qt401 Quantum Research Group, qt401 Datasheet - Page 6

• Manufacturer Part Number: qt401
• Description: Qslide? Touch Slider Ic
• Manufacturer: Quantum Research Group

electrode via induction, or in extreme cases via dielectric
breakdown. Porous materials may allow a spark to tunnel
right through the material. Testing is required to reveal any
problems. The device has diode protection on its terminals
which will absorb and protect the device from most ESD
events; the usefulness of the internal clamping will depending
on the panel's dielectric properties and thickness.
One method to enhance ESD suppression is to insert
resistors Rs1, Rs2 in series with the strip as shown in Figure
1-1; these can be as high as 1K ohms. Normally these are
not required, and in the E401 eval board they are 0 ohms.
Diodes or semiconductor transient protection devices or
MOV's on the electrode trace are not advised; these devices
have extremely large amounts of nonlinear parasitic
capacitance which will swamp the capacitance of the
electrode and cause false detections and other forms of
instability. Diodes also act as RF detectors and will cause
serious RF immunity problems.
See also Section 2.7, below
2.7 EMC and Related Noise Issues
External AC fields (EMI) due to RF transmitters or electrical
noise sources can cause false detections or unexplained
shifts in sensitivity.
The influence of external fields on the sensor can be reduced
by means of the 1K series end resistors described in Section
2.6. The Cs capacitor and Rs1, Rs2 (Figure 1-1) form a
natural low-pass filter for incoming RF signals; the roll-off
frequency of this network is defined by -
lQ
(Slave Input - MOSI)
(Slave Out - MISO)
Host Data Output
QT Data Output
DRDY from QT
CLK from Host
Acquire Burst
/SS from host
Sleep State
3-state
?
output driven
<11µs after /SS
goes low
?
Data sampled on rising edge
Data shifts out on falling edge
7
7
6
6
response byte
command byte
>13µs, <100µs
>12µs, <100µs
>12µs, <100µs
5
5
awake
Figure 3-1 SPI Timing Diagram
4
4
3
3
2
2
1
1
0
0
6
If for example Cs = 22nF, and Rs = 1K, the EMI rolloff
frequency is ~7.2kHz, which is vastly lower than most noise
sources (except for mains frequencies i.e. 50 / 60 Hz). The
resistance from the sensing strip itself is actually much higher
on average, since the strip is typically 100K ohms from end to
end. A more credible value for Rs is about 10K.
Rs and Cs must both be placed very close to the body of the
IC so that the lead lengths between them and the IC do not
form an unfiltered antenna at very high frequencies.
PCB layout, grounding, and the structure of the input circuitry
have a great bearing on the success of a design to withstand
electromagnetic fields and be relatively noise-free.
These design rules should be adhered to for best ESD and
EMC results:
output floats
before DRDY
goes low
1.
2.
3.
4.
5.
6.
<30µs
data hold >12µs
after last clock
Use only SMT components.
Keep all Cs, Rs, and the Vdd bypass cap close to the IC.
Do not place the electrode or its connecting trace near
other traces, or near a ground plane.
Do use a ground plane under and around the QT401
itself, back to the regulator and power connector (but not
beyond the Cs capacitor).
Do not place an electrode (or its wiring) of one QT401
device near the electrode or wiring of another device, to
prevent cross interference.
Keep the electrode (and its wiring) away from other
traces carrying AC or switched signals.
3-state
F
R
sleep state: 1s max
=
<1ms, ~920µs typ
>35µs
2✜R
1
S
C
3-state
S
~31ms
QT401 R10.04/0505
400µs typ
awake