Category: Hardware
Product Number: DS3231M
Hello everyone,
Together with my team we are having an issue using RTC DS3231 together with a supercapacitor connected to the VBAT pin as a backup battery. We are using the following circuit, where the capacitor "C" is a supercapacitor of 1 Farad. A DMM was connected in parallel with an imput impedance of 10 MOhms, in order to measure the charge and discharge curve of the supercapacitor:
In order to measure the discharge curve, the 5V supply was disconnected, giving the following equivalente circuit:
In this case power is supplied by the supercapacitor. Also there is a current IDMM that flows through the DMM's imput impedance of 10 MOhms. For this last case, measurements of the supercapacitor's voltage were taken for several days, giving the following plot:
The VBAT plot (blue curve) shows the voltage samples taken using the DMM. Each sample was taken once every 5 minutes. The IBAT plot (red curve) shows an estimation of the IBAT current, which was calculated from the supercapacitor's voltage samples.
Approximately after 100000 seconds, the blue curve becomes linear and the red curve becomes a constant of 2 uA. This value corresponds to what is indicated on the datasheet (IBATT), which is OK:
However, the plots show and exponential behaviour before the first 100000 seconds, with an IBAT current as high as 120 uA. I would like to know if this is an expected behaviour of the RTC or why does the IBAT behaves like this. Just by looking at the datasheet, I would expect a constant current between 1 uA and 3.5 uA during the whole discharge time of the supercapacitor.
Thank you in advance!
