What is the purpose of the PMOS switch for VBAT-SYSTEM STM32L0x1 see AN4718

Oliver R. · ‎2023-01-12

AN4718 "How to design a V BAT system based on

STM32L0/L1 ..."

Case 1b, it is stated, quote:

"V DD /V DDA can also be switched to V BACKUP when STM32L0/STM32L1 MCU goes into STOP mode, to allow user to remove main supply in very safe way."

End quote.

Question: What does that mean "very safe" ?

What is the exact purpose of that PMOS switch ?

Is is to prevent the supercap current going to a battery pin ?

What is unsafe if not used ?

Peter BENSCH · ‎2023-01-12

OK, the remark "very safe" is perhaps a bit exaggerated, but it means that in the event of a main supply failure, no current flows from the backup supply into the main supply.

The PMOS allows for low-impedance and thus low-loss switching of the main supply. Since Schottky diodes have quite large reverse currents, it makes no sense to replace the PMOS with a Schottky.

Does it answer your question?

Regards

/Peter

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View solution in original post

Peter BENSCH · ‎2023-01-12

OK, the remark "very safe" is perhaps a bit exaggerated, but it means that in the event of a main supply failure, no current flows from the backup supply into the main supply.

The PMOS allows for low-impedance and thus low-loss switching of the main supply. Since Schottky diodes have quite large reverse currents, it makes no sense to replace the PMOS with a Schottky.

Does it answer your question?

Regards

/Peter

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

Oliver R. · ‎2023-01-15

I see. The main supply might be tied to ground, for instance, if broken, and that would shourtcut the supercap.

Thank you very much.

Peter BENSCH · ‎2023-01-16

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

Regards

/Peter

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