Low Leakage Capacitor for DS3231 RTC

[scott216]

The data sheet for the Maxim DS3231 Real Time Clock says to use a low leakage decoupling capacitor between 0.1 to 1.0uF.  I don't know much about low leakage caps, but I read film capacitors are pretty low leakage, so I chose this one from Digikey:

http://www.digikey.com/product-detail/en/ECP-U1C224MA5/PCF1128CT-ND/285455

Is this a good choice?

[gxti]

An ordinary 100nF ceramic capacitor would be fine, as they have negligible leakage.

The datasheet says says "low leakage" because the RTC draws so little current that you can run it from a lithium coin cell or supercap for a very long time (months to years). If you have a leaky electrolytic cap across the supply rails though it will shorten the battery life considerably. If you are not intending to run the RTC off a small battery for a long time then the "low leakage" spec is irrelevant.

[scott216]

I figured that was the reason.  It's for an alarm clock that I'm giving a friend, so most of the time it will be plugged in, but it could also be the case they stop using it for a while and unplug it.

[TarHeelTom]

Don't know if they're still around, but ran into a 0.1 farad 5 volt tantalum capacitor at Radio Shack many years ago and bought it just for grins.

Charged it to 5 volts and put it back in the blister pack and threw it in my dresser drawer. 

For about 5 years or more I pulled it out once a year and put it to a cheap voltmeter.  Last time i tested it, it still read 5 volts.

I need to dig through my dresser and see if I've still got that thing.  Curious to see what it's got for voltage now.

Tom

[SeanB]

Use a 0.1F  5V5 supercap, it will hold the clock running for a month if the external battery was disconnected.

[scott216]

Use a 0.1F  5V5 supercap, it will hold the clock running for a month if the external battery was disconnected.

That's a good idea.  Would the supercap also take care of decoupling, or would i still need a different cap to do that?

[dannyf]

:
That's a good idea. :

read more about super caps and their limitations before you commit to it.