Quantum QT220 Technical data

lQ QT220

z Two 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

2 KEY QT

SYNC/SS

 Access systems
 Pointing devices

OUCH

SNS1A

SNS1K

n.c.

SPEED

n.c.

OUT1

OUT2

VSS

n.c.

1

2

3

4

5

6

7

8

9

10

™ S

QT220

20-SSOP

ENSOR

SNS2K/OPT1

20

19

SNS2A

OPT2

18

n.c.

17

n.c.

16

OSC

15

VDD

14

/RES

13

n.c.

12

n.c.

11

IC

 Instrument panels
 Gaming machines

The QT220 charge-transfer (“QT’”) QTouch IC is a self-contained digital sensor IC capable of detecting near-proximity or touch
on 2 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 , 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

A

SSOP-20T

QT220-ISSG-400C to +850C

LQ

Copyright © 2005 QRG Ltd

QT220R R1.02/0905

1 - OVERVIEW

QT220 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.

T

ABLE

1-1 PIN L

ISTING

- QT220-ISSG

DescriptionNamePin

Sense pin (to Rs1+ Cs1)SNS1A1
Sense pin (to Cs1 electrode)SNS1K2
Do not connectn.c.3
Speed optionSPEED4
Do not connectn.c.5
Output, key 1OUT16
Output, key 2OUT27
GroundVSS8
Sync in and/or spread spectrum driveSYNC/SS9
Unbonded internallyn.c.10
Unbonded internallyn.c.11
Do not connectn.c.12
Reset pin, active low. Can usually tie to Vdd./RES13
Power: +4.0 to +5.0V locally regulatedVDD14
Oscillator bias inOSC15
Do not connectn.c.16
Do not connectn.c.17
Option OPT2OPT218
Sense pin (to Rs2 + Cs2)SNS2A19
Sense pin (to Cs2, electrode); option OPT1SNS2K/OPT120

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

1.2 ELECTRODE DRIVE; WIRING

The QT220 has two 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 QT220 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

LQ

S1

SPEED

OPT

R1

1M

VDD

OUT1

OUT2

SYNC

2

F

IGURE

1-1 LOW P

KEY 1

RSNS1

22K

CS1

10nF

RS1

2.2K

OWER

, S

YNCHRONIZED CIRCUIT

KEY 2

RSNS2
22K

CS2

10nF

RS2

2.2K

62K R4

VDD

QT220R R1.02/0905

10 second
timeout shown

R3

1M

S3

OPT2

VDD

VDD

OPT1

R2

1M

S2

VDD

F

IGURE

1-2 F

AST

, S

PREAD-SPECTRUM CIRCUIT

KEY 2

RSNS2

22K

CS2

10nF

RS2

2.2K

10 second
timeout shown

VDD

VDD

R5

360K

C1

47nF

R3

1M

S3

OPT2

62K R4

S1

SPEED

OPT

R1

1M

VDD

OUT1

OUT2

KEY 1

RSNS1

22K

CS1

10nF

RS1

2.2K

Multiple touch electrodes connected to either 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.

1.3.1 A
S

LTERNATIVE WAYS TO INCREASE

ENSITIVITY

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

OPT1

1M

R2

1.3.2 D

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

S2

strategies: a) making the electrode smaller, b)

ECREASING SENSITIVITY

making the electrode into a sparse mesh using a
high space-to-conductor ratio, or c) by
decreasing the Cs capacitors.

VDD

1.3.3 KEY B

VDD

A number of factors can cause sensitivity

ALANCE

imbalances. Notably, SNS wiring to electrodes
can have differing stray amounts of capacitance
to ground. Increasing load capacitance will

R6

220K

cause a decrease in gain. Key size differences,
and proximity to other metal surfaces can also
impact gain.

The two keys may thus require ‘balancing’ to
achieve similar sensitivity levels. This can be
best accomplished by trimming the values of the

two Cs capacitors to achieve equilibrium. The
two 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 - QT220 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.

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.

F

IGURE

2-1 D

RIFT COMPENSATION

Signal

Hysteresis

Threshold

Reference

Output

LQ

2.1.1 D

RIFT COMPENSATION

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 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.

3

QT220R R1.02/0905

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 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.

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
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.

ETECTION INTEGRATOR

2.1.2 T

The internal threshold level is fixed at 12 counts for both
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.

HRESHOLD LEVEL

URATION

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.

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 QT220 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

LQ

4

QT220R R1.02/0905