Clean power supply for RTC

[pk2010]

I made a device which uses 18650 battery for runing RTC. A coin battery was not possible because the customer did not want the hassle of changing batteries for at least 3-4 years. The 12v is converted to 4.2v (because there are some other things on board which run @4.2v)

I connected the VBAT pin of STM32F103 directly to the battery and since datasheet said that no CAP should be added to vbat pin if there is a battery since the leakagae current of CAP will be roughly of the order of current in vbat pin, so it will have shorten the battery life.

60 devices are already installed in remote sites and we are facing problems of RTC resetting to random values when main power supply is turned off and ON. Some people have suggested that since the battery has protection circuit , it might be momentarily disconecting the batteries when tp4056 starts charging. So the power supply to RTC is not stable.

Thus I want to add a few components to make the power supply to RTC clean. I am thinking of adding a 1uf and 100uf by scratching the PCB traces. Of course too much tinkering wont be possible. So I want to ask are there some other components which i should definitely add ? A pseudo schematic is attached

 

[KlausST]

Hi,

I don't expect longer livetime with a 18650 battery due to it's chemistry livetime and self discharge.

Coin battery: as long as the system is mains powered there shoukd be no current drain from V_bat.
Thus the coin cell need only deliver power when mains is off.
How long is the longest expectable main off period? Maybe a goldcap as an alternative.

There is a charger. What is the expectable peak (!!) voltage at the battery? Are you sure this is within STM32 specification?

To minimize capacitor leakage current you may use a ceramics capacitor with much higher voltage rating, or use a foil capacitor...and for sure a PCB and layout with low leakage current, maybe the PCB needs to be carefully cleaned after solder process.

Klaus

 

[schmitt trigger]

Yes
At least a schottky to decouple the RTC supply from the remaining loads.

The caps should be on cathode side such that they will decouple the RTC.