RTC super capacitor is discharging very fast

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I am using an PCA85073A RTC in my design. On some boards w the supercapacitor is getting discharged very fast (5V to 1.1V in half an hour.) This happens when we turn off 5V_uC.

The circuit is given below.



All the signals in the right hand side are going to a microcontroller. On the I2C line one more device is connected. A 100K pullup is connected for RTC_INTP, RTC_SCl, RTC_SDA. Below is the block diagram of right hand side signals. All the signals are going to the microcontroller. The FET RESET signal is coming from the microcontroller.
I checked the MOSFET and it is off.




When 5V_uC is off the RTC will be working from supercapacitor and the microcontroller getting power from VDD_MCU_5V time. The clock output of the RTC will be in enabled state.

I am seeing only two paths for the super capacitor to discharge. One is via the MOSFET and the other via C260 (if C260 is shorted.)

Will the discharge happen via any other paths?
 

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Hi,

There was a mistake from my side in that discussion.

In that what I told is RTC CLK is going to the microcontroller which is unpowered. This statement is wrong.
Actually the microcontroller is powered.

That is why I opened a new thread.

Regards
 

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What does this mean?
That´s why we ask for a complete schematic.

Did you try the recommended circuit of the datasheet?
If NOT: why?
(If you keep on the datasheet recommendations you are on the safe side)

Did you calcualte the required discharge current?

Klaus
 

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Makes no sense. 5V_uC is off but uC is powered? If this is special test setup, you should describe it in detail.

Generally speaking, I would identify the discharge path by measurements.
 

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FvM said:
Generally speaking, I would identify the discharge path by measurements.
Click to expand...
I agree.
.. with different test setups. Like:
* disconnected MOSFET
* switched OFF CLK
* and so on

Klaus
 

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That net name was accidently 5V_uC .It is not powering the Microcontroller.

Microcontroller is powered with a separate power supply

When my system is off 5V_uC will not me coming to RTC.

RTC will be powered from Supercapacitor. The Microcontroller supply voltage (VDD_MCU_5V)also present.

I checked for discharge paths. There is only one path via mosfet.(will completely remove the mosfet and test)

I checked the MOSFET and it is off. Other than that I did not see any discharge path.

I am not sure about reverse leakage via diode D64 when 5V_uC is off.
 

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I checked the datasheet.
I did not find any information about reverse leakage via diode D64 when 5V_uC is off.
Leakage can happen via diode.
The present diode has an Ir of 1uA max.
Typical is not given.
At 5V it is not causing much Ir
 
Last edited:

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