This 32x21mm breakout is based on the DS3231 Real Time Clock (RTC) from Maxim Integrated and a FT0H474ZF 0.47F super capacitor from Kemet for backup power.
The design powers the DS3231 via the Vbat input rather than Vcc, this results in a current consumption of ~3uA, with spikes to 575uA every 65 seconds when temperature conversion occurs. See the electrical characteristics for Vcc = 0V, Vbat = 2.3V to 5.5V on page 3 of the data sheet.
- 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 ~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 design files can be found at https://oshwlab.com/chrisb2/ds3231-low-power and the Gerbers are included in this repository.
| Designator | Value | Part |
|---|---|---|
| - | DS3231SN | 1 |
| C1 | FT0H474ZF 0.47F | 1 |
| C2 | 0.1uF | 1 |
| R1/R2 | 4.7k | 2 |
| R3 | 15 (500mW) | 1 |
| D1 | BAT54H | 1 |