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  • Case ID: 294097
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Protection diodes on NRF52840: clamping current

nRF52_user

I'm developing an application using the NRF52840. I'm using the ADC, and there may be times where the signal to be measured is up to 1.8V, but the NRF52840 is not yet powered (i.e. VDD = 0). One option I am considering is to put a resistor in series with the ADC input to limit the current through the protection diodes (see the diagram below). What current can I safely expect the protection diodes to sink when V_I/O = VDD + 0.3?

Alternatively, can you suggest an alternative approach that will not affect a high-impedance (~100 kOhm) signal when the NRF52840 is powered?

I recognize this question is similar to Protection diodes on NRF52832: max current, nRF52832 Maximum GPIO Input Voltage and Body Diode Clamping Current, but none of them have provided an an alternative to the series resistor, or recommended a safe value for that series resistor.  The ESD ratings in the "Absolute maximum ratings" table do not seem to be relevant as those reflect a single high-voltage pulse rather than a constant low level.

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    There is no safe value low enough to allow the SAADC to acquire an accurate reading. The reason is interference with the reset functionality as before reset occurs current consumption can be extremely low such that the shottky clamp diode current is high enough (> few 10s nA) to start the power up but interfere with reset, most notably on a brown-out or VDD dip where the brown-out condition reset never completes as the voltage is held too high at (say) 20nA supply. Alternatives require an external switch: FET normally off or Ideal Diode normally off or Flying Capacitor or ...The "on" signal doesn't need to be a port pin, it can be the output of an on-board regulator on the nRF52 or even VDD. 20nA at 3V is 150MOhm. Is this a problem for every powerup/VDD dip? No. Is it a problem with shipped product on site? Yes .. eventually. There is no specification for this condition as maximum ratings are being violated. Nordic might disagree (hopefully) :-)

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