AN3960

Protecting against ESD

Doc ID 022082 Rev 1

13/21

Figure 4.

Ground ring

3.4

Adding resistance

The most common method of external ESD protection is to add a small serial resistor in-line

between the ESD energy source and the touch sensing device pin to be protected. A

resistor as small as 50

protection is somewhat proportional to increased serial resistance so, higher immunity is

possible.

This method works for two reasons. First, the serial resistor works with the parasitic pin

capacitance (typically 5 to 10 pF) of the device to create a single-pole low pass filter with a

cutoff frequency below 1 GHz. This causes the serial resistor to attenuate most of an ESD

event's high-frequency energy (as much as 90 % of the rising-edge power in an HBM

discharge). Second, when the protection circuits of the device are operating normally, their

impedance is very low (in the order of tens of ohms or less). This low impedance works with

the serial resistor to create a voltage divider, so that the high voltage from an ESD stress

can only bias the built-in protection circuits of the device with a portion of the total ESD

voltage. This attenuation is in addition to rising-edge filtering. The sum of these effects from

a simple external serial resistor dramatically improves ESD performance in a demanding

application.