Electronics > Projects, Designs, and Technical Stuff
Choosing an appropriate LDO for low power device
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schmitt trigger:
I've personally witnessed what Siwastaja posted, although at the time I did not understand what was going on.
magic:

--- Quote from: Siwastaja on June 19, 2019, 12:40:43 pm ---Look how much charge your cell has left at the cutoff of your choice, and divide it by the worst case "off current". You need to decide one parameter, which will be "time from turn-off to self-destruction", which, unfortunately, cannot be infinite. If you don't calculate it but do it based on hand-waving or just ignore the issue altogether, it may accidentally become just a week or two. Yet you'd like to have something around a year or so.

--- End quote ---
I would rather have 10-100 years to be honest. Are there no simple protection ICs you just put in series with the cell and connect a 3rd sense terminal to the opposite pole that would solve it while drawing nanoamps or less on their own? Basically, I would want a MOSFET with well defined threshold at 3V and very high transconductance, latching and hysteresis could be a bonus ;)
Siwastaja:

--- Quote from: magic on June 20, 2019, 07:42:46 am ---I would rather have 10-100 years to be honest. Are there no simple protection ICs you just put in series with the cell and connect a 3rd sense terminal to the opposite pole that would solve it while drawing nanoamps or less on their own?

--- End quote ---

Of course, but the market is full of protection devices (ICs or professional modules) that utterly fail to do that job. So the designer needs to know that such traps exist, and needs to understand the importance of the current draw after a low-voltage cutoff event.

In some cases, it is very appealing to do it without an extra switch and extra controller, for example if all loads in the system (often just a single microcontroller, really) can be turned to low-power or sleep modes reliably, with low enough off current. Extra switches and extra controllers bring extra problems to be solved (cost and PCB real estate being the most obvious ones, but things like inrush current in high-power devices can be very demanding traps for young players "just using a protection IC").
ThomasDK:

--- Quote from: Siwastaja on June 19, 2019, 12:40:43 pm ---
--- Quote from: ThomasDK on June 19, 2019, 10:52:23 am ---Anyway, since this the OP intends to use a LiPo, low Iq doesn't matter. The self discharge will probably be in the hundreds of μA,

--- End quote ---

A nice example of the classical false assumption which leads to all those nasty crap devices which self-destruct.

Okay, take a small li-ion pouch cell, say around 1Ah. The self-discharge of an empty cell is not "hundreds of uA", not even "tens" - if it was, the cell would self-destruct by self-discharging itself beyond low-voltage cutoff, finally causing low-voltage damage (copper dissolution).
--- End quote ---
Self discharge is not constant, it tapers off over time. The first day or so is the worst.

To take your 1Ah example (first hit on Google):

https://www.sparkfun.com/products/13813


--- Quote ---Excellent long-term self-discharge rates (<8% per month)
--- End quote ---

That's 80 mAh in a month = 80/(31×24)=108μA.

--- Quote from: Siwastaja on June 19, 2019, 12:40:43 pm ---
Now, if you indeed assume that the cell self-discharges at "hundreds of uA" anyway, and so it is OK to add external load in a similar range, you end up with this classical broken-by-design gadget:
--- End quote ---


--- Quote ---The self discharge will probably be in the hundreds of μA, so who cares if the regulator uses 1, 2 or 5μA? It's smaller than the tolerance of the battery...
--- End quote ---

5μA is not a similar range...


--- Quote from: Siwastaja on June 19, 2019, 12:40:43 pm ---Regulators / MCU sleep modes with Ioff/Isleep < 10uA, even <5uA are very much needed in the li-ion battery powered low power device market.

--- End quote ---

...but you seem to agree ;)
Siwastaja:

--- Quote from: ThomasDK on June 20, 2019, 04:05:56 pm ---
--- Quote ---Excellent long-term self-discharge rates (<8% per month)
--- End quote ---

--- End quote ---

Well yeah, a typical li-ion cell (I measured about ten different cells) self-discharges at approximately 0.1 to 0.5% per month. Which, of course, is less than 8%!

Excellent ones are those near to that 0.1% rate or below, the Sony VTC5 cell was the best in this regard in my tests and some others were close.

However, if any cell self-discharges at rates near 8% of month (at room temperature at least), they are completely dead crap. No one would accept that, it's more than an order of magnitude worse than industry average.

I'm quite positive even the cheapest no-brand cell sold by Sparkfun doesn't do that. This specification is just total bullshit. Or is it? A destroyed cell (caused by manufacturing failure, or abuse such as a serious overdischarge event) may leak more, of course. So it is possible that Sparkfun sells failed cells from a QC reject bin of a Chinese li-ion cell factory, and specify their leakage this high to avoid complaints. Such cells should be disposed of, however. They may be dangerous as well. In this context, "excellent" is an insider joke. Like: "excellent sports car, top speed 15 mph".

The exact self-discharge figure measured from the full cell doesn't matter, however, because the leakage rate goes to basically zero when the cell is near empty, which is the relevant SoC for this question. All cells I tested were equal with completely non-measurable leakage below about 50%, even the "worst" ones, even at elevated temperatures. And, the samples deliberately overdischarged to approx. -1% SoC did not leak below the original voltage measured at the start of the storage period, not a single one of them, during 1.5 years.

Only seriously damaged cells would continue leaking while near-empty.

So just don't assume that you can add external leakages because the "cells would leak anyway". They won't - your external load is what kills the cells. They would survive otherwise, unless they are bad to begin with.

Yes, I agree with you that most often it doesn't matter whether Ioff is 1, 2 or 5 µA (although 5µA is starting to get into the range of mattering in some cases). I wasn't commenting on that part.

In any case, let's assume a 1Ah cell, with 5% SoC (50mAh) margin at low-voltage cutoff event:

1 µA: time to self-destruction: 5.7 years
2 µA: 2.8 years
5 µA: 1.1 years
10 µA: 7 months
100 µA: 20 days

A smaller gadget, with a 200mAh cell, with the same 5% margin, would already die in less than 3 months given 5µA external leakage. So yes, sometimes it matters.
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