QST104 Datasheet, PDF (7/45 Pages) STMicroelectronics – Capacitive touch sensor device 4 keys with individual key state outputs or I2C interface

QST104
QST touch sensing technology
3
QST touch sensing technology
3.1
Functional description
QST 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. Signals are processed using algorithms pioneered by
Quantum which are specifically designed to provide reliable, trouble-free operation over the
life of the product.
The QST switches and charge measurement hardware functions are all internal to the
device. An external C_S capacitor accumulates the charge from sense-plate C_X, which is
then measured. Larger values of C_X cause the charge transferred into C_S 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 C_S reduce the rise
of differential voltage across it, increasing available resolution by permitting longer QST
bursts. The value of C_S can thus be increased to allow larger values of C_X to be tolerated.
The device is responsive to both C_X and C_S, and changes in either can result in substantial
changes in sensor gain.
Figure 2. QTouch™ measuring circuitry
SNSK_SCKn
SNS_SCKn
Sense capacitor
C_S (a few nF)
C_T (~5 pF)
Earth
C_x (~20 pF)
Ai12569
3.2
Spread-spectrum operation
The bursts operate over a spread of frequencies, so that external fields will have minimal
effect on key operation and emissions are very weak. Spread-spectrum operation works
with the Detection Integrator mechanism (DI) to dramatically reduce the probability of false
detection due to noise.
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