QPROX QT140-AS, QT140-D, QT150-AS, QT150-D Datasheet

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Completely independent QT touch circuits

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Individual logic outputs per channel (open drain)

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Projects prox fields through any dielectric

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Only one external capacitor required per channel

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Sensitivity easily adjusted on a per-channel basis

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100% autocal for life - no adjustments required

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3~5.5V, 5mA single supply operation

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Toggle mode for on/off control (strap option)

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10s, 60s, infinite auto-recal timeout (strap options)

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AKS™ Adjacent Key Suppression (pin option)

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Sync pin for multi-chip sync or line sync

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Less expensive per key than many mechanical switches

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Eval board with backlighting - p/n E160

APPLICATIONS

QProx™ QT140 / QT150

AND

5 KEY QT

SSOP

Vss

Vdd

SNS1A

SNS1B

SNS2A

SNS2B

SNS3A

SNS3B

SNS4A

SNS4B

SNS5A

Vss

QT150 shown - NOTE: Pinouts are not the same!

OUCH

/RST

OSC_I

OSC_O

OPT2

OPT1

SYNC

OUT5

OUT4

OUT3

OUT2

OUT1

AKS

SNS5B

™ S

Vdd

Vss

SNS1A

SNS1B

SNS2A

SNS2B

SNS3A

SNS3B

SNS4A

SNS4B

SNS5A

ENSOR

DIP

/RST

OSC_I

OSC_O

OPT2

OPT1

SYNC

OUT5

OUT4

OUT3

OUT2

OUT1

AKS

SNS5B

 PC Peripherals  Backlighted buttons

 Appliance controls  Security systems

 Access systems  Pointing devices

 Instrument panels  Gaming machines

QT140 / QT150 charge-transfer (“QT’”) QTouch ICs are self-contained digital controllers capable of detecting near-proximity or touch on 4 or 5 electrodes. They allow electrodes to project independent sense fields through any dielectric like glass, plastic, stone, ceramic, and wood. They can also turn metal-bearing objects into intrinsic sensors, making them responsive to proximity or touch. This capability coupled with their continuous self-calibration feature can lead to entirely new product concepts, adding high value to product designs.

Each of the channels operates independently of the others, and each can be tuned for a unique sensitivity level by simply changing its sample capacitor value.

The devices are designed specifically for human interfaces, like control panels, appliances, gaming devices, lighting controls, or anywhere a mechanical switch or button may be found; they may also be used for some material sensing and control applications.

These devices require only a common inexpensive capacitor per sensing channel in order to function. They also offer patent pending AKS™ Adjacent Key Suppression which suppresses touch from weaker responding keys and allows only a dominant key to detect, for example to solve the problem of large fingers on tightly spaced keys.

These devices also have a SYNC I/O pin which allows for synchronization with additional similar parts and/or to an external to suppress interference.

The RISC core of these devices use signal processing techniques pioneered by Quantum which are designed to survive numerous real-world challenges, such as ‘stuck sensor’ conditions, component ageing, moisture films, and signal drift.

By using the charge transfer principle, these devices deliver a level of performance clearly superior to older technologies yet are highly cost-effective.

C to +700C

C to +1050C

C to +700C

C to +1050C

AVAILABLE OPTIONS

QT140-AS-40

QT150-AS-40

DIP-28SSOP-28T

QT140-D-0

QT150-D-0

QT140/150 1.01/1102

Table 1-1 Pin Listing

Supply negative rail (ground)Vss1

Option Mode (Input pin - see Table 2-1)OPT12

QT140 / QT150 DIP-28QT140 / QT150 SSOP-28

DescriptionNamePinDescriptionNamePin

Positive powerVdd1Negative power (Ground)Vss1 Positive powerVdd2Negative power (Ground)Vss2 Negative power (Ground)Vss3Positive powerVdd3 Negative power (Ground)Vss4Positive powerVdd4

