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A project log for Universal LiPo/Li-ion UVLO (undervoltage lockout)

Feature-packed over-discharge protection. Set the cell-count (2S to 7S), minimum cell voltage and dip-time below threshold. Fully hackable!

JanJan 06/21/2019 at 09:037 Comments

So, after having so much trouble with the failing regulator I finally found the culprit: the regulator needs at least 4.7uF of ceramic capacitance at its INPUT to keep it from blowing up. 

This does generate another problem which is arcing when connecting a battery, caused by the near short circuit behavior of an discharged cap... 

So to keep things short I found a few other regulators that I want to try:

I'll decide for one and come back to you with a fresh design. 

Discussions

Xasin wrote 06/22/2019 at 07:24 point

God, I know that problem. For my DSKorder, the battery charge controller I'm using just randomly decided to blow up. It's the MCP7 something. 

It didn't just do this one time, it happened to my lasertag too, and I'm pretty sure it's either that the chip is just plain bad (as the USB to UART converters, powered by the same USB lines, are fine), or because I'm lacking a very similar low ESR input smoothing capacitor like in your design. 

My personal choice is to go for a chip that, frankly, blows up less, but adding more caps might also help? 

In any case, good luck finding a suitable replacement! 

Small side note:

I think digikey has tiny DC DC converter modules for a fairly cheap price. They could be useful to reduce waste heat of the LDO for higher battery voltages, but since we aren't talking about currents above a few mA, I'm sure that's not a concern ^^'

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Jan wrote 06/23/2019 at 12:49 point

Thanks @Xasin !

Yeah, stuff like that really slows down projects. You're totally right just choosing a new regulator to get rid of the problems. That's why I posted the printed datasheet pages above :) 

Currents will be under 10mA to keep it working even at 8S input voltages which are around 34V...

Will so some parts ordering soon! 

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Xasin wrote 06/23/2019 at 13:14 point

Still sounds like it'll have to drop a good bit of power - but I'm sure you've already selected a good regulator for that. 

As for me, I think I'll just get a LiPo USB charge board and slap it in instead.

Keep us posted!

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Jan wrote 06/23/2019 at 14:05 point

I looked into DC DC buck converters as well. Those I looked at had awful efficiency at those super low currents. I need to test it maybe to see how effective this would be. Some special linear regulators have only like 100mV drop in the region I want to use them in, it might work. Need to do some thermal calculations anyway, especially with the SOT23 cases which have no pad underneath...

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Xasin wrote 06/25/2019 at 13:04 point

Oh damn, thanks for linking those parts! I didn't even know you could get these kinds of fairly niche regulators.

The external component count also doesn't seem too bad, and most should be doable with a few SMD chips here and there - but good luck either way!

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Xasin wrote 06/23/2019 at 14:53 point

Look at some of the TRACO converters - they might have good efficiencies even at lower currents! Then again, their standby current should be considered...

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Jan wrote 06/25/2019 at 11:06 point

As you said, their standby current is high (like 2mA).
I want to have it draw less than say 5...6µA in OFF mode to keep battery drain low. Found some very promising chips:
- http://www.ti.com/lit/ds/symlink/tps54062.pdf

- http://www.ti.com/lit/ds/symlink/tps54061.pdf

- https://datasheet.lcsc.com/szlcsc/ROHM-Semicon-BD9G101G-TR_C108761.pdf

- https://datasheet.lcsc.com/szlcsc/Microchip-Tech-MCP16331T-E-CH_C105695.pdf

They all have around 50 to 60% efficiency converting 34V (8S LiPo) to 5V/10mA.

Off-currents (EN = low) are somewhere between 0.1 and 10µA. Perfect for my application.

The board will be more expensive using those as they need additional parts...

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