QPROX QT60040-D, QT60040-IS Datasheet

LQ QT60040

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Creates 4 ‘touch buttons’ through any dielectric

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Only 1 inexpensive capacitor required

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Simple 4x1 matrix key geometry

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100% drift compensation for lifetime reliability

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'2' key rollover: senses any 2 keys at same time

 Back-lit keys possible with ITO electrodes

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Simple direct 'per key' active-high drive outputs

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Auto recalibration after 10 or 60 seconds of touch

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2.5 - 5.5V single power supply operation

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CMOS design - very low power consumption

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14-pin SOIC package

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E604 Evaluation reference design board available

4-KEY C

OPT1

OPT2

HARGE-TRANSFER

VDD GND

Q3 Q4

APPLICATIONS -

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Security keypanels

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Industrial keyboards

The QT60040 digital charge-transfer (“QT”) QMatrix™ IC is designed to detect touch on up to 4 keys in a scanned 4x1 matrix. It will project the keys through almost any dielectric, like glass, plastic, stone, ceramic, and even most kinds of wood, up to thicknesses of 6mm. The touch areas are defined as simple 2-part interdigitated electrodes of conductive material, like copper, Indium-Tin-Oxide (ITO), or screened silver or carbon deposited on the rear of a control panel. Alternatively the keys can be implemented on a stick-on flex circuit that can be adhered to the rear of most panels.

The IC is designed specifically for domestic appliances, computer and peripheral control buttons, ATM machines, security panels, portable instruments, machine tools, or similar products that are subject to environmental challenges or physical attack. It permits the construction of 100% sealed, watertight keypanels that are immune to environmental factors such as humidity and condensation, temperature, dirt accumulation, or the physical deterioration of the panel surface from abrasion, chemicals, or abuse. The QT60040 contains Quantum-pioneered self-calibration, drift compensation, and digital filtering algorithms that make its sensing function extremely robust and survivable.

The device can easily control keys over graphical LCD panels or LEDs when used with clear, conductive ITO electrodes. It does not require 'chip on glass' or other exotic fabrication techniques, thus allowing the OEM to source the keymatrix from multiple vendors.

External circuitry consists only of a single, inexpensive capacitor. The sensitivity of the keys can be set by simply changing the value of this capacitor. The device has 4 outputs which indicate detection on the keys; up to 2 keys can be sensed at any one time.

The QT60040 features automatic recalibration timeouts which will cause the device to recalibrate keys on an individual basis when they are 'stuck on' for intervals of either 10s or 60s, depending on a jumper option.

QT60040 technology makes use of an important new variant of charge-transfer sensing, transverse charge-transfer, in an XY format that minimizes the number of required scan lines and external components. Unlike older technologies it does not require one IC per key, and is cost competitive even with some rubber membrane technologies. A distinct advantage is an accelerated time to market due to the fact that custom molded membranes are not required; the entire system can be designed using common PCB materials.

The E604 board available from Quantum is a reference design that permits full evaluation of the QT60040

Appliance controls

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Vandal-proof keypads

ATM machines

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Touch-screens

Automotive controls

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PC / peripheral controls

C to +700C

AVAILABLE OPTIONS

QT60040-IS-400C to +850C

DIPSOICT

QT60040-D-0

QT60040 / R1.04 / 0303

©Quantum Research Group Ltd.

1 - OVERVIEW

The QT60040 is a CMOS charge-transfer (QT) sensor designed specifically for matrix touch controls; it includes all signal processing functions necessary to provide stable sensing under a wide variety of changing conditions. Only one low cost external capacitor is required for operation.

The QT60040 uses burst-mode charge transfer methods pioneered and patented by Quantum. This revolutionary new technology allows the construction of entirely new forms of

Figure 1-1 Field flow between X and Y elements

overlying panel

element

cmos d

ive

keypanels which can include back-illumination, arbitrary shapes of keys, 'morphed' keys wrapped onto complex surfaces, and keys having unique textures and feel, all at very low cost.

The QT60040 uses a 4x1 matrix, having 4 'X' drive lines and 1 'Y' receive line. This configuration reduces interconnect requirements and also lowers the external component count to one charge sampling capacitor which is sequentially shared by the four keys.

The QT60040 has four simple active-high CMOS outputs that go high when the corresponding key is touched. Up to 2 keys can be touched at the same time; three or more keys touched will limit to the first two touch outputs. An option pin allows this to be restricted to only one key if desired.

The device operates on a 2.5 to 5.5 regulated power supply which can be from a common 78L05-type IC regulator or a simple 2-stage zener regulator supply.

element

1.2 CIRCUIT MODEL

An electrical circuit model is shown in Figure 1-4. The coupling capacitance across the X and Y electrodes and from each to a finger is represented by Cx1, Cx2a, and Cx2b. The sampling capacitor Cs is used to accumulate charge during the course of a burst. An important parasitic capacitance from the Y line to ground, Cx3, is also shown.

QT switch timing action is shown in Figure 1-5.

Initially, switch S3 is closed to reset Cs then re-opened. After S3 is opened, S1 is closed to charge the capacitances associated with the Y-line, including all Y-to-X capacitances. After S1 is closed, one of the four X lines is raised high, so that there is then a zero differential potential from the selected X line to the Y line. Then, S1 is opened and S2 is closed, causing charge to flow from the Cx capacitances into Cs; Cs charges up slightly with the polarity shown. Then the selected X line is driven low, causing a step-function decrease in charge on Cs whose magnitude is proportionate to the amount of coupling from X to Y.

