18V to 18V 1A DC to DC CONVENTER?

I making a project of Flashlight which
- NEED 18V 1A Power.
- Batteries source is 18V-20.4V UNSTABLE (Current is no issues here, the batteries can be discharges up to 40A simultaneously)

THE PROBLEM
I SEARCHED for MONTHS and cannot find the circuit for this.

Please help me with the circuits
Thanks.

Answer:
You are correct that you will need a DC to DC converter. The typical "simple" regulator circuit has to have at least a bit more input voltage than output in order to regulate, and you've specified that it could be 18 in and 18 out.

You REALLY want to get hooked up with the WEBENCH at National Semiconductor. They've got a facility there that will let you plug in your 18~20.4V input for an 18V output and design the whole DC to DC for you while you watch. I'm looking at YOUR schematic as I type. I wish to heck we could do graphics here, I'd just give you the schematic. They show all of the associated values (and even manufacturers part numbers !) for caps, inductors, etc. to build the supply.

This thing will even let you run simulations of transient changes in input, load, etc. and see the results!

Here's where you want to go:

www.national.com

Click on the Webench Tools at the top along the yellow line.
Give them the info they want -- it's a free registry, and they don't spam you for giving them info.

On the Webench section, select "Power".

Have at it. You're gonna love this. It is SO easy!


NOTE:

I'd use the next to last (SEPIC) recommendation using the LM3488. It uses a few more caps and resistors, but uses a *standard* transformer from Coiltronics so you won't have to be screwing around winding your own. It'll provide 85% efficiency at 5A.
All you need concernig matters like these is this book: http://www.amazon.com/principles-electro...
While a DC-DC converter is doubtlessly the most "elegant" solution, I am thinking of a simpler and cheaper one.
You must have a tolerance which your flashlight can accept, rather than 18 Volt dead-on...?!
Say if you can live with 17.1 ...18.9 Volt then I would put a 1 Ohm 5W resistor in-line with your battery supply, and a 18V 5W Zener Diode across.

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