Datasheet

CY8C21x34/B CapSense
®
Design Guide Document No. 001-66271 Rev. *B 37
5. Design Considerations
When designing capacitive touch sense technology into your application, it is crucial to keep in mind that the
CapSense device exists within a larger framework. Careful attention to every level of detail from PCB layout to user
interface to end-use operating environment will lead to robust and reliable system performance. For more in-depth
information, see Getting Started with CapSense.
5.1 Overlay Selection
In CapSense Fundamentals, Equation 1 was presented for finger capacitance as follows:
Where:
ε
0
= Free space permittivity
ε
r
= Dielectric constant of overlay
A = Area of finger and sensor pad overlap
D = Overlay thickness
To increase the CapSense signal strength, choose an overlay material with higher dielectric constant, decrease the
overlay thickness, and increase the button diameter.
Table 5-1. Overlay Material Dielectric Strength
Material Breakdown Voltage (V/mm) Min. Overlay Thickness at 12 kV (mm)
Air 1200–2800 10
Wood – dry 3900 3
Glass – common 7900 1.5
Glass – Borosilicate (Pyrex
®)
) 13,000 0.9
PMMA Plastic (Plexiglas
®
) 13,000 0.9
ABS 16,000 0.8
Polycarbonate (Lexan
®
) 16,000 0.8
Formica 18,000 0.7
FR-4 28,000 0.4
PET Film (Mylar
®
) 280,000 0.04
Polymide film (Kapton
®
) 290,000 0.04
Conductive material cannot be used as an overlay because it interferes with the electric field pattern. For this reason,
do not use paints containing metal particles in the overlay.
An adhesive is typically used to bond the overlay to the CapSense PCB. A transparent acrylic adhesive film from
3M™ called 200MP is qualified for use in CapSense applications. This special adhesive is dispensed from paper-
backed tape rolls (3M™ product numbers 467MP and 468MP).