--- Russell McMahon <apptech@paradise.net.nz> wrote:
> Did you mean 90 mA ???
> That's only about 1000/90 x (Vmax-Vmin)
> If micro will run on 1.8v max and say 1.5v min that's about 3.3 
> seconds at 90 mA.
> If that's 9mA then that's about 33 seconds.
> 
It was definately 90ma that's max typical is more like 32ma with all
the periferals on in the uC (it's not a PIC).  Given operating range is
1.95 to 1.65V which is the same range as you guesstimated. IO is at
3.3V so it has Vio Vref VPLL Vin and VCore  Vout is the built in 1.8V
regulator.  I guess I could just use a 3.3V supple instead of getting
fancy.

> Even ay 50 uA standby, 1F will give you 1E6/50 = about 20,000 seconds
> 
Hmmm interesting point well acording to the data sheet is' 36.2 uA but
50uA is probably a better estimate considering leakage etc.  This
doesn't do well does it?

> per volt of drop in the cap or under 2 hours with 0.3v of droop.
> 
> If using a supercap you would probably be better off using it with a 
> low dropout, low quiescent current regulator to allow it to droop 
> more.
> 
> Two AAA alkaline plus a regulator would allow 1000 mAH or 20,000
> hours or about 2.5 years at 50 uA. Or half that if the regulator 
> added 50uA to the load.
> 
> Even the very smallest of 3v Lithium coin cells would far exceed the 
> capacity of a supercap. (A 1F supercap with 1 volt drop gives about 
> 0.3 mAH).
> 
I'm dealing with battery phobic people they do not like the idea of
replacing anything ever. However I don't believe I need more than an
hours worth of backup time.  There is no memory to preserve  I merely
wanted the uC to be in stand bye mode for a while in case power resumed
from a shutdown or other unforeseen condition. I would rather not
include yet another part into the equation so the internal RTC system
was it. More to contemplate I suppose.

Stephen




		
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