AK4160 Datasheet, PDF (9/40 Pages) Asahi Kasei Microsystems – 16-channel Capacitive Touch Sensor IC

[AK4160]
OPERATION OVERVIEW
■ Operation of Touch Sensor
The touch switch (capacitor) that is connected to the sense input is charged up with direct current during a given period of
time. The switch is connected to ground before the measurement. As a result, the touch switch capacitance is completely
discharged before start being charged. When the touch switch is fully charged, the voltage is inversely proportional to the
capacitance. When the touch switch is touched, this charge voltage decreases because the capacitance value when the
switch is touched is larger than when not touched. The charged voltage is converted to a digital data by ADC. The data is
get through the noise reduction filter, and compared to a touch threshold value. When the measurement value exceeds the
threshold that is corrected environmental drifts, the AK4160 updates the status register to the touch detected state.
Environmental
Drifts
Correction
VDD
Control Logic
(Touch Detection)
Second
Noise Reduction
Filter
First
Noise Reduction
Filter
SAR
ADC
Touch Switch
Figure 5. Touch Sensor Block Diagram
VSS VSS
■ Capacitance – Voltage Converter
The touch switch (capacitance C), that is connected to the sense terminal, is charged with a direct current I during the
period T. The voltage of the sense terminal is V=(I × T)/C, and if the values of I and T are constant, the charged voltage is
inversely proportional to the value of capacitance C. The charge voltage is decreased by V=(I × T)/(C+dC) when the
capacitance C is increased by dC by touching the touch switch comparing with the not touched status. After the voltage is
charged, the AK4160 discharges the sense terminal by a direct current I, during T period. At the same time, the ADC
converts the terminal value. The sense terminal must be connected to ground before the next measurement. The next
measurement should be started when the sense terminal is discharged completely.
Measurement
value
V = ^{I ×T} (Not Touched)
C
V = ^{I ×T}
(Touched)
V
C + dC
T
Charge
T
Discharge Ground
A/D Convert
Next Measurement
Figure 6. The Voltage Transaction of a Sense Terminal
MS1313-E-01
-9-
2011/11