QT100-ISG - Quantum Research Group

The QT100 charge-transfer (‘QT’) touch sensor is a self-contained digital IC capable of detecting near-proximity or touch. It will

project a touch or proximity field through any dielectric like glass, plastic, stone, ceramic, and even most kinds of wood. It can

also turn small metal-bearing objects into intrinsic sensors, making them responsive to proximity or touch. This

capability, coupled with its ability to self-calibrate, can lead to entirely new product concepts.

It is designed specifically for human interfaces, like control panels, appliances, toys, lighting controls, or anywhere a

mechanical switch or button may be found.

Power consumption is less than 500µA in most applications when running in Fast response mode. This typically drops to

5-10µA at 2V VDD in Low Power mode, depending on the burst length. In most cases the power supply need only be minimally

regulated; for example, by Zener diodes or an inexpensive three-terminal regulator. The QT100 only requires a common

inexpensive X7R ceramic capacitor in order to function.

The QT100’s Reduced Instruction Set Computer (RISC) core employs signal processing techniques pioneered by Quantum;

these are specifically designed to make the device survive real-world challenges, such as ‘stuck sensor’ conditions and signal

drift.

The Quantum-pioneered HeartBeat™ signal is also included, allowing a microcontroller to monitor the health of the QT100

continuously, if desired. By using the charge transfer principle, the IC delivers a level of performance clearly superior to older

technologies in a highly cost-effective package.

LQC

QT100-ISG R3.06/0606

LQQT100

HARGE

-T

RANSFER

OUCH™

SNSK

VSS

OUT

VDD

SNS

SYNC/MODE

!2V to 5V single supply operation

!10µ

µµ

µA low power mode @ 2V

!Self-calibrating on power-up

!Sensitivity easily adjusted

!Consensus filter for noise immunity

!Autorecalibration timeout

!HeartBeat™ health indicator on output

!Only a few passive external parts required

!RoHS compliant SOT23-6 package

QT100-ISG-40ºC to +85ºC

6-pin SOT23-6T

AVAILABLE OPTIONS

1 Overvie

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 70ms 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.

QT100-ISG R3.06/0606

Figure 1.1 Basic Circuit Configuration

SYNC/MODE

SNS

SNSK

VSS

OUT

VDD

CS10nF

SENSE

ELECTRODE

+2.5 to 5V

Figure 2.4 SYNC Mode (Short Pulses)

SNSK

QT100

SYNC

>10us >10us >10us

2.1.4 SYNC Mode

It is possible to synchronize the device to an external clock

source by placing an appropriate waveform on the SYNC pin.

SYNC mode can synchronize multiple QT100 devices to

each other to prevent cross-interference, or it can be used to

enhance noise immunity from low frequency sources such as

50Hz or 60Hz mains signals.

The SYNC pin is sampled at the end of each burst. If

the device is in Fast mode and the SYNC pin is

sampled high, then the device continues to operate in

Fast mode (Figure 2.2). If SYNC is sampled low, then

the device goes to sleep. From then on, it will operate in

SYNC mode (Figure 2.1). Therefore, to guarantee entry

into SYNC mode the low period of the SYNC signal

should be longer than the burst length (Figure 2.3).

However, once SYNC mode has been entered, if the

SYNC signal consists of a series of short pulses

(>10µs) then a burst will only occur on the leading edge

of each pulse (Figure 2.4) instead of on each change of

SYNC signal, as normal (Figure 2.3).

In SYNC mode, the device will sleep after each

measurement burst (just as in LP mode) but will be

awakened by a change in the SYNC signal in either

direction, resulting in a new measurement burst. If

SYNC remains unchanged for a period longer than the

LP mode sleep period (about 70ms), the device will

resume operation in either Fast or LP mode depending

on the level of the SYNC pin (Figure 2.3).

There is no DI in SYNC mode (each touch is a

detection) but the Max On-duration will depend on the

time between SYNC pulses; see Sections 2.3 and 2.4.

Recalibration timeout is a fixed number of

measurements so will vary with the SYNC period.

2.2 Threshold

The internal signal threshold level is fixed at 10 counts

of change with respect to the internal reference level,

which in turn adjusts itself slowly in accordance with the

drift compensation mechanism.

