STM32L476 RTC with SuperCap

[gokhannsahin]

Posted on January 14, 2018 at 10:34

Hi everyone,

Should I connect a any series resistor to the super capacitor while using a super capacitor that is 1F 5.5V instead of a battery? Or I can connect directly the capacitor to VBat pin?

#power-backup-supercapacitor-rtc #rtc

[Szymon PANECKI]

Posted on January 14, 2018 at 10:43

Hello sahin.gokhan,

Voltage range, which can be applied directly to VBAT pin in STM32L4 is 1.55V - 3.6V. Applying higher voltage like 5.5V, which you mentioned, would be out of specification and may lead to MCU damage.

Regards

Szymon

[gokhannsahin]

Posted on January 14, 2018 at 11:50

However, at the begining the voltage of capacitor is zero, it will be charged through the VBat pin of mcu. So it will never exceed the higher voltage of 3.6V. Am I wrong?

[Szymon PANECKI]

Posted on January 14, 2018 at 12:07

Yes, you are right. I found a webpage, which describes this use case. Here is a

http://embedded-lab.com/blog/stm32s-internal-rtc/

. On this webpage you can find such design:

Regards

Szymon

[gokhannsahin]

Posted on January 14, 2018 at 12:48

Thank you for answer. Well, can I connect directly the capacitor to the Vbat pin of Mcu as following image and call the function in order to enable battery charging module?

In the datasheet of it says that there is a internal resistor and inverse diode to vdd. So, I understand that it can charge itself without any materials. 

[Szymon PANECKI]

Posted on January 14, 2018 at 13:06

Yes, you can do it. Below I attach a slide, which refers to such scenario:

Regards

Szymon

[Uwe Bonnes]

Posted on January 14, 2018 at 14:35

Why a supercapacitor and not some CR2032? CR2032 will also run a long time, with less hassle and I think is cheaper.

[Tesla DeLorean]

Posted on January 14, 2018 at 16:53

Shipping stuff with batteries is problematic.

A lot of people won't use batteries in designs due to leakage, shelf life and replaceability/access issues, or assorted bad experiences.

[Uwe Bonnes]

Posted on January 14, 2018 at 17:02

And supercapacitor never leak?

[Tesla DeLorean]

Posted on January 14, 2018 at 17:20

Not sure there is a documented case where they have caught fire and brought down a plane...

I listed a number of reasons people eschew batteries, not why they choose super-caps.

Some people won't use tantalum capacitors because they can explode, others because they use conflict minerals.