I have question on STM32F777 RTC and super cap

[Chiku]

I wanted to use RTC Circuit for Supercap.

I have designed circuit (Check Attached snapshot).

but i dont get back up time? can anyone explain do we need to do any provision in controller.

It seems that capacitor is discharging very fast

[SStor]

Seems that the high leakage current of zener diode discharges the supercap.

Try to decouple zener diode from supercap with a diode with very low reverse current.

[Chiku]

Yes.. by removing zener.. Supercap voltage is decreasing as per expectations.

BZX84-B zener diode has reverse current of 5uA at Reverse leakage voltage of 1V.

in our case we have zero reverse voltage.

Still i dont understand which parameter of Datasheet causing this problem.

https://assets.nexperia.com/documents/data-sheet/BZX84_SER.pdf

Can you please which parameter we need to check so that alternate zener could be selected.

[SStor]

Fig.5 on page 10 in BZX84 datasheet shows the dependency of reverse current IR from reverse voltage VR. So for type 3V3 IR is ~20mA at 3V (limited with R140), ~100µA at 2V and ~5µA at 1V (STM32F777 requires VBAT > 1.65V).

VR is the same voltage as the supercap because the Z-Diode is parallel. Therefore the supercap will discharge very fast over 1V through zener diode beaviour.

The current over D3 in Z-Diode is also not limited in the current circuit.

You should simply place the Z-Diode on the other side of D2 to eliminate these problems with Z-Diode.