Quantum QT240 DATA SHEET

lQ QT240

z Four independent charge-transfer (‘QT’) touch keys

z Individual outputs per channel - active high

z Projects prox fields through any dielectric

z Sensitivity easily adjusted on a per-channel basis

z 100% autocal for life - no adjustments required

z 3.9V ~ 5.5V single supply operation

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

z Sync pin for line sync to suppress noise

z Spread spectrum operation

z Pin options for auto recalibration timings

z Extremely low cost per key

z 20-SSOP Pb-free package

APPLICATIONS

 PC Peripherals  Backlighted buttons

 Appliance controls  Security systems

4 KEY QT

SNS2

SNS1

SNS1K

OUT1

OUT2

OUT3

VSS

SS/SYNC

n.c. n.c.

 Access systems  Pointing devices

OUCH

™ S

QT240

20-SSOP

ENSOR

SNS3K

SNS3SNS2K

SNS4K

SNS4

n.c.

OSC

VDD

/RES

OUT4

 Instrument panels  Gaming machines

The QT240 charge-transfer (“QT’”) QTouch IC is a self-contained digital sensor IC capable of detecting near-proximity or touch on 4 electrodes. It allows electrodes to project independent sense fields through any dielectric like glass, plastic, stone, ceramic, and wood. It can also turn metal-bearing objects into intrinsic sensors, making them responsive to proximity or touch. This capability coupled with its 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 other s, and each can be tuned for a unique sensitivity level by simply changing its sample capacitor value. Two speeds are supported, one of which consumes on ly 90µA of typical current at 4V. Unique among capacitance sensors, the device incorporates spread spectrum modulation for unsurpassed EMC compliance.

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 feature a SYNC pin which allows for synchronization with additional similar parts and/or to an external source to suppress interference. This pin doubles as a drive pin for spread-spectrum modulation. Option pins are provided which allow different timing and feature settings.

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.

AVAILABLE OPTIONS

SSOP-20T

QT240-ISS-G-400C to +850C

QT240R R1.10/0905

1 - OVERVIEW

QT240 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 1-1 and 1-2 show basic circuits for these device s. See Table 1-1 for device pin listings.

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 six consecutive confirmations of detection.

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. 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 either can result in substantial changes in sensor gain.

Unused channels: If a channel is not used, a dummy sense capacitor (nominal value: 1nF) of any type plus a 2.2K series resistor must be connected between unused SNS pin pairs ensure correct operation.

ABLE

1-1 PIN L

ISTING

- QT240-ISS

DescriptionNamePin

Sense pin (to Rs2 + Cs2)SNS21 Sense pin (to Cs2, electrode)SNS2K2 Sense pin (to Rs1 + Cs1)SNS13 Sense pin (to Cs1, electrode); speed optionSNS1K4 Output, key 1OUT15 Output, key 2OUT26 Output, key 3OUT37 GroundVSS8 Sync in and/or spread spectrum driveSYNC/SS9 Unbonded internallyn.c.10 Unbonded internallyn.c.11 Output, key 4OUT412 Reset pin, active low. Can usually tie to Vdd./RES13 Power: +4.0 to +5V locally regulatedVDD14 Oscillator bias inOSC15 Ground or no connectVSS16 Sense pin (to Rs4 + Cs4)SNS417 Sense pin (to Cs4, electrode); OPT2SNS4K18 Sense pin (to Rs3 + Cs3)SNS319 Sense pin (to Cs3, electrode); OPT1SNS3K20

optional passive parts (if desired ). Sync operation is not supported in this mode.

1.2 ELECTRODE DRIVE; WIRING

The QT240 has four completely independent sensing channels. The conversion process treats Cs on each channel as a floating transfer capacitor; as a direct result, sense electrodes can be connected to either SNS pin and the sensitivity and basic function will be the same; however electrodes should be connected to SNSnK lines to reduce EMI susceptibility.

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

1.1 OPERATING MODES

The QT240 features spread-spectrum acquisition capability, external synchronization of acquire bursts, and fast and slow acquisition modes. These modes are enabled via high-value resistors connected to the SNS pins to ground or Vdd. These resistors are required in every circuit.

There are two basic modes as shown in Figures 1-1 and 1-2.

Low-power Sync mode: In this mode the device operates with about a 100ms response time and very low current (about 90µA average at 4.0V). This mode allows the device to be synchronized to an external clock source, which can be used to either suppress external interference (such as from 50/60Hz wiring) or to decrease response time (which will also increase power consumption). Spread-spectrum operation is not directly supported in this mode. Sync usage is optional; the Sync pin should simply be grounded if unused.