Negative power (Ground)Vss5Sense pin (to Cs1, electrode)SNS1A 5 Sense pin (to Cs1, electrode)SNS1A6Sense pin (to Cs1)SNS1B 6 Sense pin (to Cs1)SNS1B7Sense pin (to Cs2, electrode)SNS2A7 Sense pin (to Cs2, electrode)SNS2A8Sense pin (to Cs2)SNS2B8 Sense pin (to Cs2)SNS2B9Sense pin (to Cs3, electrode)SNS3A9 Sense pin (to Cs3, electrode)SNS3A10Sense pin (to Cs3)SNS3B10 Sense pin (to Cs3)SNS3B11Sense pin (to Cs4, electrode) SNS4A11 Sense pin (to Cs4, electrode) SNS4A12Sense pin (to Cs4)SNS4B12

Sense pin (to Cs4)SNS4B13Sense pin (to Cs5, electrode) n/c on QT140NC/SNS5A13 SNS5A1 SNS5B15Sense pin (to Cs5) n/c on QT140NC/SNS5B15

OUT522Channel 5 output, o-d or p-p (n/c on QT140)NC/OUT522

Sense pin (to Cs5, electrode) n/c on QT140NC

Sense pin (to Cs5) n/c on QT140NC

Output Option (input pin; 1= open drain)OC16Output Option (input pin; 1= open drain)OC16

Adjacent Key Suppression Opt. (input ; 1=AKS)AKS17Adjacent Key Suppression Opt. (input; 1=AKS)AKS17

Channel 1 output, o-d or p-pOUT118Channel 1 output, o-d or p-pOUT118

Channel 2 output, o-d or p-pOUT219Channel 2 output, o-d or p-pOUT219

Channel 3 output, o-d or p-pOUT320Channel 3 output, o-d or p-pOUT320

Channel 4 output, o-d or p-pOUT421Channel 4 output, o-d or p-pOUT421

Channel 5 output, o-d or p-p (n/c on QT140)NC

Synchronization pin (I/O pin - pull high with 10K)SYNC23Synchronization pin (I/O pin - pull high with 10K)SYNC23

Option Mode (Input pin - see Table 2-1)OPT12

Option Mode (Input pin - see Table 2-1)OPT225Option Mode (Input pin - see Table 2-1)OPT225

Oscillator outputOSC_O26Oscillator outputOSC_O26

Oscillator inputOSC_I27Oscillator inputOSC_I27

Reset pin (active low input)/RST28Reset pin (active low input)/RST28

1 - OVERVIEW

QT140/150 devices are burst mode digital charge-transfer (QT) sensor ICs designed specifically for touch controls; they include all hardware and signal processing functions necessary to provide stable sensing under a wide variety of conditions. Only a single low cost capacitor per channel is required for operation.

Figures 1-6 and 1-7 show basic circuits for these devices. See Table 1-1 for device pin listings.

SSOP

Vss

Vdd

SNS1A

SNS1B

SNS2A

SNS2B

SNS3A

SNS3B

SNS4A

SNS4B

Vss

/RST

OSC_I

OSC_O

OPT2

OPT1

SYNC

OUT4

OUT3

OUT2

OUT1

AKS

Fig 1-1 QT140 Pinouts

NOTE: SSOP / DIP Pinouts are not the same!

Vdd

Vss

SNS1A

SNS1B

SNS2A

SNS2B

SNS3A

SNS3B

SNS4A

SNS4B

DIP

/RST

OSC_I

OSC_O

OPT2

OPT1

SYNC

OUT4

OUT3

OUT2

OUT1

AKS

The DIP and SOIC pinouts are not the same and serious damage can occur if a part is miswired.

1.1 BASIC OPERATION

The devices employ bursts of charge-transfer cycles to acquire signals. Burst mode permits low power operation, dramatically reduces RF emissions, lowers susceptibility to RF fields, and yet permits excellent speed. Internally, signals are digitally processed to reject impulse noise using a 'consensus' filter that requires three consecutive confirmations of detection. Each channel is measured in sequence starting with Channel 1.

