Protecting against ESD

AN3960

12/21

Doc ID 022082 Rev 1

Equation 1

Where p is the pressure in atmospheres and d is the distance in millimetres.

This spark gap can be expected to have a peak voltage of about 2000-2500 V.

Figure 3.

PCB with spark gap

1.

The contact area of this spark gap needs to be free of solder resist, in order to function as a spark gap.

3.3

Ground rings

To protect against ESD stress on the touch sensing surface, a low impedance path to

ground must exist through the device. The touch sensor can be protected using a ground

ring (also called a guard ring) which is a ring around all the system electrodes (see Figure 4:

Ground ring). It is placed in the border area. The ground ring can be a simple metal foil. It is

necessary to ensure that there is a firm connection between the ground ring and the device

system ground.

If the product is densely packed, it may not be possible to prevent an ESD stress.

Consequently, the touch sensing device can be protected by controlling where the discharge

occurs. This can be achieved through a combination of the:

●

PCB layout

●

Mechanical layout of the system

●

Conductive tape or other shielding material

These three items avoid an ESD stress reaching the electrodes (and therefore the MCU)

because they form a sufficient shield. For example, an ESD stress goes directly to ground if

it occurs in the ground ring.

As recommended in “PCB Layout Guidelines”, providing a hatched ground plane around the

touchkey or rotary or linear sensors (other types of electrode with different shapes) can

redirect the ESD stress away from the electrodes and touch sensing device.

V

3000

p

×

d

×

() 1350

+

=