Fast, Spread-Spectrum mode: In this mode the device operates with ~40ms response times but higher current drain (~1.5mA @ 4.0V). This mode also supports spread-spectrum operation via a few

IGURE

SPEED

OPT

VDD

OUT1

OUT2

OUT3

SYNC

OUT4

1-1 LOW P

KEY 1

RSNS1

22K

CS1

10nF

RS1

2.2K

KEY 2

CS2

10nF

RSNS2

22K

RS2

2.2K

OWER

QT240-ISS

, S

YNCHRONIZED CIRCUIT

KEY 3

KEY 4

RSNS4

RSNS3

22K

CS3

10nF

CS4

10nF

RS3

2.2K

RS4

2.2K

62K

VDD

10 second timeout shown

OPT2

VDD

2 QT240R R1.10/0905

OPT1

VDD

IGURE

1-2 F

AST

, S

PREAD-SPECTRUM CIRCUIT

RSNS4

CS4

10nF

RS4

2.2K

360K

22nF

10 second timeout shown

OPT2

VDD

62K R4

180K

SPEED

OPT

KEY 3

RSNS3 22K

CS3

10nF

RS3

2.2K

KEY 4

22K

VDD

CS1

10nF

RS1

2.2K

KEY 2

RSNS2

22K

CS2

10nF

RS2

2.2K

QT240_ISS

KEY 1

RSNS1

22K

VDD

OUT1

OUT2

OUT3

OUT4

Multiple touch electrodes connected to any SNSnK 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 SNS terminals, for example by minimizing trace lengths and widths to allow for higher gain without requiring higher values of Cs. Under heavy delta-Cx loading of one key, cross coupling to another key’s trace can cause the other key to trigger. Therefore, electrode traces from adjacent keys should not be run close to each other over long runs in order to minimize cross-coupling if large values of delta-Cx are expected, for example when an electrode is directly touched. This is not a problem when the electrodes are working through a plastic panel with normal touch sensitivity.

1.3 SENSITIVITY

Sensitivity can be altered to suit various applications and situations on a channel-by-channel basis. The easiest and most direct way to impact sensitivity is to alter the value of each Cs; more Cs yields higher sensitivity. Each channel has its own Cs value and can therefore be independently adjusted.

Figure 2-1 Drift Compensation

Signal

Threshold

Reference

Output

Hysteresis

1.3.1 A

LTERNATIVE WAYS TO INCREASE SENSITIVITY

Sensitivity can also be increased by using bigger electrode areas, reducing panel thickness, or using a panel material with a higher dielectric constant .

1.3.2 D

ECREASING SENSITIVITY

In some cases the circuit may be too sensitive. Gain can be lowered further by a number of

OPT1

VDD

strategies: a) making the electrode smaller, b) making the electrode into a sparse mesh using a high space-to-conductor ratio, or c) by decreasing

the Cs capacitors.

1.3.3 KEY B

ALANCE

A number of factors can cause sensitivity imbalances. Notably, SNS wiring to electrodes can

VDD

have differing stray amounts of capacitance to ground. Increasing load capacitance will cause a decrease in gain. Key size differences, and proximity to other metal surfaces can also impact gain.

The four keys may thus require ‘balancing’ to achieve similar sensitivity levels. This can be best accomplished by trimming the values of the four Cs capacitors to achieve equilibrium. The four Rs resistors have no effect on sensitivity and should

not be altered. Load capacitances can also be added to overly sensitive channels to ground, to reduce their gains. These should be on the order of a few picofarads.

2 - QT240 SPECIFICS

2.1 SIGNAL PROCESSING

These devices process all signals using 16 bit math , using a number of algorithms pioneered by Quantum. These algorithms are specifically designed to provide for high survivability in the face of adverse environmental changes.

2.1.1 D

Signal drift can occur because of changes in Cx , Cs, and Vdd over time. If a low grade Cs capacitor is chosen, the signal can drift greatly with temperature. If keys are subject to extremes of temperature or humidity, the signal can also drift. It is crucial that drift be compensated, else false detections, non-detections, and sensitivity shifts will follow.

Drift compensation (Figure 2-1) is a method that makes the reference level track the raw signal at a slow rate, only while no detection is in effect. The rate of reference adjustment

RIFT COMPENSATION

must be performed slowly else legitimate detections can also be ignored. The IC drift compensates each channel independently 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 signal 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

3 QT240R R1.10/0905

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 for, 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.

Drift Compensation in Slow Mode: Drift compensation rates in Slow mode are preserved if there is no Sync signal, and the rates are derived from the ~90ms Sleep interval. However if there is a Sync signal, then drift compensation rates are derived from an assumption that the Sync periodicity is ~18ms (which is corresponds to 55.5Hz). Thus, drift compensation timings in Sync mode are correct for an ~18ms Sync period but different (slower or faster) for other Sync periods. For example a Sync period of 36ms would halve the expected drift compensation rates.

