I'm doing an application using RTC on STM32F091. I'm using a small rechargable Li-Ion battery (such as MS920SE-FL27E) as the Vbat power source for the RTC.

My concern is, how can I protect the battery from deep discharge?

  • Battery is small (due to physical restrictions), thus changing it after one 'season' (3-6 months of OFF time) is not a very good option.
  • hoping to reuse the battery more by cutting off the battery usage when V level is getting dangerously low.

I'm aware I can measure the Vbat voltage level to know when the battery is getting too low. What would be a good way to cut power from the Vbat entirely (and thus 'kill' the system completely) in RTC mode?

My one idea was to abuse the RTC_ALARM output. Set it high (during normal application) to keep a transistor open in-line with VBAT (ALARM should keep the state when VDD cycles on/off...). If VBAT is measured too low, the ALARM would be LOW and killing the connection to VBAT. Haven't been able to think of any other idea though :/.

EDIT 22.06 From 1st Answer I made a PMOS equivalent circuit. Current consumption for such circuits seems to be an issue for me. RTC usually consumes <3uA.

EDIT 28.06 Took some time to look for appropriate MOSFET and I believe I have found it. NX1029X (using the P-channel one). I set the gate voltage so that the MOSFET should turn itself off around 2.3V-2.4V of RTC Battery voltage. Seems to work. Only issue is, can I reliably use MOSFET's Vgs, to make the cutoff? Have used it before though but on higher voltages (was easier to set the gate-source voltage).

I'd go for this: RED (Chl A), BLUE (Chl B), GREEN (Ch C) (capacitor is for avoiding convergence issues)

ON current is ~470nA extra, so 3.47uA. OFF consumption is ~20nA (around the transition 0.5uA or so). I think I can live with that :P.

P-MOSFET cutoff circuit for VBAT enter image description here

  • \$\begingroup\$ There is only a problem when the device is off for >5000 hours. You can charge this battery with a resistor and some diodes. \$\endgroup\$
    – Jeroen3
    Jun 22, 2017 at 9:19
  • \$\begingroup\$ @Jeroen3 Jup, and it IS a problem. I've currently estimated approx 2160h of working hours from 5.5mAh cell @~2uA RTC. After which battery could be danerously low. Device might not be used for even longer period - thus cell can die. Cutting consumption down to some nA can be solution, because I've currently figured, I can't really unplug the battery (with trans. switch), to make the consumption near zero... \$\endgroup\$
    – crypton
    Jun 28, 2017 at 12:17
  • \$\begingroup\$ The chip is unable to function on the Vbat only. It can only sustain the RTC. Is your device really completely off for 2160h? \$\endgroup\$
    – Jeroen3
    Jun 28, 2017 at 12:20
  • \$\begingroup\$ ermm, to clarify: only RTC runs on Vbat, yes. Main (external) power Can be switched off for extended periods of time (3 months min) - thus no recharging the VBAT cell for a while. \$\endgroup\$
    – crypton
    Jun 28, 2017 at 12:24

1 Answer 1


There are tons of ways to do it. I like the idea of using an transistor (MOS, BJT, doesn't matter) and a zener. The zener will conduct as long as the battery is greater than the zener voltage. Remember that the zener in this must be small enough to turn on the transistor. When the voltage in the circuit below gets below 2V, the zener won't conduct at all. This is just off the top of my head, there may be better options.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Cool, nice thought! I tried initial simul. with this, NPN cricuit and made a voltage equivalent PMOS circuit aswell (which I'd rather use). Right off the bat falstad circuit simulation (will link this in my initial question) shows rather high current consumption for both circuits. Meaning when the RTC would take ~3uA (usually less), then the cut-off circuit consumes about 5uA. Basically double the consumption - that degrades the expected lifetime >2x :S Guess I can't get around from this trade-off, zeners do consume quite a bit. Or some other ideas concerning current consumption? \$\endgroup\$
    – crypton
    Jun 22, 2017 at 7:59
  • \$\begingroup\$ You'd need a zener that has a reverse leakage of less than 1uA (typical rtc current) a few mV from threshold to have any effect. Let us know if you find any. \$\endgroup\$
    – Jeroen3
    Jun 22, 2017 at 9:10
  • \$\begingroup\$ You may want to look into PMICs that have such features available. TI has the BQ2970. \$\endgroup\$ Jun 22, 2017 at 20:17
  • \$\begingroup\$ @Jeroen3 Damn haven't seen Zeners with low working voltage AND with low reverse leakage current. The leakage for 2.4Vz almost always stays around 50 - 100uA. Geesh! I wonder If I can rely on MOSFET's Vgs consistently, for this to turn itself off, when cell coltage goes too low ? Doing stuff on Multisim now, with more precise models, FLASTAD is greatly simplified. Edited one MOSFET simulation in the inital post. \$\endgroup\$
    – crypton
    Jun 28, 2017 at 13:45

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