The QT100 employs a hysteresis dropout of two counts

of the delta between the reference and threshold levels.

2.3 Max On-duration

If an object or material obstructs the sense pad the

signal may rise enough to create a detection, preventing

further operation. To prevent this, the sensor includes a

timer which monitors detections. If a detection exceeds

the timer setting the sensor performs a full recalibration.

This is known as the Max On-duration feature and is set to

~60s. This will vary slightly with Cs and if SYNC mode is

used. As the internal timebase for Max On-duration is

determined by the burst rate, the use of SYNC can cause

dramatic changes in this parameter depending on the SYNC

pulse spacing.

2.4 Detect Integrator

It is desirable to suppress detections generated by electrical

noise or from quick brushes with an object. To accomplish

this, the QT100 incorporates a ‘detect integration’ (DI)

counter that increments with each detection until a limit is

reached, after which the output is activated. If no detection is

sensed prior to the final count, the counter is reset

immediately to zero. In the QT100, the required count is four.

In LP mode the device will switch to Fast mode temporarily in

order to resolve the detection more quickly; after a touch is

either confirmed or denied the device will revert back to

normal LP mode operation automatically.

The DI can also be viewed as a 'consensus' filter, that

requires four successive detections to create an output.

QT100-ISG R3.06/0606

Figure 2.3 SYNC Mode (triggered by SYNC edges)

SYNC

SNSK

QT100

SNSK

QT100

slow mode sleep period

sleep

sleepsleep

Revert to Fast Mode

Revert to Slow Mode

slow mode sleep period

Figure 2.2 Fast Mode Bursts (SYNC held high)

SNSK

QT100

SYNC

~1ms

Figure 2.1 Low Power Mode (SYNC held low)

SYNC

SNSK

QT100 sleepsleep sleep

fast de tect

integrator

OUT

Key

touch

~70ms

2.5 Forced Sensor Recalibratio n

The QT100 has no recalibration pin; a forced

recalibration is accomplished when the device is

powered up or after the recalibration timeout.

However, supply drain is low so it is a simple

matter to treat the entire IC as a controllable load;

driving the QT100's VDD pin directly from another

logic gate or a microcontroller port will serve as

both power and 'forced recal'. The source

resistance of most CMOS gates and

microcontrollers are low enough to provide direct

power without problem.

2.6 Drift Compensation

Signal drift can occur because of changes in Cx

and Cs over time. It is crucial that drift be

compensated for, otherwise false detections, nondetections,

and sensitivity shifts will follow.

Drift compensation (Figure 2.5) is performed by making the

reference level track the raw signal at a slow rate, but only

while there is no detection in effect. The rate of adjustment

must be performed slowly, otherwise legitimate detections

could be ignored. The QT100 drift compensates using a

slew-rate limited change to the reference level; the threshold

and hysteresis values are slaved to this reference.

Once an object is sensed, the drift compensation mechanism

ceases since the signal is legitimately high, and therefore

should not cause the reference level to change.

The QT100's drift compensation is 'asymmetric'; the

reference level drift-compensates in one direction faster than

it does in the other. Specifically, it compensates faster for

decreasing signals than for increasing signals. Increasing

signals should not be compensated for quickly, since an

approaching finger could be compensated for partially or

entirely before even approaching the sense electrode.

However, an obstruction over the sense pad, for which the

sensor has already made full allowance, could suddenly be

removed leaving the sensor with an artificially elevated

reference level and thus become insensitive to touch. In this

latter case, the sensor will compensate for the object's

removal very quickly, usually in only a few seconds.

With large values of Cs and small values of Cx, drift

compensation will appear to operate more slowly than with

the converse. Note that the positive and negative drift

compensation rates are different.

2.7 Response Time

The QT100's response time is highly dependent on run mode

and burst length, which in turn is dependent on Cs and Cx.

With increasing Cs, response time slows, while increasing

levels of Cs reduce response time. The response time will

also be a lot slower in LP or SYNC mode due to a longer time

between burst measurements.

2.8 Spread Spectrum

The QT100 modulates its internal oscillator by ±7.5% during

the measurement burst. This spreads the generated noise

over a wider band reducing emission levels. This also

reduces susceptibility since there is no longer a single

fundamental burst frequency.

