Low voltage warning followed by auto cut off circuit

Hi,

This is my first post in here so I hope not to annoy you too much with my small request for some help.

I am looking to design a small circuit to sit between a Li-Po battery (14.8V) and the rest of the modules (Video Processor, TFT Panel, etc) which require regulated 12V@2A.
It also needs to have a low voltage output that can fire an LED/Buzzer before the over discharge circuitry within the Li-Po battery cuts everything off.
On my journey through Google I came across a MAX932 which could be used as a window comparator, then maybe use it to fire up Linear 12V reg (obviously rated around 3A)?? Just wanted to run it by you guys in here to see if this might be a decent way of doing it or whether or not there are pitfalls I should be aware of by going down this route.
Alternatively if there are other better options I would dearly love to hear them (cheapest of course).

Thanks in advance.

There is a really simple chip that can be configured for your purposes, the ICL7665. It's made by maxim and is extremely versatile, I am currently using one of these in one of my designs. (you can look in my albums if your curious). It has 2 outputs so it can warn and cut off separately.

Many thanks for that..it was much appreciated

You'll need to use a low dropout regulator if you need regulated 12V supply, but I suspect the electronics you need to power will already have built-in regulation and will probably be find up to 15V.

What's the maximum voltage from the batteries? Probably 16.4V, if so you'll need to regulate it down.

A simple comparator and a voltage reference can be used as a low voltage alarm and shut down - no need for any proprietary ICs. I'll post a basic schematic if you like.

Thanks for that Hero, I would like to stay away from Maxim if possible due to the high cost of the IC and use a generic but fairly robust IC.

The max voltage can hit 16.8V when fully charged, but some of the modules require fairly close to 12V as possible, so I was going to use a low drop out Switching Regulator because I just realised that the turn on current for some of the devices can peak at around 6A.

The final requirement is this equipment has to pass fairly high EMC testing in a chamber (120V/m @ 400MHz) and wide storage/operating temperatures (-20 to +60 deg C)...

I posted my favorite low-voltage alarm circuit here.

You could use the LM311, that's probably what I'd do because it can drive up to 50mA which is quite a loud piezo siren.

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Hi thanks for that guys, but I think I failed to mention that I need a second output to turn everything off before the Li-Po auto cut off circuitry turns everything off.
The reason is to allow the hardware enough juice in the batteries to shutdown the video recording system and allow a disc to be ejected with the remaining juice in the batteries without any other operation of the equipment, a bit like a small backup battery which can do the last rites even though the equipment is officially dead.

I hope this clears everything up, apologies if it looks like I mislead you, I have just read my first post and I realised my mistake now.

Just make two LM431 cutoff circuits like I posted, where one goes off before the other. Actually, it seems you should use only one, with a fixed time delay to the second function.

I don't understand why you think that the ICL7665 is too expensive. I got mine for free.

mrballistic said:

Hi thanks for that guys, but I think I failed to mention that I need a second output to turn everything off before the Li-Po auto cut off circuitry turns everything off.

Click to expand...

Then you need two comparators, the LM393 will do.

It won't be able to drive a huge piezo, only up to around 10mA, if you need a more powerful siren, you'll need a booster transistor.