2020-08-13 01:11 AM
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
2020-08-13 03:28 AM
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.
2020-08-13 04:57 AM
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.
2020-08-13 05:45 AM
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.
2020-08-13 09:35 AM
maybe you need this:
apply inputs to R3 , R4