I'm designing a ATmega-based microcontroller test board. One of the features I want to include is a Real Time Clock with a Maxim DS1307 IC. Instead of including a traditional coin cell battery backup, however, I want to use a really small supercapacitor.
The power draw of the DS1307 is typically around 500nA in backup mode. Panasonic makes a really small 0.015F 2.6v supercap which looks like it would work. How can I estimate how long the RTC will run on this supercap?
Best Answer
Like David says supercaps leak their charge to a certain extent, which is mainly a problem over longer periods of time. Let's make the required calculations ignoring the leakage.
The voltage drop across a capacitor at a constant current is given by
or rearranging for time:
\$V_{BAT}\$ is typically 3V, but for the given supercap is maximum 2.6V. Minimum for the RTC is 2V, so the allowable voltage drop is 0.6V. Filling in the other numbers this gives
which isn't very long, but then you also chose a rather small supercap. A 1F/3V cap would increase your time to 23 days, but there we would have to take the cap's leakage into account, so in practice this may be around a week to a fortnight.
edit
Just picking the right RTC and supercap will dramatically improve longevity. The PCF2123 RTC can operate down to 1.1 V, and a PAS311HR supercap not only has a higher capacitance of 30 mF, but can also operate at 3.3 V. Then the equation becomes
or just short of one week. A 1F/3.3V cap would be good for 7 months, or probably 2 to 3 months taking self-discharge into account.