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This question is a continuation of my previous question, but different from that.

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

The circuit is given below.

enter image description here

All the signals on the right hand side are going to a microcontroller. On the I2C line, one more device is connected. A 100 kΩ pullup is connected for RTC_INTP, RTC_SCl, RTC_SDA. Below is the block diagram of right hand side signals. All of the signals go go to the microcontroller. The FET RESET signal comes from the microcontroller.

I checked the MOSFET and it is off.

enter image description here

When 5V_uC is off, the RTC runs from the supercapacitor and the microcontroller is powered from VDD_MCU_5V. (In my previous question I wrote that the microcontroller is not getting any power, that was wrong.) The clock output of the RTC will be in enabled state.

I see 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?

UPDATE

XTAL Datasheet

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    \$\begingroup\$ You haven't stated what SCL/SDA activity there is nor what the capacitance loading the RTC_CLK_OUT line is. You should disable RTC_CLK_OUT and see what happens. You should also disconnect R420 to unambiguously disconnect the MOSFET. Plenty of things you should do to solve this. \$\endgroup\$
    – Andy aka
    Commented Jun 11 at 8:34
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    \$\begingroup\$ Additionally, the manufacturer specifically recommends to connect all the resistive pull ups to the same VDD that also powers the chip itself - not sure how that might have an impact, but it is worth investigating in my opinion. \$\endgroup\$
    – Vladimir Cravero
    Commented Jun 11 at 8:46
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    \$\begingroup\$ checked the MOSFET and it is off why then ask May I know any leakage current will flow through the MOSFET, how did you establish it is off? \$\endgroup\$
    – greybeard
    Commented Jun 11 at 9:33
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    \$\begingroup\$ Why is the symbol of your crystal apparently wired incorrectly? Please provide a data sheet link. Why is C249 (or the crystal) not connected? \$\endgroup\$
    – Andy aka
    Commented Jun 11 at 11:19
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    \$\begingroup\$ XTAL datasheet is added.RTC is working as expected when powered with main supply \$\endgroup\$
    – Confused
    Commented Jun 12 at 4:32

1 Answer 1

Reset to default
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Well the current is going somewhere .Remember that 1mA is significant here because 1F means 1 mA for 1000s for 1 volt drop .First measure volts on R316 to eliminate D1 leakage ,low volt shotties leak a lot. Then do R420 to check mosfet leak .If things appear good then place 10 ohm resistor in series with pin 8 Vdd to check the chip ,it could be drawing more current than expected .Now that you have all your currents computed from measured volt drops across the resistors then you can ballpark that the supercap is really 1F .They can go leaky and can fail to lower values .

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