I create a RTC backup circuit for a STM32L412CBT6. Circuit work but not as long as I estimate during design.
Here is the schematic (VBKP go directly to STM32 VBAT pin):
Here is my estimation :
Tc = RcxC
Tc = 100 * 0.33
Tc = 33 s
To fully charge the super capacitor at 5xTc I theoretically need 165 s (2 min 45 s)
I do measurement and It's OK, my super capacitor from 0V take about 2 min 30 to go to 3.2V.
To get the Rd, the internal resistance of the VBAT circuit of STM32, I see in datasheet RTC theoretically need 300nA at 3V, but when I take voltage across R17 during discharge I got 5mV, so current is 50µA (166 times more but it's an other problem so let's take 50µA)
Rd=3 / 0.00005
Rd = 60000 ohms (60 kohms)
Td = Rd x C
Td = 60000 x 0.33
Td = 19800 s
To fully discharge the super capacitor at 5xTd I theoretically need 99000 s (27 h 30 min)
To discharge super capacitor at 0.7xTd (50% is about VBAT min for RTC) I theoretically need 13860 s (3 h 51 min)
I do measurement and I'm totally out :
HOUR SC1 Voltage [V] 22:00 3,21 22:12 2,62 22:22 2,44 22:33 2,29 22:49 2,10 22:59 2,00 23:12 1,86 23:24 1,74 23:34 1,64 23:47 1,52 0:15 1,25 0:24 1,16 0:42 0,99 0:55 0,86 1:26 0,58 1:35 0,51 1:53 0,37 2:34 0,18 2:56 0,14 3:03 0,13
I take about 5H to discharge the super capacitor (far away from 27H)
- I think my estimation problem is from Rd (internal VBAT circuit resistance of STM32) determination, but why I'm so far of my estimation ? Can you help me to find my error ?
- If someone got an idea for why my RTC sink 50µA instead of 300nA, let me known (Voltage measurement device error ?)
- If someone got an idea to connect (without regulator) the 3V3 of this circuit to a LiPo (3.7V) battery (charge at 4.2V), let me known.
Thanks in advance
EDIT 1 :