Minimum self discharge battery powered device - Page 1
Forum section: electronics / beginners

author: Veketti
author_rank: Regular Contributor
timestamp: February 28, 2023, 05:57:55 pm
content: I’ve been trying to find the holy grail of battery powered device regulator solution that consumes close to zero energy when MCU is on sleep. Closest my goal I’ve reached with solution where there is no voltage regulator at all, wide voltage PIC MCU & OLED display. Shut down all pull-ups etc. with transistor before sleep. However sometimes you need the voltage regulation and those are the problem. Some LDO regulators have EN pin, but you cannot control that if it is powering the MCU as well. Assuming there are components which need 5V. I know this is tricky to professionals as well as most of the tools I have, do drain batteries pretty quickly and then they might leak etc. First comes to my mind Stahlwille 4AA battery powered torque wrench which empties batteries in two months. Who removes the batteries after use anyway? Of course you could always have toggle switch which cuts the supply, but nobody uses such thing nowadays. Everything needs to wake up by momentary push button, which has other functions as well.So the question is what sort of approach is the best (other than the toggle switch)?Thank you in advance.

author: fourfathom
author_rank: Super Contributor
timestamp: February 28, 2023, 06:25:46 pm
content: Quote from: Veketti on February 28, 2023, 05:57:55 pmI thought you were going to use a PIC which can run directly from the battery? I am looking at specs for a MIC5225 low-Q current regulator (I've never used it, but the spec popped up), and the enable pin can have a pull-down to keep it shut off when the uC is sleeping. When the uC wakes up it can drive the enable to anything greater than 2V to turn it on.. The disabled current is typ 0.1 uA.The LP5907 regulator has similar specs (but a slightly higher "off" current.)So why won't this work?

author: CountChocula
author_rank: Unknown
timestamp: February 28, 2023, 06:32:03 pm
content: Under what conditions does the PIC come out of sleep? If there is some kind of external stimulus, you could use it to turn on the LDO, and then have the MCU keep it on until it needs to shut itself off, without using sleep at all.

author: Manul
author_rank: Super Contributor
timestamp: February 28, 2023, 07:42:53 pm
content: From what I know, TPS7A02 and TPS7A03 are as good as it gets regarding low quiescent LDOs. Usually there is no need to disable them, they use so little.

author: Veketti
author_rank: Regular Contributor
timestamp: February 28, 2023, 08:57:15 pm
content: fourfathom: In case I need the 5V I need to run it with four 1.5V batteries so with full batteries around 6.6V which the PIC can't handle. In case I don't need 5V I can run with three 1.5V batteries and that is no problem.CounChocula: Excellent idea, I haven't considered this if the button would wake up the LDO and MCU would latch the EN pin until it needs to shut down. However it would be beneficial if that button could be used also as an input for the MCU. Need to think about that how it would be possible. If it were DP button it would be no brainer..Manul: Those LDO's really have truly low quiescent current. However absolute max for TPS7A03 is 6.5V which with four alkaline 1.5 cells might be bit unhappy? There seem to be also TPS7A24 (input max 18V) which has quiescent current around 3uA with 6V. Is that current when enabled or at shutdown?

author: mariush
author_rank: Super Contributor
timestamp: February 28, 2023, 09:17:02 pm
content: How much current is the circuit going to use?May get more efficiency boosting 3v...4.2v from a single lithium cell to 5v, using a synchronous rectifier.Can fit quite a lot of mAh in the volume of 4 AA batteries.For example MAX17220, boost 0.9v to 4.5v up to 5v, up to 95% efficiency, up to 225mA :https://www.digikey.com/en/products/detail/analog-devices-inc-maxim-integrated/MAX17220ELT-T/7667862300nA quiescent current, output completely disconnected when enable is off ... unless I'm reading it wrong, have to get enable high to turn on regulator, so your momentary button could pull up the enable just long enough for the microcontroller to turn on and keep the enable high when user released the button.If that's too expensive, there's also MCP16251 (with true input-output disconnect) or MCP16252 (input-output bypass)https://www.digikey.com/en/products/detail/microchip-technology/MCP16251T-I-MNY/4079879https://www.digikey.com/en/products/detail/microchip-technology/MCP16252T-I-MNY/4079880

author: Veketti
author_rank: Regular Contributor
timestamp: March 01, 2023, 07:08:47 pm
content: That MAX17220 would be interesting but it doesn't have any package which is hand solderable... That MCP16252 is very interesting. If I understood correctly I could in bypass mode have only the wide operating voltage PIC to work and then the PIC could pull the EN high when wake up from sleep and other devices could operate as well. I'll order bunch of these. Thanks everybody. There are so many IC's that it is impossible to just search something you didn't know exists.

author: MrAl
author_rank: Super Contributor
timestamp: March 01, 2023, 07:49:45 pm
content: Quote from: Veketti on February 28, 2023, 05:57:55 pmHi,One thing you can try is to use a transistor and capacitor such that when the cap charges up it turns on the transistor and the transistor turns on the regulator and thus the microcontroller. When the microcontroller gets started up it should be in reset mode, and follow the program as you have coded.The code can then use an i/o pin to turn on an SCR which discharges the capacitor which turns off the transistor which turns off the uC chip. After the cap discharges, the SCR will have low hold current, too low to keep it on and so it will turn off and start the process all over again. You'll need to check that you have brownout mode in the uC chip set or whatever you need during power up and power down for the uC chip.There are some details you would have to work out such as what value resistor to use for the cap charging. You dont want it too big. You cant go too small in value either or it will keep the SCR on forever. Of course testing over temperature range is called for also.I have the same issue with mine but i have a much bigger battery so it can take a 1ma drain without too much trouble, but i may want try this idea also to lower the current even more. I went to a part LP2905 (i think that's the right number) and it has low enough quiescent current for my app, but yours may need even lower current.I did a refrigerator monitor using a uC some time ago and i got 2 years out of the AA batteries used to run it. It went to sleep and only woke once in a while.

author: mariush
author_rank: Super Contributor
timestamp: March 02, 2023, 08:45:18 pm
content: Quote from: Veketti on March 01, 2023, 07:08:47 pmMaybe I have a hard time understanding you... but I'm not sure that's what bypass mode does.I think the idea is that in bypass mode, the chip just passes the input voltage to the output and consumes very little current doing that (600uA or thereabouts)For example, you may have a microcontroller that can run on 1.8v or 3.3v at a lower frequency like 1-4 Mhz / stand-by / sleep but which may need 3.3v or 5v at higher frequencies.So you could enable the regulator to boost the lower voltage to your desired voltage, wait a few ms (or however much it takes, see datasheet) for output to ramp up and then wake up the microcontroller and push the clocks higher to do the job fast and when done, disable the regulator again.So you could have a 3v...4.2v lithium battery and give whatever voltage when running at low frequency / standby or boost to 5v as needed.Or maybe have 2 AA batteries ( 2v .. 3v range, depending if you use alkaline or rechargeable) and want to boost to 5v only when needed and your micro can work with as low as 2v otherwise (lots of PICs for example work with even 1.8v at low clocks)