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'[EE] Very Low voltage Power Backup circuit'
2005\02\07@122246 by Stephen R Phillips

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In this era of 1.8V microcontrollers, I've come across a dilema.
I need something LIKE a memory backup circuit, it's not for memory but
a microcontroller instead. I need some way to indcate a power failure
(not too hard there) so that I can put it in low power operating mode
(50uA power consumption). However I need a circuit that can switch over
from the 'main' 1.8V supply to a backup supply (I was considering one
of those 2.5V super caps as a backup source 1F should last a while).
Current draw is 90ma max at the 1.8V core voltage.

My problem is 1.8V is a difficult and rather low voltage to switch, and
then there is the voltage sensing and switching system as well.  The
whole thing seems rather difficult to improbable to do. Suggestions,
places to bang ones head?

Stephen


               
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2005\02\07@152613 by Jinx

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> However I need a circuit that can switch over from the 'main'
> 1.8V supply to a backup supply (I was considering one of
> those 2.5V super caps as a backup source.
>
> My problem is 1.8V is a difficult and rather low voltage to switch

You might not need to switch it at all. The supercap is across
the PIC's supply lines all the time, and you monitor its voltage
compared to a reference. If the 1.8V fails, you'll see a drop in
Vdd as the 90mA starts to discharge the supercap, so you
change to low-power mode, albeit with a little lost reserve
capacity. This assumes that the supercap can't discharge into
the dead 1.8V line, eg it has in-line semi(s) that turn off and
isolate it. If it doesn't, possibly you could add a series P-ch
FET which the 1.8V both keeps on and passes through

2005\02\07@163538 by Russell McMahon

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>(I was considering one
> of those 2.5V super caps as a backup source 1F should last a while).
> Current draw is 90ma max at the 1.8V core voltage.

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.

Even ay 50 uA standby, 1F will give you 1E6/50 = about 20,000 seconds
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 50 uA
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).



       RM

2005\02\07@170505 by Stephen R Phillips

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--- Russell McMahon <spam_OUTapptechTakeThisOuTspamparadise.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?

{Quote hidden}

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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2005\02\07@174805 by Mike Hord

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{Quote hidden}

What Russell said, plus this:

I considered using a "supercap" for awhile, but in my case, when the
power loss occured (say, the battery was pulled) the desired behavior
was to save some data in flash, then switch off until further notice.  I
realized quite quickly that in order to meet the current/timing demands
of my flash chip, the size and cost of adding the capacitance to the
project would outweigh the benefit.  The result?  A standard "removing
the battery may cause data loss" warning, plus measures to ensure
that only a minimal amount of data could be lost.

I think supercaps are mainly intended to keep SRAM from losing its
mind during off cycles; for example, a sleeping nanowatt PIC draws
<1 uA.  As long as it doesn't wake up and start trying to do something,
a supercap will help it retain its SRAM contents for a long time.

YMMV, and your application may require different considerations.

Mike H.

2005\02\07@180414 by Russell McMahon

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>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.

In such a case a Farad or few cap across the power line and a fast
power failure detection system could well do what you want.



       RM

2005\02\07@181820 by Bob Axtell

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Which brings me to a battery Q:

I am using a PIC at 32K and a 3V Rechargeable Cell (VL2020).
If I run the PIC at 8.5uA, I wonder how long I'd get with a topped-
off VL2020? I don't have the VL2020 specs handy... Anybody used it like
that before?

--Bob

Russell McMahon wrote:

{Quote hidden}

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2005\02\07@190031 by Russell McMahon

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> I am using a PIC at 32K and a 3V Rechargeable Cell (VL2020).
> If I run the PIC at 8.5uA, I wonder how long I'd get with a topped-
> off VL2020? I don't have the VL2020 specs handy... Anybody used it
> like
> that before?

Simplistically:

20 mAH so ABOUT 20,000/10 =~~~~ 2000 hours.
About 3 months.

Using the graphs at
http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_Lithium_VL1220_VL2020.pdf

About 130 days = about 4 months

That's a very costly battery!




       RM


2005\02\07@192904 by Bob Axtell

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Russell McMahon wrote:

{Quote hidden}

Its rechargeable, Russell.

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2005\02\07@195515 by Russell McMahon

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Here is an excellent application note on the care and feeding of
Vanadium Pentoxide Lithium rechargeable cells.
They have a number of excellent features - and some very negative ones
as well. Used properly they appear superb. Applied incorrectly they
can become expensive junk very quickly.

Discharge current for the VL2020 cell that Bob mentioned should not be
above 0.2 mA while the cell is above 3v. Charging voltage should be
strictly controlled at 3.4 V. Higher, it dies. Lower, it doesn't fully
charge. And they are costly.

Against these finnicky specs there is the superb 2% per year self
discharge rate. This implies a 50 year shelf life - unlikely to be
true BUT should be in excess of the 10 years or so of many other
Lithiums.

   http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_Lithium_VL.pdf

The VL3032 rated at 100 mAh provides about 11 uA-years of capacity.
You'd have to be drawing very little current to make use of any
lifetime over 10 years. At those levels some sort of solar or thermal
cell may be a better alternative.


       RM

2005\02\07@200141 by Russell McMahon

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>>> I am using a PIC at 32K and a 3V Rechargeable Cell (VL2020).


> Its rechargeable, Russell.

I know - it's still very dear. But it has lovely self discharge
characteristics. If I needed backup over vast periods of time I'd
possibly use one too. Over more normal periods I'd desperately search
for a cheaper solution :-)


       RM

2005\02\07@201127 by Bob Axtell

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I parallel the battery with a low-leakage tantalum. The tantalum
delivers the energy when I turn on the FRAM and write something. Then I
turn it off. I write something once every 30 minutes.

Thanks for help, RM. I wish you coulda helped the Phillies, tho...

--Bob

Russell McMahon wrote:

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2005\02\07@201323 by Bob Axtell

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Russell McMahon wrote:

>>>> I am using a PIC at 32K and a 3V Rechargeable Cell (VL2020).
>>>
>
>
>> Its rechargeable, Russell.
>
>
> I know - it's still very dear. But it has lovely self discharge
> characteristics. If I needed backup over vast periods of time I'd
> possibly use one too. Over more normal periods I'd desperately search
> for a cheaper solution :-)
>
>
>        RM

They are used in law enforcement motorcycle applications as power for a
timekeeper chip.

But I have another application in mind.

BTW: at 32K, which PIC uses the LOWEST power?

--Bob

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2005\02\07@224831 by Stephen R Phillips

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--- Russell McMahon <apptechspamspam_OUTparadise.net.nz> wrote:

> I know - it's still very dear. But it has lovely self discharge
> characteristics. If I needed backup over vast periods of time I'd
> possibly use one too. Over more normal periods I'd desperately search
>
> for a cheaper solution :-)

Indubidably!

I just had to concur with that.  Cost is the trouble with many
applications these days. Sure there is an ultimate backup solution but
can one afford it (sigh).


Stephen

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