Purpose of series resistor in TSC

[CTimm.1]

Hi,

I'm wondering what the purpose of the series resistor (Rs) is for touchkey applications. I have found in documentation and presentations that it is for extra ESD immunity, but can someone be more precise?

Thanks,

CT

[Trevor Jones]

The touch input is inherently exposed to the outside world, unlike the onboard pin interfaces like QSPI,

being exposed externally like that, the pin can be hit by a high energy ESD spike.

if you have a 10k resistor in series, then the energy cannot get into the pin input.

exactly, the pin diodes push a high voltage ( > 3V3 ) to the Vdd rail and any negative voltage to the ground rail

if the spike is 1000V eventually you will kill these diodes and then after a few more hits, kill the pin input.

The theory is to block the high voltage energy so the pin diodes can absorb the spike completely without damage.

then the pin input remains protected indefinitely.

generally 1k or 10k in series is enough to protect the protection diodes.

[CTimm.1]

Hi Trevor, Thanks for the answer – that all makes sense. CT HOTSTART Carter Timm Electrical Engineer Spokane, WA Ph: 509.534.6930 ext 247 www.hotstart.com<> [cid:2019BestPlacesEmailSignatureRev2_d6333210-d5e1-4455-a2a3-5b789e5e47f9.jpg]