Supercap charging circuit for RTC backup on STM32

Question

I'm asking for a reliable/simple/cheap circuit for the V_BAT section of a STM32 design, powered at V_DD=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 V_DD=3.3V, V_BAT<2V)
• does not add significantly to the STM32 power drain on V_BAT;
• delivers proper V_BAT 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 V_DD 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:

Answer 1

I find the circuit proposed in the picture perfectly fine.

Your worry about VBAT being lower than VCC is mostly mitigated by your diode right across those two rails. The power-on delay introduced by this diode would be so minuscule, that it is not worth considering.

Also, frying the supercap mostly depends on a particular type. Some of these have rather low allowed currents. This is mainly due to their high ESR. During current flow, this causes heating and damages the part. Some types don't have their maximum currents explicitly stated. In such case you need to rely on the ESR value and a wattage the package can reasonably deal with (sometimes also given in datasheets).

In the end, you adjust your \$R_1\$ to be a compromise between charging time and charging current. \$22\Omega\$ sounds good for starters. Keep in mind that you will almost never discharge to 0 volts. Also, charging current in this case drops exponentially, so you will only draw \$100\tilde- 200 mA\$ for a few seconds.

Answer 2

I've put together a circuit very similar to this in the past and I think, based on that, you'll be just fine with something like this, especially if it's not a critical application.

A couple of considerations: Check out your specific supercap to see how best to charge it. You'll want to avoid charging to too quickly if it has a high ESR. Also, certain types of supercaps hold more charge if you charge them slowly (I'm thinking of Panasonic's "Gold" electric double layer supercaps). Although you'll be losing more as you discharge your capacitor, these two considerations could mean it's worth increasing the value of R1. For maximum lifetime you'll have to balance these factors.

Also, I think a Schottky is a great idea if you're looking to get a little more charge out of your cap. The one thing I'd look out for there is that Schottky's can have horrendous reverse leakage current at high temperatures, so if this would be in an environment that gets toasty you could lose a lot of battery life.

Answer 3

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)

Looking at the Device power supply scheme of STM32F3XX you can see there is a power switch. I guess it's all the same for all the ST32 families, but you better check It's controlled by Vdd, if it's present Vbat is only "monitorable" with ADC but does not take any action for what concerns RTC etc. So just for this question, I think with your circuit you can play safe

Answer 4

The STM32G070CB/KB/RB datasheet chapter 3.7.6 VBAT operation writes: "An internal circuit for charging the battery on VBAT pin can be activated if the VDD voltage is within a valid range."

So you might not need the diode if you activate this feature.