2.9 Output Features

2.9.1 Output

The output of the QT100 is active-high upon detection. The

output will remain active-high for the duration of the

detection, or until the Max On-duration expires, whichever

occurs first. If a Max On-duration timeout occurs first, the

sensor performs a full recalibration and the output becomes

inactive (low) until the next detection.

QT100-ISG R3.06/0606

Figure 2.5 Drift Compensation

Threshold

Signal Hysteresis

Reference

Output

Figure 2.7

Using a micro to obtain HeartBeat pulses in either output state

Figure 2.6

Getting HeartBeat pulses with a pull-up resistor

~ 5

OUT

SNS

SYNC/MODE

SNSK

VSS

VDD

HeartB eat™ Pulses

Microcontroller

PORT_M.x

PORT_M.y

OUT

SNS

SYNC/MODE

SNSK

2.9.2 HeartBeat™ Output

The QT100 output has a HeartBeat™ ‘health’ indicator

superimposed on it in both LP and SYNC modes. This

operates by taking the output pin into a three-state mode for

15µs once before every QT burst. This output state can be

used to determine that the sensor is operating properly, or, it

can be ignored using one of several simple methods.

The HeartBeat indicator can be sampled by using a pull-up

resistor on the OUT pin, and feeding the resulting

positive-going pulse into a counter, flip flop, one-shot, or

other circuit. The pulses will only be visible when the chip is

not detecting a touch.

If the sensor is wired to a microcontroller as shown in

Figure 2.7, the microcontroller can reconfigure the load

resistor to either VSS or VDD depending on the output state of

the QT100, so that the pulses are evident in either state.

Electromechanical devices like relays will usually ignore the

short Heartbeat pulse. The pulse also has too low a duty

cycle to visibly affect LEDs. It can be filtered completely if

desired, by adding an RC filter to the output, or if interfacing

directly and only to a high-impedance CMOS input, by doing

nothing or at most adding a small noncritical capacitor from

OUT to VSS.

2.9.3 Output Drive

The OUT pin is active high and can sink or source up to 2mA.

When a large value of Cs (>20nF) is used the OUT current

should be limited to <1mA to prevent gain-shifting side

effects, which happen when the load current creates voltage

drops on the die and bonding wires; these small shifts can

materially influence the signal level to cause detection

instability.

3 Circuit Guidelines

3.1 Application Note

Refer to Application Note AN-KD02, downloadable from the

Quantum website for more information on construction and

design methods. Go to http://www.qprox.com, click the

Support tab and then Application Notes.

3.2 Sample Capacitor

Charge sampler capacitor Cs should be a stable type, such

as X7R ceramic or PPS film. The normal Cs range is from

2nF to 50nF depending on the sensitivity required; larger

values of Cs demand higher stability to ensure reliable

sensing.

For more consistent sensing from unit to unit, 5% tolerance

capacitors are recommended. X7R ceramic types can be

obtained in 5% tolerance at little or no extra cost.

Values of Cs above 100nF will only be required for large

values of Cx. Sensing may become unstable if Cx is small

and Cs is large; for example, in attempting to implement

proximity fields.

3.3 Power Supply, PCB Layout

The power supply can range between 2.0V and 5.0V. At 3V

current drain averages less than 500µA in Fast mode.

If the power supply is shared with another electronic system,

care should be taken to assure that the supply is free of

digital spikes, sags, and surges which can adversely affect

the QT100. The QT100 will track slow changes in VDD, but it

can be badly affected by rapid voltage fluctuations. It is highly

recommended that a separate voltage regulator be used just

for the QT100 to isolate it from power supply shifts caused by

other components.

If desired, the supply can be regulated using a Low Dropout

(LDO) regulator, although such regulators often have poor

transient line and load stability. See Application Note

AN-KD02 (see Section 3.1) for further information on power

supply considerations.

Parts placement: The chip should be placed to minimize the

SNSK trace length to reduce low frequency pickup, and to

reduce stray Cx which degrades gain. The Cs and Rs

resistors (see Figure 1.1) should be placed as close to the

body of the chip as possible so that the trace between Rs

and the SNSK pin is very short, thereby reducing the

antenna-like ability of this trace to pick up high frequency

signals and feed them directly into the chip. A ground plane

can be used under the chip and the associated discretes, but

the trace from the Rs resistor and the electrode should not

run near ground to reduce loading.

