QST104 STMicroelectronics, QST104 Datasheet - Page 26

• Manufacturer Part Number: QST104
• Description: Capacitive touch sensor device 4 keys
• Manufacturer: STMicroelectronics

Design guidelines
5
5.1
5.2
5.2.1
5.2.2
5.2.3
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Design guidelines
C
The C
sensitivity. Higher values of C
The values of C
key or to balance unequal sensitivities. Unequal sensitivities can occur due to key size and
placement differences and stray wiring capacitances. More stray capacitance on a sense
trace will desensitize the corresponding key. Increasing the C
for the loss of sensitivity.
The C
The normal C
C
range may be exceeded. Acceptable capacitor types for most uses include PPS film,
polypropylene film, and NP0 and X5R / X7R ceramics. Lower grades than X5R or X7R are
not recommended.
Sensitivity tuning
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 C
therefore be independently adjusted.
Increasing sensitivity
Sensitivity can also be increased by using larger electrode areas, reducing panel thickness,
or using a panel material with a higher dielectric constant.
Decreasing sensitivity
In some cases the circuit may be too sensitive. Gain can be lowered further by a number of
strategies:
●
●
●
Key balance
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.
The keys may thus require “balancing” to achieve similar sensitivity levels. This can be best
accomplished by trimming the values of the C
resistors have no effect on sensitivity and should not be altered. Load capacitances to
ground can also be added to overly sensitive channels to reduce their gain.
These should be in the order of a few picofarads.
S
S
require better quality to ensure reliable sensing. In certain circumstances the normal C
making the electrode smaller
making the electrode into a sparse mesh using a high space-to-conductor ratio
decreasing the C
sense capacitor
S
S
S
sense capacitors accumulate the charge from the key electrodes and determine
capacitors can be virtually any plastic film or low- to medium-K ceramic capacitor.
: more C
S
S
range is 1nF to 50nF depending on the sensitivity required: larger values of
S
can differ for each channel, permitting differences in sensitivity from key to
yields higher sensitivity. Each channel has its own C
S
capacitors
S
make the corresponding sensing channel more sensitive.
S
capacitors to achieve equilibrium. The R
S
for that key will compensate
S
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value and can
QST104
S
S