Need to invert voltage detector behavior 1

[loicalset]

Greetings,

I have a voltage detector IC (Seiko) that outputs an active low (N-ch) as a result of a low voltage state (<5VDC). I also have a voltage regulator (LM2931) that can be turned ON via active low. The problem is that we would like the active low (which represents a low-battery or power fault condition) to DISABLE the regulator. As it is, the active low during the low battery condition would turn ON the regulator.

Any clever ideas on how to wire this to make it work? I have been looking for some sort of inverted voltage detector IC (active N-ch low when voltage is greater than detection voltage) to no avail. Power consumption needs to be kept very low, as this is a battery disconnect circuit of sorts.

Any suggestions would be much appreciated!

 

[loicalset]

And please let me know if there is more information I can provide to help with solving the problem. Thanks.

 

[loicalset]

Possible solution... MAX810 active high reset monitor!

 

[itsmoked]

That would be my first suggestion. Just use one of the zillion battery monitors Maxim or Linear Tech or National has.

Baring that you can use a P-FET or an N-FET to reverse a logic signal. However the problems this can cause are pretty numerous as the failing power can cause 'buzzing' of your control, or galloping, etc.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[loicalset]

I have been searching through the Digi library for a while and have yet to find a reset IC that can take an input voltage over 7V (just noticed that the IC I poseted earlier has this limitation too). That is uncomfortably close to the max battery voltage.

I think I will use a small n-fet to invert the logic at the shutdown pin. Thanks for the suggestion. I am a bit new to electrical design - could you explain to me about the potential 'galloping' problem? I'm looking forward to absorbing some new information.

 

[itsmoked]

You just have to watch out for all the boundary conditions.

For instance a really common problem is;
The battery voltage drops.
The disable triggers.
The regulator is shut down.
The load evaporates.
The unloaded battery voltage rebounds.
The system turns ON.
The load returns.
The battery voltage drops....................

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

Why not use a device that does a bunch of this at once?

Check these parts. They go to 70V in some cases and I'm sure some will auto shutdown.

http://para.maxim-ic.com/cache/en/results/4671.html

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[loicalset]

Ah, yes I had considered the unstable behavior earlier. There is a bit of hysteresis built in to the comparator, and also the current draw is quite low compared to the resistance of the battery. But it is always good to keep these things in mind as you mentioned.

And I wish I knew about that part earlier - it looks very nice. Live and learn! The n-fet appears to be working well but hopefully I can simplify the design in the next round.

Thanks for the help.