sensitive. In this case

gain can be easily lowered further by decreasing Cs.

2 Operation Specifics

2.1 Run Modes

2.1.1 Introduction

The QT100 has three running modes which depend on the

state of SYNC, pin 6 (high or low).

2.1.2 Fast Mode

The QT100 runs in Fast mode if the SYNC pin is permanently

high. In this mode the QT100 runs at maximum speed at the

expense of increased current consumption. Fast mode is

useful when speed of response is the prime design

requirement. The delay between bursts in Fast mode is

approximately 1ms, as shown in Figure 2.2.

2.1.3 Low Power Mode

The QT100 runs in Low Power (LP) mode if the SYNC line is

held low. In this mode it sleeps for approximately 85ms at the

end of each burst, saving power but slowing response. On

detecting a possible key touch, it temporarily switches to Fast

mode until either the key touch is confirmed or found to be

spurious (via the detect integration process). It then returns to

LP mode after the key touch is resolved as shown in

Figure 2.1.

3

QT100_3R0.09_0707

Figure 1.1 Basic Circuit Configuration

Cs

OUT

VDD

SNSK

SNS

SYNC/MODE

VSS

2

6

4

3

1

5

VDD

Rs

Cx

SENSE

ELECTRODE

Note: A bypass capacitor should be tightly wired

between Vdd and Vss and kept close to QT100 pin 5.