The QT switches and charge measurement hardware functions are all internal to the device. A single-slope switched capacitor ADC includes the QT charge and transfer switches in a configuration that provides direct ADC conversion; an external Cs capacitor accumulates the charge from sense-plate Cx, which is then measured.

Larger values of Cx cause the charge transferred into Cs to rise more rapidly, reducing available resolution; as a minimum resolution is required for proper operation, this can result in dramatically reduced gain. Conversely, larger values of Cs reduce the rise of differential voltage across it, increasing available resolution by permitting longer QT bursts. The value of Cs can thus be increased to allow larger values of Cx to be tolerated. The IC is responsive to both Cx and Cs, and changes in Cs can result in substantial changes in sensor gain.

2 QT140/150 1.01/1102

Unused channels: If a channel is not used, a dummy sense capacitor (nominal value: 1nF) of any type must be connected between the unused SNSnA / SNSnB pins ensure correct operation.

Unused pins: Unused device pins labeled NC should remain unconnected.

1.2 ELECTRODE DRIVE

These devices have completely independent sensing channels. The internal ADC treats Cs on each channel as a floating transfer capacitor; as a direct result, sense electrodes can be connected to either SNSnA or SNSnB and the sensitivity and basic function will be the same; however there is an advantage in connecting electrodes to SNSnA lines to reduce EMI susceptibility.

The PCB traces, wiring, and any components associated with or in contact with SNSnA and SNSnB will become touch sensitive and should be treated with caution to limit the touch area to the desired location.

Multiple touch electrodes connected to SNSnA can be used, for example to create control surfaces on both sides of an object.

It is important to limit the amount of stray capacitance on the SNSnA and SNSnB terminals, for example by minimizing trace lengths and widths to allow for higher gains and lower values of Cs.

1.3 KEY DESIGN

1.3.1 K

Like all capacitance sensors, these parts rely on Kirchoff’s Current Law (Figure 1-2) to detect the change in capacitance of the electrode. This law as applied to capacitive sensing requires that the sensor’s field current must complete a loop, returning back to its source in order for capacitance to be sensed. Although most designers relate to Kirchoff’s law with regard to hardwired circuits, it applies equally to capacitive field flows. By implication it requires that the signal ground and the target object must both be coupled together in some manner for a capacitive sensor to operate properly. Note that there is no need to provide actual galvanic ground connections; capacitive coupling to ground (Cx1) is always

IRCHOFF’S CURRENT LAW

Figure 1-2 Kirchoff's Current Law

sufficient, even if the coupling might seem very tenuous. For example, powering the sensor via an isolated transformer will provide ample ground coupling, since there is capacitance between the windings and/or the transformer core, and from the power wiring itself directly to 'local earth'. Even when battery powered, just the physical size of the PCB and the object into which the electronics is embedded will generally be enough to couple a few picofarads back to local earth.

1.3.2 KEY G

EOMETRY

, S

IZE, AND LOCATION

There is no restriction on the shape of the key electrode; in most cases common sense and a little experimentation can result in a good electrode design. The devices will operate with long thin electrodes, round or square ones, or keys with odd shapes. Electrodes can also be on 3-dimensional surfaces. Sensitivity is related to the amount of electrode surface area, overlying panel material and thickness, and the ground return coupling quality of the circuit.

If a relatively large touch area is desired, and if tests show that the electrode has more capacitance than the part can tolerate, the electrode can be made into a sparse mesh (Figure 1-3) having lower Cx than a solid plane.

Since the channels acquire their signals in time-sequence, any of the electrodes can be placed in direct proximity to each other if desired without cross-interference.

1.3.3 B

ACKLIGHTING KEYS

Touch pads can be back-illuminated quite readily using electrodes with a sparse mesh (Figure 1-3) or a hole in the middle (Figure 1-4). The holes can be as large as 4 cm in diameter provided that the ring of metal is at least twice as wide as the thickness of the overlying panel, and the panel is greater than 1/8 as thick as the diameter of the hole. Thin panels do not work well with this method as they do not propagate fields laterally very well, and will have poor sensitivity in the middle. Experimentation is required.

