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This is from an Adafruit RTC module.

What purpose is the R3 1k resistor serving in this circuit? Is this a RC charging circuit? Would the capacitor C1 not get any charge if the resistor wasnt there? How is the value of R3 determined?

Thanks!

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Section 14.1 of the datasheet tells you why - the supply is monitored to allow it to switch over to the backup battery if and when it fails, and needs to not fall by more than a specified rate, such that the monitor circuit has time to make the switch over before power is lost.

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The voltage will drop at a rate of dV/dt = Ic/C and if current is limited by 1k then Ic (max) =V/R where a power interruption or switch to battery becomes worst case dV=V so the time constant for 64% voltage sag = RC=Tau = 10ms so a 1ms interruption is only 6% and less for a FET bridge.

Also any ripple > 1/RCf is attenuated by 20 dB/decade in f, which may be of more benefit.

The datasheet spec says d Vdd/dt = 0.7 V/ms max

Thus Tau = 10ms and Vdd=5V gives 0.5 V/ms and satisfies the spec. with only 40% margin. Best case. I once worked with a great consultant who discovered this weakness and notified the IC OEM chip maker >20 yrs ago which may have been the source of this spec.

An abrupt failure might be a sudden disconnect or a Vdd short so you need temporary storage so it has time to switch to battery in this logical OR function of battery backup. Often it arises due to the ESR of a tiny battery dropping its voltage momentarily to charge the load cap.

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