Electrical and Mechanical Design Considerations
AN66308 - CY8CMBR2044 CapSense® Design Guide, Doc. No. 001-66308 Rev. *F
31
Bonding Overlay to PCB
Because the dielectric constant of air is very low, an air gap between the overlay and the button degrades the
performance of the button. To eliminate the gap, use a nonconductive adhesive 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).
ESD Protection
Robust ESD tolerance is a natural byproduct of thoughtful system design. By considering how the contact discharge
occurs in your end product, particularly in your user interface, you can withstand an 18-kV discharge event without
incurring any damage to the CapSense controller.
CapSense controller pins can withstand a direct 12-kV event. In most cases, the overlay material provides sufficient
ESD protection for the controller pins.
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 ESD protection, apply countermeasures in the following order: Prevent, Redirect, Clamp.
Prevent
Make sure all paths on the touch surface have a breakdown voltage greater than potential high-voltage contacts. In
addition, design your system to maintain an appropriate distance between the CapSense controller and possible
sources of ESD. 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.
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. Place a guard ring on the perimeter of the circuit
board that is connected to chassis ground. As recommended in
using a hatched ground
plane around the button can redirect the ESD event away from the button and CapSense controller.
Clamp
Because CapSense sensors are purposely placed in close proximity to the touch surface, it may not be practical to
redirect the discharge path. In this case, consider including series resistors or special-purpose ESD protection
devices.
The recommended series resistance value is 560
Ω.
A more effective method is to provide special-purpose ESD protection devices on the vulnerable traces. Note that
ESD protection devices for CapSense need to be low in capacitance. Table 4-2 lists devices recommended for use
with CapSense controllers.
Table 4-2. Low-Capacitance ESD Protection Devices Recommended for CapSense
ESD Protection device
Input
Capacitance
Leakage
Current
Contact
Discharge
maximum limit
Air Discharge
maximum limit
Manufacturer
Part Number
Littlefuse
SP723
5 pF
2 nA
8 kV
15 kV
Vishay
VBUS05L1-DD1
0.3 pF
0.1 µA <
±15 kV
±16 kV
NXP
NUP1301
0.75 pF
30 nA
8 kV
15 kV