Electronic – RTC battery deep discharge protection

lithium ionrtc

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

Best Answer

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.

schematic

simulate this circuit – Schematic created using CircuitLab

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