# Wihtout power supply,how many times can keep on the internal clocking with the Super CAP ( VDD_RTC of the Jetsom TX2 4G)

Hello. I am having problems with the VDD_RTC PIN,I want to konw how many times can keep on the internal clocking with the Super CAP ( VDD_RTC of the Jetsom TX2 4G).then I can know the hold time of the core-board

Hi, the working current of RTC part is about 2uA, you can calculate the working time accordingly based on capacitance of super cap.

Hi, there.

Let us start from

I(t) = C x dV(t) / dt.

Since the working current of RTC is about 2uA, as Trumany pointed out, I(t) can be simplified as I = 2uA, a constant independent of time. We further assume V(t), when discharging, is dropping linearly with time, so dV(t) / dt = delta V / delta t. From datasheet, C72 is a 80000uF super cap, and VDD_RTC won’t work when the input voltage drops below 1.65V.

Now, putting everything together we have

delta t = C x delta V / I
= 80000uF x (3.3V - 1.65V) / 2uA
= 66000s
= 18.33h

Math tells me TX2, when disconnected from power, could maintain its internal clock for 18.33 hours. But my personal experience is that the system normally cannot survive one night. If I’ve left mistakes along the line of my thought, please feel free to let me know.

Additionally, what is the best practice (with minimum changes made to the TX2 carrier board) to extend the working time (to one week for example)?

1 Like

The full charged backup cap voltage is typical 2.5V not 3.3V.

delta t = C x delta V / I
= 80000uF x (2.5V - 1.65V) / 2uA
= 34000s
= 9.44h

OK, that’s more like it.
Then, again, would you please kindly recommend the best practice (with minimum changes made to the TX2 carrier board) to extend the working time (to one week for example)?