Do you have experience with low power step down 100uA for up to 10 year backup ? - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 13, 2020, 07:41:08 am
content: Hi folks,for the current project, I am looking for the best solution to backup memory and I need roughly 100uA at 1.1-1.2Vand considering to use some small step down converter instead of LDO to utilize lithium cell more reasonablyAlso as I wan about 10 year backup time, and 9-10Ah battery is a huge beastAll main manufacturers now have step down converters, offering over 80% efficiency at this load, this looks promising to decrease battery size to reasonable 3-4AhDo you have any experience with hem in long term and reliability ?

author: richard.cs
author_rank: Super Contributor
timestamp: October 13, 2020, 09:42:47 am
content: I have used ADP5304 and found it to be very good. In our design it idles at a measured 300 nA powering a microcontroller from a lithium primary cell.https://www.analog.com/en/products/adp5304.html

author: voltsandjolts
author_rank: Unknown
timestamp: October 13, 2020, 01:09:17 pm
content: For memory backup?How many KB/MB/GB?Why not use non-volatile memory?

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 13, 2020, 01:48:08 pm
content: Quote from: voltsandjolts on October 13, 2020, 01:09:17 pmIt holds a secure access keyAnd key must be discarded when intrusion is detectedAnd it must be able to just lay in box for years without power

author: ejeffrey
author_rank: Super Contributor
timestamp: October 13, 2020, 03:27:45 pm
content: I'm sure you have considered other options but 100 uA seems like a lot for maintaining SRAM. Is this special tamperproof parts that don't support low leakage states or something?

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 13, 2020, 03:45:13 pm
content: It is technically not sram alone, it hold whole encryption chip in standby with key in registerTo prevent any key transfer outside, cannot be read any way (paranoid way to prevent cloning)

author: Peabody
author_rank: Super Contributor
timestamp: October 13, 2020, 04:03:46 pm
content: Another option might be to power your circuit directly from a lithium/iron disulfide battery, which is nominally 1.5V, but probably closer to 1.7V at no load.https://data.energizer.com/pdfs/l91.pdf

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2020, 08:07:41 pm
content: Micropower switching regulators are available from the usual suspects. Internally they operate in burst mode by shutting down the switching clock to save power when the output voltage is in regulation, so quiescent currents in the microamp range are feasible. The only parts which are powered continuously are the reference and error amplifier.

author: jbb
author_rank: Super Contributor
timestamp: October 13, 2020, 10:16:53 pm
content: 100uA is a lot! I would expect more like 10uA for maintaining key registers and RTC.Do you also have some intrusion detection electronics running?

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 16, 2020, 06:25:38 pm
content: It is a worst case scenario consedered, it might go down to 30uA for key register itselfBut I want some headroom

author: tszaboo
author_rank: Super Contributor
timestamp: October 16, 2020, 10:42:49 pm
content: I've used DC-DC converters for extremely low currents. They surprisingly work, though the switching frequency gets very very low, and you might have a lot of ripple. I was measuring one as design verification, and with light load the switching frequency was 30 Hz. Yes, thats not a typo.At 10 year, you will run into issues with the battery itself. Passivation, self discharge that sort of things.

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 17, 2020, 07:47:53 am
content: I also think about Supercap with some coin cell in socket and changing it 2-3 yearsAs an alternative, but it can lead to user/maintenance issues

author: voltsandjolts
author_rank: Unknown
timestamp: October 17, 2020, 08:38:32 am
content: Quote from: Miyuki on October 13, 2020, 01:48:08 pmWhy do you believe you must use battery backed SRAM for secure key storage?If your tamper detection hardware is bypassed, your key is in plain text in SRAM?It is possible to store information securely without power and there are many products for this purpose.

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 17, 2020, 09:36:55 am
content: Quote from: voltsandjolts on October 17, 2020, 08:38:32 amIt is not about storing key secure.It is a key device to protect again physical theft and intrusion.Key must be safely destructed if is any suspicion. So must be not only secure agains key extraction/cloning, must also be able to discard it in any time.This cannot be reliably done without volatile solution.

author: voltsandjolts
author_rank: Unknown
timestamp: October 17, 2020, 10:38:13 am
content: Quote from: Miyuki on October 17, 2020, 09:36:55 amYes it can.

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 17, 2020, 11:05:19 am
content: Quote from: voltsandjolts on October 17, 2020, 10:38:13 amI am open to any idea, if you want to shareBut in my eyes waking up and trying to replace data in flash is a delicate operation requiring steady power supply for some time

author: voltsandjolts
author_rank: Unknown
timestamp: October 17, 2020, 12:01:23 pm
content: There are many solutions out there to protect again physical theft and intrusion, without a battery. Your car fob, for example, has a battery for the transmitter, not for storing a secure code. You know the details of system requirements, I don't know whether your system would be more suited to secure microcontroller, or secure EEPROM, or encrypted rolling code transponder, such as KeeLoq, or secure RFID access systems. I'm just suggesting you actually do some research on available solutions before you blindly assume BBSRAM is the only way to go.Also, you are assuming your 'tamper detection' is 100% reliable so that your BBSRAM key is guaranteed to be wiped before being accessed. That won't be true.

author: BrianHG
author_rank: Super Contributor
timestamp: October 17, 2020, 12:37:30 pm
content: Quote from: Miyuki on October 13, 2020, 07:41:08 amBesides looking at non-battery key security methods, I do not get that ridiculously high 100ua figure?If I were to statically secure an encrypted key within a Microchip PIC featuring XLP, the internal sleep current where battery can retain the ram is 20na. That's 5000x less power than what you are asking for. A tiny lithium coin cell which could fit in a watch could power and retain that PIC's core static ram for over 10 years easily. A medium sized one could even have it processing at 32KHz for 10 years waking up twice a second to monitor inputs. No regulator needed.

author: voltsandjolts
author_rank: Unknown
timestamp: October 17, 2020, 04:21:00 pm
content: If, by chance, you are looking to implement some keyfob-style security access system, then this thread might be of interest:https://www.eevblog.com/forum/microcontrollers/microchip-keeloq-classic-new-firmware-for-garage-door-receiver-new-fobs/

author: bson
author_rank: Unknown
timestamp: October 17, 2020, 07:01:24 pm
content: This is what EEPROM and small-sector flash areas are for... things like factory-installed public keys.