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Do I Need a Diodes When Powering STM32F407VET6 VBAT with a CR2032? Any Official Reference Circuit?

chirpy
Visitor

‎2025-12-11 1:06 AM

Hi everyone,
I’m using the STM32F407VET6 VBAT pin to power the RTC, and I plan to use a CR2032 coin cell directly.

I noticed that many schematics add a diode between the CR2032 and the VBAT pin to prevent reverse charging, but after checking the datasheet I’m still not fully sure whether this is actually required.

I’d like to ask two questions:

  1. When the main supply is present, can the F407’s VBAT pin back-charge the coin cell? Is a diode necessary?

  2. Does ST provide an official reference design for powering VBAT with a CR2032? If so, where can I find it?

Thanks for your help!

chirpy_0-1765443496987.png

 

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mƎALLEm
ST Employee

‎2025-12-11 1:17 AM - edited ‎2025-12-11 1:23 AM

Hello,

This is a schematic from the hardware getting started application note AN4488: either you select the battery or VDD. Generally, if the battery is used, it's connected directly to VBAT.

mALLEm_0-1765444375823.png

But that depends on the usage. Why are you connecting VBAT to both power supplies? do you need to save the power consumption from the VBAT while VDD is present?

I think it's doable but you need to select diodes with low voltage drop and ensure you have VDD > VBT1

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mƎALLEm
ST Employee

‎2025-12-11 2:39 AM - edited ‎2025-12-11 2:43 AM

Boff!! I completely forget that there is an internal switch (the Power switch) that switches from VBAT to VDD when VDD is present. 

From RM0090 figure 9:

mALLEm_0-1765449329921.png

So there is no need for these diodes! Need to connect the battery directly to VBAT pin.

Thank you @Peter BENSCH for the notification for that very basic information!

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Ozone
Principal II
In response to mƎALLEm

‎2025-12-11 3:03 AM

As I understand it, you can't really power the MCU from VBat.
The DS says :
> The VBAT pin allows to power the device VBAT domain from an external battery, an external supercapacitor, or from VDD when no external battery and an external supercapacitor are present.
> VBAT operation is activated when VDD is not present.
> The VBAT pin supplies the RTC, the backup registers and the backup SRAM.
Supplying the MCU or the VBat domain are different use cases.
Although I think powering a F407 from a CR3032 is not a a viable idea in the first place.

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