Cypress CapSense AN64846, Getting Started

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Design Considerations
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X 41
CapSense controller pins can withstand a direct 2-kV event. In most cases, the overlay material provides sufficient ESD
protection for the controller pins.
Table 3-3 lists the thickness of various overlay materials required to protect the
CapSense sensors from a 12-kV discharge as specified in IEC 61000-4-2. If the overlay material does not provide
sufficient protection, ESD countermeasures should be applied in the following order: Prevent, Redirect, Clamp.
3.2.1 Preventing ESD Discharge
Preventing the ESD discharge from reaching the CapSense controller is the best countermeasure you can take. Make
certain that all paths on the touch surface have a breakdown voltage greater than any voltage to which the surface may
be exposed. Also, design your system to maintain an appropriate distance between the CapSense controller and
possible sources of ESD. In the example illustrated in
Figure 3-2 , if L1 and L2 are greater than 10 mm, the system will
withstand 12 kV.
Figure 3-2. ESD Path
Air-Filled Space
PCB
CapSense
N
o
n
-C
o
n
d
u
c
tiv
e
M
a
te
rial
ESD Event
Mechanical Structure
Exposed
Mounting
Hardware
ESD Event
L2
L1
If it is not possible to maintain adequate distance, place a protective layer of a high-breakdown voltage material between
the ESD source and CapSense controller. One layer of 5 mil-thick Kapton tape will withstand 18 kV. See
Table 3-3 for
other material dielectric strengths.
3.2.2 Redirect
If your product is densely packed, it may not be possible to prevent the discharge event. In this case, you can protect
the CapSense controller by controlling where the discharge occurs. This can be achieved through a combination of
PCB layout, mechanical layout of the system, and conductive tape or other shielding material. A standard practice is to
place a guard ring on the perimeter of the circuit board. The guard ring should connect to chassis ground.
Figure 3-3. Guard Ring
CapSense
Controller
Ground With Conductive
Material on the Perimeter to
Direct Discharge Away From
CapSense Controller
As recommended in PCB Layout Guidelines , providing a hatched ground plane around the button or slider sensor can
also redirect the ESD event away from the sensor and CapSense controller.