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

MBrei.3
Associate II

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

4 REPLIES 4
prain
Senior III

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
Associate II

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
Associate II

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
Senior III

maybe you need this:

apply inputs to R3 , R4​

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