I have an existing project which uses the Adafruit DS3231 Precision RTC Breakout which is plugged in via a header. Measurements of the deep sleep current showed that the RTC was consuming about 100uA, but looking at the DS3231 datasheet it is possible to obtain about 3uA by powering the RTC via the Vbat input instead of Vcc.
I did not want to rebuild the project to use the Vbat input on the Adafruit breakout, so I decided to build my own breakout and choose to use a super capacitor instead of a battery as backup as I did not need long term backup.
- The schematic shows a DS3231SN which operates accurately below freezing, but the DS3131S can be used if the device is used at room temperature.
- The BAT54H Schottky diode prevents the super capacitor from powering the connected MCU. It has a forward current of 200mA to allow for the current to charge the super capacitor.
- R3 has been chosen to limit the super capacitor charge current to about 150mA based on Vbat being 3.3V. If the input voltage is higher a correspondingly larger value will be required to maintain the maximum current of 150mA.
The breakout worked correctly in my project, the next step is to do some measurements of current and the discharge time for the super capacitor which should be about 48 hours.