Regulator?

[antknee]

I have a chip that has a maximum voltage rating of 16V and I want to give it 2 PP3 batteries supplying 18V.

What is the best way of keeping the voltage at 16V?

I know I can use a pot but then when the the voltage from the batteries actually drops to 16V with the pot I'll only be getting say 14V.

Thanks.

 

[bountyhunter]

What load current? If under 1A, you could use a low dropout regulator like the LM2941.

 

[antknee]

It'll be 1.2A load current. Perhaps a bit more. I'll have a look at the LM2941, it'll highlight the principle. Thanks.

 

[Dx3]

I don't think so. PP3 batteries have too much internal resistance to sustain 1.2 amps at full voltage. However, four rectifier diodes will reduce the voltage to a nice, safe place, even if you are using a reasonable current flow.

 

[antknee]

How would I use the 4 rectifier diodes? I can sink 1A for 5 minutes from a PP3 and 0.6A for 10 minutes and I only need them for 5 minutes. Thanks.

 

[Torben]

Won't your PP3s burn up, trying to provide 1.2A? That's a lot of juice from PP3 9Vs, which are pretty weak to start with.

How long do you expect to run this thing from the batteries?


Torben

 

[antknee]

We replied at the same time. Its just 5 minutes I need from them. Thanks.

 

[Torben]

We replied at the same time. Its just 5 minutes I need from them. Thanks.

Saw that. Too tired right now but I'll come back to this.


Cheers,

Torben

 

[bountyhunter]

I know both Linear Technology and maxim make higher voltage rated (like 25V) low dropout linear regulators that can do 3A. The part numbers LT1085 and LT1086 pop into my head but my brain is old, so you should go to their sites and use their selector tools and see what they currently offer.

 

[antknee]

The part numbers were right. The LT1086 seems pretty good, I can't say I really understand the datasheet or exactly how it is going to work, but I know it will work so I am going to buy one and test it. Regards.

 

[antknee]

I have read the datasheet and the typical application below looks like it would give me the option of using different batteries. For example 4 AAA batteries rather than 2 PP3's. Perhaps I could get it down to 2 AAA's, I'm not sure.

 

[Nigel Goodwin]

Just try it directly off the two PP3's, they won't be above 16V for very long at all anyway - it will most probably be perfectly fine.

 

[antknee]

A few minutes a couple of volts over maximum probably wouldn't do any harm, but I'm going to get the LT1086 because it will protect my chip and give me power supply options.

I'll be using the regulator with a chip that has a current limiting pin so I should be able to get a very adjustable and reproduceable power output from a range of different batteries.

Thanks for the input.

 

[Dx3]

It's a bit late, but just to clarify, putting 3 amp diodes in series with the batteries would drop the delivered voltage by .65-.85 volts each. It looks like 3 will do the job.