QT60248C-ASG Atmel, QT60248C-ASG Datasheet - Page 12

• Manufacturer Part Number: QT60248C-ASG
• Description: IC TOUCH SENSOR 24KEY 32TQFP
• Manufacturer: Atmel
• Series: QMatrix™, QProx™r
• Type: Capacitiver

Specifications of QT60248C-ASG

• Number Of Inputs/keys: 24 Key
• Resolution (bits): 9, 11 b
• Data Interface: Serial, SPI™
• Voltage - Supply: 3 V ~ 5 V
• Current - Supply: 25mA
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFP, 32-VQFP
• Output Type: *
• Interface: *
• Input Type: *
• For Use With: 427-1087 - BOARD EVAL QT60248-AS QMATRIX
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

10620D–AT42–04/09
General Advice
2-2
Figure 2-1.
Figure 2-2.
As a rule, the overall RC time constant of each sensor should be reduced as far as possible, while trying
to preserve at least a 1 k series resistor close to the sensor chip. A good rule-of-thumb is given in
Equation
Equation 2-3. Rule Of Thumb for RC Time Constant
The “Marginal” example in
about 5 time constants.
1. The factor of 5 ensures that each charge-transfer pulse settles to better than 99 percent, leaving just 1 percent of
charge subject to second order effects, such as the Rseries value, chip output drive strength, and so on..
2-3.
Measuring the Charge-transfer Pulses
Good and Bad Charge Pulses
Rseries_total * (Cx + Cp) <= 5 x Tcharge

Where:



Bad
Rseries_total is the total series connection from the sensor chip’s pin to the farthest
point on the sensor.
Cx + Cp represents the total capacitance that the sensor chip detects for the sensor
(that is, the sensor itself plus any parasitic capacitance). Measuring this can be done
for a first order approximation using a commercial capacitance bridge. A more
practical approach is to measure the pulse shapes as described above.
Tcharge varies somewhat from sensor chip to sensor chip, but a good estimate is
1 µs for zero- and one-dimensional sensors, and 2 µs for two-dimensional sensors.
Figure 2-2
represents about 3 time constants; the “Good” example represents
Marginal
(1)
Good
Touch Sensors Design Guide
Excellent