This article describes an application where two requirements were combined (high ESD protection level and low leakage). Our customer is developing a fully-encapsulated wireless button. The user can program the button to control any connected appliance. You can turn on lights, start the dishwasher, open the curtains, start the Tesla or all those things combined.
Fully encapsulated also means that the coin-battery is included in the package molding and that it is thus not possible to change the battery during the product life time. The entire circuit has to last for 20 years from a single coin battery. The standard on-chip ESD protection was not able to meet those ultra-low leakage requirements. The radio chip is only powered when communication to internet is required.
Because the button can be used on different locations and is touched frequently it needs to be able to sustain severe ESD stress. The requirements include 15kV air discharge IEC 61000-4-2 compatibility. The spark could run through the plastic and damage the internal circuitry. There is no ground connection.
The intial peak current of the IEC pulse happens in a very short time-frame (1-5ns). We used VF-TLP (Very Fast Transmission Line Pulser) measurements to obtain ESD clamp candidates.
An SCR (silicon controlled rectifier or Thyristor) based protection scheme was selected as ESD protection.
The SCR clamp has a turn-on time of 0.7-1ns , has very low leakage during normal operation (~100pA) and is capable to conduct high peak currents – which makes this clamp a good candidate for this application.
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