How to handle transient violence of maximum ratings on 5V-tolerant Pins (STM32F405)

[MBrei.3]

Good morning,

I'm using 5V-tolerant Pins on the STM32F405xx for my 5V Pulled-up I2C Bus. The Vdd for the STM32 will be generated by a LDO out of a 5V supply.

According to AN4899 ("GPIO electrical characteristics and definitions") [p. 19/20], you should use a zenner diode in parallel to your LDO to ensure, that the maximum Rating of Vdd+4.0V on the 5V - tolerant Pins never gets violated.

How should you dimension the zenner diode and the series resistor to ensure safe operation (on power up)?

Best wishes and thanks in advance

[prain]

for fast transient spikes use TVS diodes​ in parallel with each pin. Be aware that if transition is fast, normal diodes can not react in time.

[MBrei.3]

Hello prain,

thank for the answer! What do you mean by each pin? I don't expect spikes on the 3V3 / 5V supply line, it's allready protected by plenty of TVS Diodes.

[MBrei.3]

Just for Clarification.

I'm not worried about Spikes over 5V on the Fife-Volt-Tolerant Pins. I'm worried about the situation, when 5V are supplied to the Fife-Volt-Tolerant Pin and Vdd < 1V.

[prain]

maybe you need this:

apply inputs to R3 , R4​