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Battery backup current draw and resistor


#1

Recall, do I that the RTC/SRAM backup battery should have a resistor in series. Two questions on this that apply to both the Domino and the Panda II. I ask because I need to squeeze as much life as possible out of the backup batteries.

First what size resistor? I’m using a 15K and measuring a 70 microamp current draw on the Domino, but a 130 microamp current draw on the Panda II.

Second question is are these typical current draws for battery backup on a FEZ? I think the LPC2388 and LPC2387 data sheets imply that the RTC needs about 20 microamps. I don’t see a spec for the SRAM.

As always, thanks for any insights you can provide.


#2

In answer to my own question I contacted NXP since it didn’t look like the TINYCLR Forum was generating a lot of results. If anyone has a similar problem or need to know the is an errata on the LPC 2388 regarding current draw on VBat. You can access it at:
http://ics.nxp.com/support/documents/microcontrollers/pdf/errata.lpc2388.pdf

There is also a work around. I can answer questions about how I solved it if needed.


#3

Looks like we missed it. Can you please remove trailing dot in the url. Thanks.


#4

Dot is removed - sorry about that. Also forgot to mention that NXP verified that the typical current draw is around 20 microamps.