2.1.2 T

The internal threshold level is fixed at 12 counts for all four channels. The hysteresis is fixed at 2 counts (17%).

2.1.3 MAX ON-D

If a sufficiently large object contacts a key for a prolonged duration, the signal will trigger a detection output preventing further normal operation. To cure such ‘stuck key’ conditions , the sensor includes a timer on each channel to monitor detection duration. If a detection exceeds the maximum timer setting, the timer causes the sensor to perform a full recalibration (if not set for infinite) . This is known as the Max On-Duration feature.

After the Max On-Duration interval, the sensor channel will once again function normally, even if partially or fully obstructed, to the best of its ability given electrode conditions. There are three timeout durations available via strap option: 10s, 60s, and infinite (Table 2-2).

Max On-Duration works independently per channel; a timeout on one channel has no effect on another channel. Note also that the timings in Table 2-2 are dependent on the oscillator frequency in fast mode. Doubling the recommended frequency will halve the timeouts. This is not true in Slow mode.

Infinite timeout is useful in applications where a prolonged detection can occur and where the output must reflect the detection no matter how long. In infinite timeout mode, the designer should take care to be sure that drift in Cs, Cx, and Vdd do not cause the device to ‘stick on’ inadvertently even when the target object is removed from the sense field.

Timeouts are approximate and can vary substantially over Vdd and temperature, and should not be relied upon for critical functions. Timeouts are also dependent on operating frequency in Fast mode.

Max On-Duration in Slow Mode: When Sync mode is used in Slow mode, the Max On-Duration timings are derived from the Sync period. The device assumes the Sync periodicity is 18ms (midway between 50Hz and 60Hz sync timings). Thus, Max On-Duration timings in Sync mode are correct for an

HRESHOLD LEVEL

URATION

18ms Sync period but different (shorter or longer) for other Sync periods. For example a Sync period of 36ms would double all expected Max On-Duration timings.

2.1.4 D

It is desirable to suppress false detections due to electrical noise or from quick brushes with an object. To this end, these devices incorporate a per-key ‘Detection Integrator’ counter that increments with each signal detection exceeding the signal threshold (Figure 2-1) until a limit count is reached, after which an Out pin becomes active. If a ‘no detect’ is sensed even once prior to the limit, this counter is reset to zero and no detect output is generated. The required limit count is 6.

The Detection Integrator can also be viewed as a 'consensus' vote requiring a detection in successive samples to trigger an active output.

In slow mode, the detect integrator forces the device to operate faster to confirm a detection. The six successive acquisitions required to affirm a detection are done without benefit of a low power sleep mode between bursts.

2.1.5 F

Pin 13 is a Reset pin, active-low, which in cases where power is clean can be simply tied to Vdd. On power-up, the device will automatically recalibrate all channels of sensing.

Pin 13 can also be controlled by logic or a microcontroller to force the chip to recalibrate, by toggling it low for 10µs or more, then raising it high again.

2.1.6 F

If the sensed capacitance becomes lower by 5 counts than the reference level for 2 seconds, the sensor will consider this to be an error condition and will force a recalibration on the affected channel.

ETECTION INTEGRATOR

ORCED SENSOR RECALIBRATION

AST POSITIVE RECALIBRATION

2.2 OPTIONS

These devices are designed for maximum flexibility and can accommodate most popular sensing requirements via option pins.

The option pins are read on power-up and about once every 10 seconds while the device is not detecting touch on any channel. Options are set using high value resistors connected to certain SNS pins, to either Vdd or Vss. These options are read 25 times over 250µs to ensure that they are not influenced by noise pulses. All 25 samples must agree. However, large values of Cx on the SNS wires can load down the pins to the point where the 1M pullup resistors cannot pull high fast enough, and the pins are read erroneously as a result. Cx should be below 50pF to prevent errors; this value can be read with a conventional capacitance meter with the QT240 removed.

The option setting resistors are mandatory and cannot be deleted. The must be strapped to either Vdd or Vss.

Speed option (Strap S1): This jumper selects whether the device acts in a slower, low power mode with a response time of approximately 100ms, or in a fast mode with a response time of 40ms typical. Fast mode consumes substantially more power than the slow mode, but also enables the use of spread-spectrum detection. Only slow mode supports the use of external Sync (Section 2.3).

4 QT240R R1.10/0905

+ 8 hidden pages

Quantum QT240 DATA SHEET

Specifications and Main Features

Frequently Asked Questions

User Manual