Post by thread:[EE] Very Low voltage Power Backup circuit

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 __________________________________ Do you Yahoo!? The all-new My Yahoo! - What will yours do? http://my.yahoo.com

> 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

>(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

--- Russell McMahon <spam_OUTapptechTakeThisOuTparadise.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}> 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 __________________________________ Do you Yahoo!? Yahoo! Mail - now with 250MB free storage. Learn more. http://info.mail.yahoo.com/mail_250

{Quote hidden}> >(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). 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.

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}>> 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 -- Note: To protect our network, attachments must be sent to .....attachKILLspam@spam@engineer.cotse.net . 1-866-263-5745 USA/Canada http://beam.to/azengineer

> 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

Russell McMahon wrote: {Quote hidden}>> 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 > > Its rechargeable, Russell. -- Note: To protect our network, attachments must be sent to attachKILLspamengineer.cotse.net . 1-866-263-5745 USA/Canada http://beam.to/azengineer

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

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: {Quote hidden}> 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 -- Note: To protect our network, attachments must be sent to .....attachKILLspam.....engineer.cotse.net . 1-866-263-5745 USA/Canada http://beam.to/azengineer

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 -- Note: To protect our network, attachments must be sent to EraseMEattachspam_OUTTakeThisOuTengineer.cotse.net . 1-866-263-5745 USA/Canada http://beam.to/azengineer

--- Russell McMahon <apptechspam_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 __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com