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

[T J]

Posted on February 16, 2018 at 06:22

oops, forgot to calc in the diode to stop reverse current back to the power supply..

since we have to charge the Supercap with a diode, the Supercap voltage will sit at 2.7V when charged.

recalculating;

V=IR.  we can drop   2.7 - 1.8 = 0.9V                     before the RTC input voltage is below spec.1.8V

R = V/I         0.9 / 35e-6  =  25k7                            R needed in series (maximum)

if you use a 22k, R will drop 22k x 35e-6 = 0.77v    across the resistor.

the pin should sit at 2.7 - 0.77 =  2V,                     after power off is ok, above 1.8v

22K should be great....

sorry for the after thought....

otherwise, I think you may want to replace that processor, it may be already damaged.

:(