A good example of backlighting can be found in the E160 evaluation board.

1.3.4 V

IRTUAL CAPACITIVE GROUNDS

When detecting human contact (e.g. a fingertip), grounding of the person is never required. The human body naturally has several hundred picofarads of ‘free space’ capacitance to the local environment (Cx3 in Figure 1-2), which is more than two orders of magnitude greater than that required to create a detection. The sensor’s PCB however may be physically small, so there may be little ‘free space’ coupling (Cx1 in Figure 1-2) between it and the environment to

Figure 1-3 Mesh Electrode Geometry

Sense Electrode

SENSOR

Surrounding environment

3 QT140/150 1.01/1102

Figure 1-4 Open Electrode for Back-Illumination

Figure 1-5 Shielding Against Fringe Fields

complete the return path. If the circuit ground cannot be earth grounded by wire, for example via the supply connections, then a ‘virtual capacitive ground’ may be required to increase return coupling.

A ‘virtual capacitive ground’ can be created by connecting the IC's circuit ground to:

(1) A nearby piece of metal or metallized housing; (2) A floating conductive ground plane; (3) A larger electronic device (to which its output might be

connected anyway).

Free-floating ground planes such as metal foils should maximize exposed surface area in a flat plane if possible. A square of metal foil will have little effect if it is rolled up or crumpled into a ball. Virtual ground planes are more effective and can be made smaller if they are physically bonded to other surfaces, for example a wall or floor.

1.3.5 F

The electrode can be prevented from sensing in undesired directions with the assistance of metal shielding connected to circuit ground (Figure 1-5). For example, on flat surfaces, the field can spread laterally and create a larger touch area than desired. To stop field spreading, it is only necessary to surround the touch electrode on all sides with a ring of metal connected to circuit ground. The ring will stop field spreading from that point outwards.

If one side of the panel to which the electrode is fixed has moving traffic near it, these objects can cause inadvertent detections. This is called ‘walk-by’ and is caused by the fact that the fields radiate from either surface of the electrode equally well. Again, shielding in the form of a metal sheet or foil connected to circuit ground will prevent walk-by; putting an air gap between the grounded shield and the electrode will help to keep the value of Cx low.

1.3.6 S

Sensitivity can be altered to suit various applications and situations on a channel-by-channel basis. The easiest and

IELD SHAPING

ENSITIVITY

Sense

wire

most direct way to impact sensitivity is to alter the value of Cs; more Cs yields higher sensitivity.

1.3.6.1 Alternative Ways to Increase Sensitivity

Sensitivity can also be increased by using bigger electrodes, reducing panel thickness, or altering panel composition.

Increasing electrode size can have diminishing returns, as high values of Cx counteract sensor gain; however, Cs can be increased to combat this up to the rated device limit. Also, increasing the electrode's surface area will not substantially increase touch sensitivity if its diameter is already much larger in surface area than fingertip contact area.

The panel or other intervening material can be made thinner, but again there are diminishing rewards for doing so. Panel material can also be changed to one having a higher dielectric constant, which will help propagate the field through to the front. Locally adding some conductive material to the panel (conductive materials essentially have an infinite dielectric constant) will also help; for example, adding carbon or metal fibers to a plastic panel will greatly increase frontal field strength, even if the fiber density is too low to make the plastic bulk-conductive.

1.3.6.2 Decreasing Sensitivity

In some cases the circuit may be too sensitive. Gain can be lowered further by a number of strategies: a) making the electrode smaller, b) making the electrode into a sparse mesh using a high space-to-conductor ratio (Figure 1-3), or c) by decreasing the Cs capacitors.

Sense

wire

4 QT140/150 1.01/1102

Figure 1-6 Basic Circuit (QT140, SSOP Package)

Figure 1-7 Basic Circuit (QT150, SSOP Package)

5 QT140/150 1.01/110

Figure 1-8 Synchronized QT140, QT150 Circuits

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