Most RTC chips have a VBATT input for backup power supply; besides a battery, you can use a supercap there, e.g. using the following scheme:
How much time would the cap provide for your RTC while the main power is not available is of course function of the RTC's current, C1's capacitance, and the various leakage currents.
- D1's leakage is in the order of 20nA and can be ignored (see below)
- My RTC consumes 365 nA, can can work with Vbatt down to 2.0V, so a 0.22F cap is enough for 18 days, if C1 was a perfect 0.22F cap.
- The C1's leakage current is, I believe, the biggest contributor, at 0.5 to more than 10µA
The leakage currents of supercaps seem to be not well characterized; in fact, a lot of datasheets omit that value at all, so it's likely nothing to write home about (20-100 or more µA).
Of the few models that actually list something, for example
- Ohmite LM055224A: 0.22F, 0.5 to 10µA.
- AVX SCMR14C474MRBA0: 0.47F, 6µA
- others, which are again 8-10+ µA for 0.2-1F, and more for larger capacitances,
it can be computed that they are unlikely to last more than a week in RTC backup operation. Their leakage is just too high.
- Is it possible to deal with this imperfection? Is a supercap suitable for this design at all?
- Can more than 1 week of autonomy be achieved? I'll be happy with at least two weeks;
- How is leakage defined? -- most of the time, manufacturers don't specify what do they mean by leakage current and in what conditions do they measure it. Might be they measure worst-case leakage (high ambient temperature, capacitor close to 5.5V, ...), while the average case at 3V and 25°C is an order of magnitude better.
Not a shopping question
I'm not searching for a specific cap model that would fit the bill; I've pored through a lot of datasheets, and haven't found any. I think I may be mistaken to use a supercap there in the first place, as per question #1 above.