lQ
Two independent charge-transfer (‘QT’) touch keys
Individual outputs per channel - active high
Projects prox fields through any dielectric
Sensitivity easily adjusted on a per-channel basis
100% autocal for life - no adjustments required
3.9V ~ 5.5V single supply operation
10s, 60s, infinite auto-recal timeout (strap options)
Sync pin for line sync to suppress noise
Spread spectrum operation
Pin options for auto recalibration timings
Extremely low cost per key
20-SSOP Pb-free package
SNS1A
SNS1K
n.c.
SPEED
n.c.
OUT1
OUT2
VSS
SYNC/SS
n.c.
1
2
3
4
QT220
5
20-SSOP
6
7
8
9
10
15
14
13
12
11
16
20
19
18
17
QT220
2 K
EY
QT
OUCH
™ S
ENSOR
IC
SNS2K/OPT1
SNS2A
OPT2
n.c.
n.c.
OSC
VDD
/RES
n.c.
n.c.
APPLICATIONS
PC Peripherals
Backlighted buttons
Appliance controls
Security systems
Access systems
Pointing devices
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 operate s 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
T
A
-40 C to +85 C
0
0
SSOP-20
QT220-ISSG
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 P
IN
L
ISTING
- QT220-ISSG
Pin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Name
SNS1A
SNS1K
n.c.
SPEED
n.c.
OUT1
OUT2
VSS
SYNC/SS
n.c.
n.c.
n.c.
/RES
VDD
OSC
n.c.
n.c.
OPT2
SNS2A
SNS2K/OPT1
Description
Sense pin (to Rs1+ Cs1)
Sense pin (to Cs1 electrode)
Do not connect
Speed option
Do not connect
Output, key 1
Output, key 2
Ground
Sync in and/or spread spectrum drive
Unbonded internally
Unbonded internally
Do not connect
Reset pin, active low. Can usually tie to Vdd.
Power: +4.0 to +5.0V locally regulated
Oscillator bias in
Do not connect
Do not connect
Option OPT2
Sense pin (to Rs2 + Cs2)
Sense pin (to Cs2, electrode); option OPT1
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
R1
1M
S1
SPEED
OPT
F
IGURE
1-1 L
OW
P
OWER
, S
YNCHRONIZED
C
IRCUIT
KEY 1
KEY 2
RSNS1
22K
RSNS2
22K
10 second
timeout shown
CS1
10nF
RS1
2.2K
CS2
10nF
R3
1M
RS2
2.2K
OPT1
R2
1M
S2
S3
OPT2
VDD
VDD
OUT1
OUT2
SYNC
VDD
VDD
62K R4
VDD
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QT220R R1.02/0905
F
IGURE
1-2 F
AST
, S
PREAD
-S
PECTRUM
C
IRCUIT
KEY 1
KEY 2
1.3.1 A
LTERNATIVE
W
AYS TO
I
NCREASE
S
ENSITIVITY
Sensitivity can also be increased by using bigger
electrode areas, reducing panel thickness, or
using a panel material with a higher dielectric
constant.
R2
RSNS1
22K
RSNS2
22K
10 second
timeout shown
CS1
R1
1M
S1
SPEED
OPT
10nF
RS1
2.2K
CS2
10nF
R3
1M
RS2
2.2K
OPT1
1.3.2 D
ECREASING
S
ENSITIVITY
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, or c) by
decreasing the Cs capacitors.
VDD
1M
S2
S3
OPT2
VDD
OUT1
OUT2
VDD
VDD
VDD
62K R4
1.3.3 K
EY
B
ALANCE
A number of factors can cause sensitivity
imbalances. Notably, SNS wiring to electrodes
can 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.
R5
360K
R6
220K
C1
47nF
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.
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.
2.1.1 D
RIFT
C
OMPENSATION
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.
F
IGURE
2-1 D
RIFT
C
OMPENSATION
Signal
Threshold
Reference
Hysteresis
Output
LQ
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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
ETECTION
I
NTEGRATOR
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.
2.1.2 T
HRESHOLD
L
EVEL
The internal threshold level is fixed at 12 counts for both
channels. The hysteresis is fixed at 2 counts (17%).
2.1.5 F
ORCED
S
ENSOR
R
ECALIBRATION
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.
2.1.3 M
AX
O
N
-D
URATION
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.
2.1.6 F
AST
P
OSITIVE
R
ECALIBRATION
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.
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
T
ABLE
2-1 S1 S
PEED
/ S
YNC
O
PTIONS
- SPEED P
IN
4
Fast / Spread Spectrum
Slow / Sync
Vss
Vdd
noise creates an ‘aliasing’ or ‘beat’ frequency effect between
the sampling rate of the QT part and the external noise
frequency. This shows up as a random noise component on
the internal signals, which in turn can lead to false activation.
