NFC Tag nRF52840 max voltage and current ratings

[Konstantin Y]

Dear Support team,

 I am using the NFC Tag of the nRF52840, CKAA F.

 What is the maximum rating of voltage and current of the NFC Tag?

-> I set NFCPINS.PROTECT = NFC which is supposed to short circuit at approximately 2 V.

Best regards,
Konstantin

[Simonr]

Hi Konstantin

The maximum input current on the NFCT pins is 80mA according to the NFCT Electrical specification. As for the voltage, the peak differential field detect threshold on NFC1-NFC2 is 1.2 Vp, while the maximum voltage on the pins themselves would be VDD. Setting NFCPINS.PROTECT = NFC will indeed protect the SoC from high voltage fields by shorting the pins when it exceeds 2V.

Best regards,

Simon

[Konstantin Y]

Thank you for the reply! Are the NFCT pins shorted with each other or are they both shorted to GND? (asking proactively for a risk discussion)

[Simonr]

The NFC protection circuit will short the two pins together if voltage difference exceeds approximately 2V.

Best regards,

Simon

[Vivia]

What does the circuit look like internally in the chip, which shorten the two pins together?

Will the pins heat up? Does the NFC-Antenna heat up? 

Does this princible work also for strong magnetic fields? Is there a limit to it? 

I need to be sure, that the chip will not be destroyed by strong voltages accross the NFC-Antenna. 

Best regards, 

Vivia

[Simonr]

Hi

It's important that the Absolute maximum ratings found in the product specification are not exceeded at any point, so I'm not sure what you mean about "strong voltages", but the NFC pins should not be exposed to more than 3.9V. You should also be in line with the NFCT electrical specifications (also in the PS). The pins or antenna should not "heat up" noticeably if kept in line with the specs, nor should it be destroyed depending on what voltages you refer to as "strong".

Best regards,

Simon

[Vivia]

Hi Simon, 

thank you for yout answer. By stong voltages I mean voltages between 4 an 10V which could appear at the antenna pins due to an external field. I read all specifications, and I realiesed that it might be critical but I was not sure if the internal clipping would be enought. What does the internal circuit look like, which clipps the pins? 
Why is there a limit to the induced voltage if the pins clip? I measured the voltage accross the antenna in a field, but I could not see any clipping with the example code nfc_peripheral_pairing. Should they clip in this application? 

[Simonr]

Hi

An update from our analog experts on the questions I weren't able to answer: NFC typically operates with fiield strengths between 1.5A/m and 7.5A/m and he nRF52840 is designed for that range. That is with the reference NFC antenna. Since the induced voltage at the antenna pins depend on the antenna it's not possible to state a general field strength limit for the chip. The relevant spec. limit is the current into the NFC pin, not the voltage.

Unfortunately, we can not disclose information on the implementation of our protection circuit.

Best regards,

Simon

[Vivia]

Hi Simon, 

thanks a lot for the explaination.

I tried to measure the clipping of the chip, but unfortunetly when voltages >2V are induced no reaction of the chip could be observed. What does it look like? What delay has the clipping circuit? Does the induced overvoltage need a specific duration for the circuit to clip the pins? 

I am looking forward to your clarifications. 

Best regards