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Old 12-30-2010, 09:26 PM #1
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Default Aware of any existing plan for LM317 + Boost circuit?

Is anyone aware of a pre-existing schematic/plan/diagram for a circuit that both boosts the voltage of a single lithium ion cell, and incorporates an LM317 for current regulation?

I'm not terribly familiar with voltage boosting circuits, but I did some searching on Digikey, and came up with the LD29300XX80

http://www.st.com/stonline/products/...d29300xx18.pdf
- Output current up to 3A
- Output voltage fixed @ 8V
- Available in TO-220 package (IE, easy to work with, not SMD)

It seems to me that this IC could be integrated between a single 18650 and an LM317, for a workable single cell driver.

But are there already tested/known driver designs that use an LM317 with some sort of boost design to run off a single cell?


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Old 12-30-2010, 09:33 PM #2
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Default Re: Aware of any existing plan for LM317 + Boost circuit?

Its possible, just far from efficient. It would be a better idea
to just use a boost that is also a current reg..
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Old 12-30-2010, 09:33 PM #3
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Default Re: Aware of any existing plan for LM317 + Boost circuit?

It just doesn't work that way. The LM317 is a linear regulator, and not meant for boost operation. That chip you cited is a linear regulator as well, so it'll need a voltage greater than its output to function. If you were to make a boost-buck circuit in the first place you could make it current adjustable and avoid the need for the LM317.

Why aren't boost-buck circuits easily buildable? They incorporate high frequency switching and an inductor in order to produce a voltage higher than the input voltage. That doesn't work so well with just wires and solder on a breadboard; you need well designed PCBs to let the function correctly. Also, the chips that are used are usually surface mount anyway.

If you need a boost-buck, you should buy a dedicated driver from Dr. Lava or something.
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Old 12-30-2010, 10:31 PM #4
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Default Re: Aware of any existing plan for LM317 + Boost circuit?

Quote:
Originally Posted by Bionic-Badger View Post
If you need a boost-buck, you should buy a dedicated driver from Dr. Lava or something.
Yep, I get that - I've read enough of the "Why an expensive dedicated driver vs XYZ?" debates to understand the value of a Microboost, Microflex, etc.

This pursuit is for the joy of experimentation and invention

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Originally Posted by Bionic-Badger View Post
That chip you cited is a linear regulator as well, so it'll need a voltage greater than its output to function.
Alright, so I understand the error in my selection of IC above. And I understand that boost-bucks are all SMD, and that's not something I want to toy with.

But what about locating a simple IC (or circuit design) that does nothing more than boost voltage? There MUST be a voltage boost design that doesn't require SMD components, because hobbiests often have to boost voltages for various purposes. In fact I actually remember creating a circuit that boosted the voltage from a hand crank LED flashlight into the 5V necessary to charge via USB... I just can't remember how I did it!

What would I be searching for if I wanted to locate an IC that was simply a boost? (that I could use in conjunction with an LM317) It wouldn't be the most efficient design, and it wouldn't be tiny like a Dr Lava driver, but it would be homemade
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Old 12-30-2010, 11:17 PM #5
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Default Re: Aware of any existing plan for LM317 + Boost circuit?

MintyBoost

Here's a somewhat simple boost driver.
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Old 12-30-2010, 11:54 PM #6
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Default Re: Aware of any existing plan for LM317 + Boost circuit?

That was one of the circuits I found initially. My concern was that the IC they use doesn't provide enough of a boost to then power an LM317-based driver - it only boosts to 5V. The current may also be insufficient depending on what you're ultimately hoping to power.

That's when I started digging a bit deeper, and found the LT1307. It comes in an N8 package (LT1307CN8) which is fairly easy to work with (same size/shape as a simply 555 timer chip)

And on page 17 of the datasheet, they actually have the plans for a single cell constant current LED driver. Wouldn't such a circuit provide both the voltage boost and current regulation functions of a good laser driver, with the benefit of existing in a package that is easy for mere mortals to work with??
http://cds.linear.com/docs/Datasheet/1307fa.pdf
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