Mains frequency is one common source of interference. A
simple AC zero-crossing detector feeding the SYNC pin is
enough to suppress this kind of periodic noise. Multiple
devices tied to SYNC can be synchronized to the mains
frequency in this fashion.
If two physically adjacent devices are to be synchronized to
each other, they should be connected via the SYNC pin to a
clock source that is slower than the burst rate of either
device. For example, a 50Hz clock can synchronize two
QT220’s running with burst spacings of up to 10ms each.
The two QT220’s should be synchronized on opposite
phases of the clock source, ie the clock source should feed
one part and its inverted phase, the other part.
A sync pulse on SYNC/SS in slow mode acts to break the
QT220 out of its sleep state between bursts, and to do
another burst. The device will then go back to sleep again
and await a new SYNC pulse. If a Sync pulse does not arrive
within about 90ms, it will wake again and run normally.
External sync pulses can be used to accelerate response
time (at the expense of power) in Slow mode. Sync pulses
running at 25Hz for example will improve response time by a
factor of 2. Sync cannot be used to slow down the device.
Sync Mode Effects on Timings:
In the absence of a Sync
signal, the Max On-Duration timings and drift compensation
rates in Slow mode are nominally correct. It should be
understood that the Max On-Duration timings and drift
compensation rates are slaved to the burst interval in Slow
mode, and that changing the burst interval will have direct
effects on these parameters.
Since the most common use of Sync is to synchronize the
device to Mains frequency (50 or 60Hz) the device makes an
assumption that the presence of a Sync signal is at 55Hz,
and the timings are made to be correct at this frequency.
Should the Sync pulses vary from this frequency, the Max
On-Duration timings and drift compensation rates will vary
proportionately. Thus, if the Sync pulses are 25Hz, the
10-second Max On-Duration timing will become 10*55/25 =
22 seconds nominal. Only at Sync=55Hz will the 10s timeout
be 10s (the same as if there were no Sync signal, or the
device was in Fast mode).
T
ABLE
2-2 OPT O
PTIONS
S2
SNS2K/OPT1
pin 20
S3
OPT2
pin 18
Output Type
Max On-Duration
Vss
Vdd
Vdd
Vss
Vdd
Vss
Vdd
Vss
DC Out
DC Out
Toggle
DC Out
10s
60s
10s
infinite
Timings assume 100 kHz operation
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).
Response time can also be modified by changing the
oscillator frequency (Section 3.3).
Recalibration / toggle select (S2, S3):
See Table 2-2.
There are 3 recalibration timing options (‘Max On-Duration’;
see Section 2.1.3) and one toggle mode option. The
recalibration options control how long it takes for a
continuous detection to trigger a recalibration on a key.
When such an event occurs, only the ‘stuck’ key is
recalibrated. S2 / S3 should be connected as shown in Table
2-2 to achieve the desired Max On-Duration of either 10s,
60s, or infinite.
Toggle option:
One option is toggle mode, which allows
both keys to behave with flip-flop action. In this mode, each
key’s corresponding OUT pin will toggle High / Low with
successive touches on the key. The underlying Max
On-Duration is 10s in this mode. If a timeout occurs in
Toggle mode, the toggle state is not affected. Toggle state
flips only when the corresponding Out pin goes High.
This is useful for controlling power loads, for example in
kitchen appliances, power tools, light switches, etc . or
wherever a ‘touch-on / touch-off’ effect is required.
2.3 SYNCHRONIZATION
Sync capability is only present in Slow mode (Section 2.2). If
SYNC is not desired, SYNC/SS should be connected to Vss.
Adjacent capacitive sensors that operate independently can
cross-interfere with each other in ways that will create
sensitivity shifts and spurious detections. Since Quantum’s
QT devices operate in burst mode, the opportunity exists to
solve this problem by time-sequencing sensing channels so
that physically adjacent keys do not sense at the same time.
Within the QT220 both channels operate synchronously, so it
is not possible for these channels to cross interfere. However
2 or more adjacent chips will cross-interfere if they are not
synchronized to each other. The same is true of the effects
of unsynchronized external noise sources.
External noise sources can also be heavily suppressed by
synchronizing the QT220 to the noise source period. External
3 - CIRCUIT GUIDELINES
3.1 CS SAMPLE CAPACITOR
Charge sampler caps Cs can be virtually any plastic film or
low to medium-K ceramic capacitor. The ‘normal’ Cs range is
4.7nF to 47nF depending on the sensitivity required; larger
values of Cs require higher stability to ensure reliable
sensing. Acceptable capacitor types for most uses include
plastic film (especially PPS film and polypropylene film) and
X7R ceramic. Lower grades than X7R are not advised;
higher-K ceramics have nonlinear dielectrics which induce
instabilities.
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QT220R R1.02/0905
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