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Choosing an appropriate LDO for low power device
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NivagSwerdna:

--- Quote from: rookie on June 16, 2019, 12:57:08 am ---I am building a wireless device that is expected to work off of a lipo battery
--- End quote ---
Why LiPo?  Capacity? Duty cycle? Iq and Iactive?
ThomasDK:

--- Quote from: magic on June 18, 2019, 08:54:31 am ---
--- Quote from: ThomasDK on June 17, 2019, 07:48:13 pm ---The TPS783 is horrible for RF applications. Been there, done that, lesson learned... The PSRR is a joke, but like me, you forget to take a look at the transient response... Over 100mV voltage sag at 10mA output step!

--- End quote ---
Throw a capacitor at it, just make sure its leakage isn't more than Iq / Ignd of a higher bandwidth LDO ;)

--- End quote ---
Too expensive ;-)
We went with another regulator with a couple of μA Iq. Can't remember which.

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, so who cares if the regulator uses 1, 2 or 5μA? It's smaller than the tolerance of the battery...
Siwastaja:
One thing everybody's missed so far: your discharge low voltage cutoff. You'd want to be able to fully discharge your cell, otherwise you'll carry around dead energy storage. At least you need to know how much you actually get.

You see, you can discharge a li-ion cell way below 3.3V; especially if you have a considerable load current, or run at low temperatures, or run with an aged cell. Just 100mV of extra dropout from a regulator can mean extra 10% loss of usable capacity.

Let's look at an example curve of Samsung INR18650-29E from https://lygte-info.dk/review/batteries2012/Common18650comparator.php . Let's look at the 0.5A curve - your discharge current being only 0.15A, but OTOH I'm assuming your cell could be smaller (1Ah-ish), so a smaller current produces a proportionally larger drop, especially if you want to leave some margin for cold weather, or aged cell.


--- Quote ---MCP1810 - marketed as lowest quiescent current ldo in the market by microchip:
Vdropout = 380mV at 150mA

--- End quote ---
Low-voltage cutoff at 3.3V + 0.38V = 3.68V -> from the curve: you only get 44% of the nominal capacity out of it! No go.


--- Quote ---MCP1811 - another low Iq LDO from microchip
Vdropout = 400mV at 150mA

--- End quote ---
This will be even worse.


--- Quote ---TPS783 - These LDOs are from TI, one of their lowest Iq offerings.
Vdropout = 130mV at 150mA

--- End quote ---
Low-voltage cutoff at 3.3V + 0.13V = 3.43V -> from the curve: you only get 81% of the nominal capacity. Acceptable I guess?


--- Quote ---NCP170 - These are low Iq offerings in IOT space from ON Semi
Vdropout = 180mV at 150mA

--- End quote ---
3.48V -> around 75% available


--- Quote ---STLQ020 - Again, IOT oriented LDOs from STMicro. This one is actually 200mA Iout:
Vdropout = 160mV at 200mA

--- End quote ---
3.46V ->around 77% available.

So if you need 3.3V out, you can clearly rule out two regulators.

In some cases, you use the LDO just to drop the voltage beyond maximum ratings (such as 3.6V max), but are OK if it drops below 3.3V.

Most switching regulators are likely out of question if you mostly sleep. It doesn't matter if your linear regulator efficiency is "only" some 60-70%. The switching regulator is likely much worse at low loads, and the zero-load consumption is easily orders of magnitude more, even with pulse-skipping modes.

200uA Ignd is actually quite a lot of no-load current, most micros would happily sleep below <20uA. This poses a problem if you have a small cell, you run it almost flat, your micro does what it should and puts the device in sleep to protect the cell, and then the user forgets the product for a while. This can damage the cell by overdischarging it. Assuming 1Ah cell, discharge stopped at 20% (200mAh left), and shelved by the end-user, it would take 0.2Ah/200e-6A = 1000h = 41 days to self-destroy beyond repair. IMHO, such a product is unacceptable, I always design for a bare minimum of half a year of margin between "run-flat" and "put into charger" events, preferably more.

So, put together, I wouldn't accept anything else than Vdrop < 0.2V and Ignd < 50uA, only satisfied by TPS783.
Siwastaja:

--- 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). But this does not happen. I have tried to measure the self discharge of empty(ish) cells, and it just is impossible to measure given standard external means. During my 1.5 years of test time, none of the dozen samples lost any measurable charge below about 50% SoC, not even at elevated temperatures.

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:
1) The user buys your battery-powered product (can be a $1 toy, a $10000 special instrument, or a $100000 EV car, the principles are the same)
2) They charge it, turn it on, play with it
3) The battery goes empty, the user turns it off,
4) The user has something else to do
5) The user comes back to the product after a month or two - with worst offenders, just after a week - it's a brick.
6) If(expensive && userbase_has_clue) warranty_repair_nightmare_ensues(); else user_throws_it_into_bin();

This is surprisingly commonplace in both cheap Chinese throw-away toys, and expensive "professional" products. This is actually a very common problem even in high-end battery management systems, I have seen many which have destroyed good packs instead of protecting them, which was their only job!

Yet it's trivially easy to calculate how to avoid this problem.

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.

Regulators / MCU sleep modes with Ioff/Isleep < 10uA, even <5uA are very much needed in the li-ion battery powered low power device market.
tszaboo:
You have to define the:
- Battery chemistry
- Expected battery life
- Battery mAh rating
- ratio of sleep time / on time
- sleep current of your device
- ambient temperature range
- typical application
- budget

Than we might be able to help. Otherwise we could suggest something, but selecting the optimal converter is not an easy task.
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