For best EMC performance the circuit should be made

entirely with SMT components.

Electrode trace routing: Keep the electrode trace (and the

electrode itself) away from other signal, power, and ground

traces including over or next to ground planes. Adjacent

switching signals can induce noise onto the sensing signal;

any adjacent trace or ground plane next to, or under, the

electrode trace will cause an increase in Cx load and

desensitize the device.

Important Note: for proper operation a 100nF (0.1µF)

ceramic bypass capacitor must be used directly between

VDD and VSS, to prevent latch-up; the bypass capacitor

should be placed very close to the device’s power pins.

QT100-ISG R3.06/0606

4 Specifications

4.1 Absolute Maximum Specifications

Operating temp........................................................................................................-40ºC to +85ºC

Storage temp........................................................................................................ -55

C to +125

................................................................................................................... -0.5 to +5.25V

Max continuous pin current, any control or drive pin.............................................................................. ±20mA

Short circuit duration to V

, any pin.............................................................................................infinite

Short circuit duration to V

, any pin............................................................................................ infinite

Voltage forced onto any pin................................................................................... -0.6V to (V

+ 0.6) Volts

4.2 Recommended Operating Conditions

..................................................................................................................... +2.0 to 5.0V

Short-term supply ripple+noise.................................................................................................. ±5mV

Long-term supply stability.................................................................................................... ±100mV

Cs value.................................................................................................................2nF to 50nF

Cx value................................................................................................................... 5 to 20pF

4.3 AC Specifications

= 3.0V, Cs = 10nF, Cx = 5pF, Ta = recommended range, unless otherwise noted

µs15Heartbeat pulse widthT

ms100Response timeT

Cs and Cx dependentmsBurst lengthT

=5V. Increases with reducing V

ms70Time between end of burst and

start of the next (LP mode)

=5Vms1Time between end of burst and

start of the next (Fast mode)

±7.5% spread spectrum variationµs2Transfer durationT

±7.5% spread spectrum variationµs2Charge durationT

Cs, Cx dependentms250Recalibration timeT

NotesUnitsMaxTypMinDescriptionParameter

4.4 Signal Processing

Will vary in SYNC modesecs30Post-detection recalibration timer duration

ms/level1,000Negative drift compensation rate

ms/level2,000Positive drift compensation rate

samples4Consensus filter length

counts2Hysteresis

counts10Threshold differential

NotesUnitsMaxTypMinDescription

QT100-ISG R3.06/0606

4.5 DC Specifications

= 3.0V, Cs = 10nF, Cx = 5pF, Ta = recommended range, unless otherwise noted

bits149Acquisition resolutionA

pF1000Load capacitance rangeC

µA±1Input leakage currentI

OUT, 1mA sourceVV

-0.7High output voltageV

OUT, 4mA sinkV0.6Low output voltageV

V2.2High input logic levelV

V0.8Low input logic levelV

Required for proper start-upV/s100Supply turn-on slopeV

DDS

Depending on supply and run modeµA6005Supply currentI

V5.252Supply voltageV

Notes

UnitsMaxTypMinDescription

Parameter

QT100-ISG R3.06/0606

4.6 Mechanical Dimensions

10º0º10º0ºØ

0.0080.0040.20.09e 0.0220.0140.550.35E 0.020.0140.50.35L 0.038 BSC--0.95 BSC--D 0.00600.150.0h 0.0510.0351.30.9H 0.0690.0591.751.5Aa 0.1180.1023.02.6W 0.1220.1103.102.8M

NotesMaxMinNotesMaxMin InchesMillimeters

Symbol

Package type: SOT23-6

4.7 Marking

02NN (where NN is variable)QT100-ISG-40C to +85C MarkingSOT23-6 Part NumberT

QT100-ISG R3.06/0606

Aa W

02NN

Pin 1

NOTES:

QT100-ISG R3.06/0606

Patented and patents pending

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Tel: 412-391-7367 Fax: 412-291-1015

This devic e is covered under one or more United S tates and corresponding i nternational patent s. QRG patent numbers can be found online

at www.qprox.com. Numerous further patents are pending, which may apply to this device or the applications thereof.

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Developer: Martin Simmons