What voltage should be clamped on for analog input, vref or VDDA? At what condition positive injected current happen when the pin is used as analog input?

[Jasmine]

Hi,

I'm using STM32H7A3, and I’m a bit confused about injected current for pins used as analog input. 

In our design, I used both 5V tolerant and 3V tolerant pins as analog input. And VDD=3.3V, VDDA is connected to VDD with a bead. And Vref is 3V. And for each input, I have clampon diode connected to VDD, not Vref.

For 5V tolerant pins, positive inject current would happen when Vin > min(Vdd, Vdda)+3.6V, with maximum 5 V. 

For 3V tolerant pins, positive inject current would happen when Vin > Vdd+0.3V In the document here, chapter 5.2.1 and 5.2.2 https://www.st.com/resource/en/application_note/an4899-stm32-microcontroller-gpio-hardware-settings-and-lowpower-consumption-stmicroelectronics.pdf

If the pins are used as analog input, the maximum operating voltage on pin cannot exceed min(VDDA, VREF+) + 0.3 V. Does it mean injection current will happen when VIN > min(VDDA, VREF+) + 0.3 V, or the injection current will happen only when VIN > VDD?

The reason I ask is that I have clamp diode connected to VDD, not Vref. If injection current will happen when VIN > min(VDDA, VREF+) + 0.3 V, then the clampon diode can not really protect the design against accidently high voltage input.

Anybody can help me with this, any response is appreciated.

With best wishes

Jasmin

[Peter BENSCH · Accepted solution]

VIN > min(VDDA, VREF+) + 0.3 V means that the injection current flows when the input voltage is greater than 300mV above the smaller value of VDDA and VREF+.

In your case, this is true at 3.0V (the lower value) +0.3V = 3.3V.

Does it answer your question?

Regards

/Peter

[Jasmine]

Thank you. I think this answers my question, this means I need to connect clampon diode to Vref, not VDD.

[Peter BENSCH]

Correct.

If the problem is resolved, please mark this thread as answered by selecting Select as best. This will help other users find that answer faster.

Regards

/Peter

[Jasmine]

Thank you very much again for the quick response.