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6V to 12V DC-DC converter
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6V to 12V DC-DC converter

by vynce on Thu Sep 24, 2009 11:38 pm

I'm looking to build a 6V to 12V DC-DC converter for powering a 12V LED strip from 4 AA NiMH batteries. I haven't measured the current draw of the LED strip yet, but I expect it'll be somewhere around 100~200 mA.

Someone suggested the MAX761 for a similar task in this thread: http://forums.adafruit.com/viewtopic.php?p=37703#p37703. The MAX761 is end-of-life, so I'm looking for a suitable substitute. I'd prefer something available in a DIP package so that I can connect the whole thing up using the dead bug approach instead of having to etch a board for a circuit with very few parts.

There's the MAX1771, but it requires an external switch, which is kind of a hassle for my relatively low-power application. I came across the MAX618, but it only comes in a QSOP package. These seem to be the only options from Maxim, unless I'm missing something. Shouldn't they have a replacement for the MAX761? (Edit: Ahh, looking at the MAX761 page again, I see that the MAX618 is its replacement)

I see that Linear Technology also makes boost regulators. There's the LT1073 that used in this circuit.

Any comments/suggestions?

vynce
 
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Re: 6V to 12V DC-DC converter

by zener on Fri Sep 25, 2009 12:22 am

You need to show the strip you are using, or the current spec. You could be very low on your guess.

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Re: 6V to 12V DC-DC converter

by vynce on Fri Sep 25, 2009 12:54 am

You're right. The LED strip that I have is the 30 LED Blue Radiantz Sidewinderz. Unfortunately, they don't provide any current or power specs and I've loaned out my multimeter.

They provide some rough current specs for some of their other products in their PDF catalog. Their 100 LEDs/foot product draws less than 0.5 A/foot. Their 60 LEDs/foot product also draws less than 0.5 A/foot. Their 24 LEDs/foot product draws less than 0.2 A/foot. The strip that I bought has 30 LEDs, so I was just interpolating from their other products. I guess 100 mA and probably even 200 mA was shooting a bit low. It probably wouldn't be more than 500 mA though, I think.

It would be good to have enough overhead to run two strips from the same power source if that doesn't push it too hard.

vynce
 
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Re: 6V to 12V DC-DC converter

by zener on Fri Sep 25, 2009 1:13 am

Well, it's hard to say. Could be 150 mA? We are guessing. Does it have any resistor built in? I assume you know what you are doing (so you don't blow them up). Here is a chip that might be good. It can do more than you want but I think it can do what you want:

http://search.digikey.com/scripts/dksea ... 441TECT-ND

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Re: 6V to 12V DC-DC converter

by zener on Fri Sep 25, 2009 1:59 am

This looks promising:

http://www.national.com/ds/LM/LM2704.pdf

See fig 4 on page 9

They don't hold your hand much in the app notes but it looks a lot more straight-forward than the Semtech part.

zener
 
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Re: 6V to 12V DC-DC converter

by vynce on Fri Sep 25, 2009 9:49 pm

I got my multimeter back today. The LED strip draws 80 mA @ 10 V, 180 mA @ 12 V, and 285 mA @ 14 V. At 12 V it's too bright to look at for more than a few seconds. 14 V doesn't appear to be that much brighter than 12 V -- certainly not enough to be worth the additional 100 mA of current. I don't think the strip consists of anything more than LEDs and resistors in some kind of series/parallel configuration.

Thanks for the suggestions. It looks like both of those parts only come in a surface mount package though and I'd rather not etch a board for such a small circuit. I have some extra MintyBoost PCBs, so if you come across a pin-compatible part, that would be convenient.

vynce
 
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Re: 6V to 12V DC-DC converter

by zener on Sat Sep 26, 2009 2:13 am

You might be able to use this:

http://www.national.com/ds/LM/LM2585.pdf

They don't show an exact schematic, but basically you put a current sense resistor in the ground leg of the load and that feeds into the feedback pin. The same as the example shown for the other National part.

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Re: 6V to 12V DC-DC converter

by Thrymr on Sat Sep 26, 2009 10:51 am

If one of the surface mount chips really is your best option, you could find a prototype board that will let you use it without etching a custom board.

E.g., for the LM2704 (SOT-23 package) you could use:

http://search.digikey.com/scripts/DkSea ... 33206CA-ND
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