Backup power

[Denys_T]

Hello, my device will work in a car and I need to arrange power for the RTC. Is the H750 processor used, is the diagram in the screenshot suitable or are there better technological solutions?

[Peter BENSCH]

 

First of all, it should be noted that no STM32 has yet received automotive approval.

In principle, such a simple circuit could be used for this purpose. However, C71 will not be able to buffer the VBAT pin for long if the 12V fails or is switched off: in the extreme case of 85°C, a current of 22µA can be drawn there, which (assuming linear discharge from 2.8V to the minimum of 1.2V) discharges the 1µF in approx. 0.073 seconds (see data sheet, table 48). Incidentally, the RTC and LSE are switched off, so no clock is running. For a buffer duration of 24 hours, for example, a capacitor with approx. 1.2 farads (not microfarads) would be required - and the RTC is not yet running.

Incidentally, you also have to take into account overvoltages of up to approx. 100V in the car, which can occur on the 12V line (so-called Load Dump). Your circuit would have to be designed accordingly - if it still comes into question after the rough calculation of the necessary capacity at VBAT.

Hope that helps?

Regards
/Peter

[Denys_T]

Thanks for your comment. Power is always supplied to the VBAT (instead of a battery); the capacitor acts as a buffer, not a source. I also assume that the zener diode will help with surges on the 12V line, or do you think it is necessary to additionally install TVS ?

[Peter BENSCH]

A TVS should not be absolutely necessary as long as you can guarantee that the absolute max ratings for VBAT are not exceeded. You have probably selected a zener with 2.8V because they remain just under 3.6V with the higher currents of surges, right?

[Denys_T]

Yes, the zener diode has a drift, so I chose a voltage of 2.8 v . Perhaps you can recommend something that uses ST in such applications.

[Peter BENSCH]

As mentioned, automotive is not a common application for STM32. In addition, pure buffering without a source function is very rarely necessary. For these reasons, I don't know of any recommendation from STMicroelectronics.

Regards
/Peter