QT320-IS Atmel, QT320-IS Datasheet - Page 11

• Manufacturer Part Number: QT320-IS
• Description: SENSOR IC TOUCH PROG 2CH 8-SOIC
• Manufacturer: Atmel
• Series: QProx™r
• Type: Capacitiver

Specifications of QT320-IS

• Rohs Status: RoHS non-compliant
• Touch Panel Interface: 2, 2-Wire
• Number Of Inputs/keys: 2 Key
• Resolution (bits): 16 b
• Data Interface: Serial
• Voltage Reference: Internal
• Voltage - Supply: 1.8 V ~ 5.25 V
• Current - Supply: 600µA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SMD (300 mil)
• Output Type: *
• Interface: *
• Input Type: *
• Other names: 427-1031

3.4 ESD ISSUES
In cases where the electrode is placed behind a dielectric
panel, the device will usually be well protected from static
discharge. However, even with a plastic or glass panel,
transients can still flow into the electrode via induction, or in
extreme cases, via dielectric breakdown. Porous materials
may allow a spark to tunnel right through the material;
partially conducting materials like 'pink poly' static dissipative
plastics will conduct the ESD right to the electrode. Panel
seams can permit discharges through edges or cracks.
Testing is required to reveal any problems. The QT320 has
internal diode protection which can absorb and protect the
device from most induced discharges, up to 20mA; the
usefulness of the internal clamping will depend on the
dielectric properties, panel thickness, and rise time of the
ESD transients.
ESD protection can be enhanced with an added resistor as
shown in Figure 3-1. Because the charge and transfer times
of the QT320 are 1us in duration, the circuit can tolerate
values of Re which result in an RC timeconstant of about
200ns. The ‘C’ of the RC is the Cx load on the distant side
from the QT320. Thus, for a Cx load of 20pF, the maximum
Re should be 10K ohms. Larger amounts of Re will result in
an increasingly noticeable loss of sensitivity.
3.5 EMC ISSUES
Electromagnetic and electrostatic susceptibility are often a
problem with capacitive sensors. QT320 behavior under these
conditions can be improved by adding the series-R shown in
Figure 3-1, exactly as shown for ESD protection. The resistor
should be placed next to the chip.
This works because the inbound RC network formed by Re
and Cs has a very low cutoff frequency which can be
computed by the formula:
If Re = 10K and Cs = 10nF, then Fc = 1.6kHz.
This leads to very strong suppression of external fields.
Nevertheless, it is always wise to reduce lead lengths by
placing the QT320 as close to the electrodes as possible.
Likewise, RF emissions are sharply curtailed by the use of
Re, which bandwidth limits RF emissions based on the value
of Re and Cx, the electrode capacitance.
Line conducted EMI can be reduced by making sure the
power supply is properly bypassed to chassis ground. The
OUT lines can also be paths for conducted EMI, and these
can be bypassed to circuit ground with an RC filter network.
4 PARAMETER CLONING
The cloning process allows user-defined settings to be loaded
into internal eeprom, or read back out, for development and
production purposes.
The QTM300CA cloning board in conjunction with QT3View
software simplifies the cloning process greatly. The E3B eval
lQ
Fc =
2✜ Re Cs
1
11
board has been designed with a connector to facilitate direct
connection with the QTM300CA. The QTM300CA in turn
connects to any PC with a serial port which can run QT3View
software (included with the QTM300CA and available on
Quantum’s web site).
The connections required for cloning are shown in Figure 4-1.
Further information on the cloning process can be found in
the QTM300CA instruction guide. Section 3.3.2 discusses
wiring issues associated with cloning.
The parameters which can be altered are shown in Table 4-1
(next page).
Parameters that can be altered for each channel
independently are:
Parameters that are common to the entire part are:
It is possible for an on-board host controller to read and
change the internal settings via the interface, but doing so will
inevitably disturb the sensing process even when data
transfers are not occuring. The additional capacitive loading
of the interface pins will contribute to Cx; also, noise on the
interface lines can cause erratic operation.
The internal eeprom has a life expectancy of 100,000
erase/write cycles.
A serial interface specification for the device can be obtained
by contacting Quantum.
OUT1
OUT2
Threshold
Hysteresis
Detect Integrator A
Detect Integrator B
Max On-Duration
Output Mode
Detect Integrator Speed
Negative Drift Compensation
Positive Drift Compensation
Sleep Cycles
Figure 4-1 Clone interface wiring
1
7
OUT1
OUT2
4
8
VDD
VSS
Vdd
S1A
S1B
S2A
S2B
3
5
6
2
QT320/R1.03 08/02
CS1
CS2
SENSOR 2
SENSOR 1