The final charge accumulated on Cs per QT cycle is thus a direct function of Cx3 minus the small amount of charge subtracted via the Cx1 / Cx2a / Cx2b / Cfinger network. Since the charge from the Cx2a / Cx2b network is highly dependent on Cfinger, which effectively forms a capacitive divider, the total charge absorbed by Cs is dependent on touch: a touch nets more charge transferred into Cs per QT cycle because less charge is transferred out of Cs per QT cycle.

The acquisition process is controlled by a state machine which continues the acquisition cycle as a burst, which finally terminates when the voltage across Cs reaches the predefined level Vref. This burst takes hundreds or even thousands of cycles

Figure 1-2 Field Flows When Touched

overlying panel

1.1 FIELD FLOWS

Figure 1-1 shows how charge is transferred across an electrode set to permeate the overlying panel material; this charge flow exhibits a rapid dQ/dt during the edge transitions of the X drive pulse. The charge emitted by the X electrode is partly received onto the Y electrode which is then captured by the Cs capacitor and processed.

The QT60040 matrix uses 4 'X' edge-driven rows and 1 'Y' sense column to detect 4 keys. The X drive occurs as a burst of pulses on each key.

The charge flows set into motion by the X drive signals are partially absorbed by the touch of a human finger (Figure 1-2) resulting in a decrease in coupling from X to Y; coupled charge increases in the presence of a conductive film like water (Figure 1-3) which acts to bridge the two elements. Increasing signals due to water films are quite easy to discern and are not detected by the QT60040.

element

cmos driver

element

Figure 1-3 Fields With a Conductive Film

Wa te r fil m

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©Quantum Research Group Ltd.

UTO

Figure 1-4 QT60040 Circuit Model

FINGER

X2A

DRIVE

Vref

1 OF 4

X2B

CHARGE

DONE START RESULT

RESET

STATE

MACHINE

POST

PROCESSOR

PTION

TRANSFER

Figure 1-5 Circuit Switch Timings

X DRIVE X

CHARGE S 1

TRANSFER S 2

to complete; the burst length depends on the value of Cs, the Cx capacitances, and Cfinger. Increasing Cs increases the burst length, increasing Cx3 decreases burst length, and increasing Cx1 and Cx2 increase burst length. Increasing Cfinger decreases the burst length. The value of the burst length is thus a variable that is dependent on these capacitances; the burst length is used to create an internal reference signal level during a calibration cycle, and to determine the presence of touch by virtue of a change in the burst length relative to the reference level.

Because the Cs capacitor is shared among all four channels it is important that the four interdigitated key designs be reasonably well matched. It is also important to keep Cx1 and Cx3 to a minimum while maximizing the values of Cx2a and Cx2b through good key design methods. These requirements also dictate that the IC be placed close to the keys to achieve good sensitivity levels; long Y traces also increase the risk of susceptibility to interference, as well as low gain. To reduce Cx3, the Y line should not be run close to other unrelated traces or over or near ground planes.

RESET S3

∆

REF

Cycle 1

Cycle 'm '

Figure 1-6 Conversion to Single Electrodes

QT60040

X2A

X2B

capacitances, possibly by using intentional mutual capacitive coupling of tracks on a PCB; traces from the intersections of these capacitors are led to solid touch pads which are implemented as metallizations on the rear of a control panel. Touching the front of the panel has the same absorptive effect on signal strength as an interdigitated electrode set.

The values of Cx2a and Cx2b should be consistent among all keys to preserve signal balance, which is required for proper operation. The surface area and geometry of this type of electrode should be adjusted to suit the desired activation area.

Typical values of Cx2a and Cx2b range from 5pF to 10pF. The traces leading from the junctions of these capacitors to the solid touch pads should not see a load of more than 10pF, thus the traces to these pads should be thin and short and not accompanied by a ground plane or other traces.

FINGE R

1.4 INTERDIGITATED ELECTRODES

Key electrodes can be made using interdigitated sets of fingers, serpentines, spirals or similar patterns (Figure 1-7). One element of each key must be connected to an X line, with the other connected to the common Y line. The pattern surface area should be similar from key to key to preserve relative key sensitivities.

It is important to prevent substantial capacitive coupling from a ‘bare’ Y line to a finger. A transient increase in Cx3 will cause a sudden disturbance common to all keys that can create unintentional detections. The connecting Y trace running between the keys should be as thin as possible, on a side of the flex circuit or pcb away from the user panel, and where possible run closely in parallel with a segment of a nearby X trace so as to suppress this effect. The problem of a bare Y line can be demonstrated by touching the Cs capacitor (which is connected to Y), which will cause one or two random keys to activate with each touch.

In cases where it is not possible to have both the X and Y traces on the same plane, the X traces should be run on the ‘finger’ side of the board. In all cases where the X and Y lines run on opposite planes, the substrate (a flex circuit, or a pcb) should be as thin as

Figure 1-7 Sample Electrode Geometries

1.3 SINGLE ELECTRODE OPERATION

An alternative mode of operation is shown in Figure 1-6. Capacitances Cx2a and Cx2b are implemented as discrete

PARALLEL LINES SERPENTINE SPIRAL

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