I'm asking for a reliable/simple/cheap circuit for the VBAT section of a STM32 design, powered at VDD=3.3V, that will safeguard the built-in RTC (based on 32768 Hz Xtall) for like 10 days.
Based on C = t⋅I/ΔU, its looks like a 1F supercap will do (I'm taking t = 864000s, I = 1.2uA, ΔU = 1.1V, see below). I need a charging circuit that
- does not fry the supercap;
- does not fry the STM32 or prevent it from starting-up reliably (in particular: I do not see a specification for what happens when VDD=3.3V, VBAT<2V)
- does not add significantly to the STM32 power drain on VBAT;
- delivers proper VBAT quickly (0.2 s) including after full discharge, so that the RTC is always available soon after power-on reset;
- brings the supercap to near full charge soon enough after VDD is applied (perhaps 80% after 50 s), so that testing leaves a decent reserve.
The simplest I can think of is
Any criticism? Suggestion on the diode (perhaps a Schottky would be better) or other component? Alternative?
A most relevant section of the STM32(F100) data sheet is: