Cypress CapSense CY8C20x36 Guía de usuario descargar pdf (Pagina 36)

36 Document No. 001-64846 Rev. *I Getting Started with CapSense

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.

3.2.3 Clamp

Because CapSense sensors are placed in close proximity to the touch surface, it may not be practical to redirect the

discharge path. Including series resistors or special purpose ESD protection devices may be appropriate. Adding a

series resistor on the vulnerable traces is a cost-effective protection method. This technique works by splitting the

dissipation between the resistor and the controller. The recommended series resistance added to the CapSense

inputs is 560 ohms. More details are available in the Series Resistor section.

Figure 3-4. ESD Protection Using a Series Resistor

A more effective method is to provide special purpose ESD protection devices on the vulnerable traces. ESD

protection devices for CapSense need to be low capacitance. Table 3-4 lists devices recommended for use with

CapSense controllers.

Table 3-4. ESD Protection Devices

ESD Protection device

Input

Capacitance

Leakage

Current

Contact Discharge

maximum limit

Air Discharge

maximum limit

Manufacturer

Part Number

Littelfuse

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

1 2 ... 31 32 33 34 35 36 37 38 39 40 41 ... 74 75

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