Cypress CapSense CY8CMBR2044, Руководство по конструкции

Страница: 10 / 50

AN66308 - CY8CMBR2044 CapSense® Design Guide, Doc. No. 001-66308 Rev. *F 10
2. CapSense Technology
CapSense Fundamentals
CapSense is a touch sensing technology that works by measuring the capacitance of each input pin on the
CapSense controller that has been designated as a sensor. As shown in
Figure 2-1 , the total capacitance on each of
the sensor pins can be modeled as equivalent lumped capacitors with values of C
X,1
through C
X,n
for a design with n
sensors. Circuitry internal to the CY8CMBR2044 device converts the magnitude of each C
X
into a digital code that is
stored for post-processing. A modulating capacitor, C
MOD
, is used b
y the CapSense controller’s internal circuitry.
C
MOD
will be discussed in more detail in Capacitive Sensing Method .
Figure 2-1. CapSense Implementation in a CY8CMBR2044 Device
CY8CMBR2044
C
MOD
Sensor
Capacitors
Each sensor input pin is connected to a sensor pad by traces, vias, or both, as necessary. A nonconductive overlay is
required to cover the sensor pad and constitutes the product’s touch interface. When a finger comes into contact with
the overlay, the conductivity and mass of the body effectively introduces a grounded conductive plane parallel to the
sensor pad. This action is represented in
Figure 2-2 . This arrangement constitutes a parallel plate capacitor, whose
capacitance is given by the following equation:
Equation 1
Where:
C
F
= The capacitance affected by a finger in contact with the overlay over a sensor
ε
0
= Free space permittivity
ε
r
= Dielectric constant (relative permittivity) of overlay
A = Area of finger and sensor pad overlap
D = Overlay thickness