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Old 06-05-2008, 08:07 AM #1
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Default Boost driver ideas...

So I'm really into the idea lately of building a boost driver in the most simple configuration possible.

What I'm looking for is some sort of power management IC that can boost ~3.6v to somewhere around 9v.

The chip needs to utilize as little external components as possible.

It doesn't really matter how 'clean' the output is, as long as its around 9v with a decent current.

My plan is to incorporate one of these little boosting IC's into the beginning of an LM317 or some other constant current driver. I'm sure this can be done, I'm just looking for the simplest way to do it.

What do you guys think? Does this post make any sense? Its late for me ;D


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Old 06-05-2008, 11:38 AM #2
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Default Re: Boost driver ideas...

It can be done, but it's not worth it. The efficiency of a 317 is like 40%.. A boost driver can be 70-90%, depending on which one it is, and how you make it. So you're left with 30%, if you're luckyl..


I've had no luck with all through hole part boost circuits. Hardly any efficiency and no stability. It's much better to do it all in one step with a constant current boost driver.


You can either make your own (etching PCB, soldering SMD components) from plans here, or you can just buy the LavaDrive... It's not that expensive for what it does, and the efficency is three times higher, than a boost + 317 combination could ever be.


Otherwise, there is a similar thread here already. Adgmeijin is trying to do just this. I sent him a boost driver from DX, to try it with a 317. It should work for small loads i think..

I wanted to do it this way in the beginning as well, but it's really better to make a real driver.
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Old 06-05-2008, 01:40 PM #3
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Default Re: Boost driver ideas...

Quote:
Originally Posted by IgorT
It can be done, but it's not worth it. The efficiency of a 317 is like 40%.. A boost driver can be 70-90%, depending on which one it is, and how you make it. So you're left with 30%, if you're luckyl..


I've had no luck with all through hole part boost circuits. Hardly any efficiency and no stability. It's much better to do it all in one step with a constant current boost driver.


You can either make your own (etching PCB, soldering SMD components) from plans here, or you can just buy the LavaDrive... It's not that expensive for what it does, and the efficency is three times higher, than a boost + 317 combination could ever be.


Otherwise, there is a similar thread here already. Adgmeijin is trying to do just this. I sent him a boost driver from DX, to try it with a 317. It should work for small loads i think..

I wanted to do it this way in the beginning as well, but it's really better to make a real driver.
I have no problem just using a switching converter and basing a driver off that, I'm just not sure which one to use. I've found a bunch that I think will work well (mostly in QFN packages) but I've read the drawings and they don't really say if its a constant current chip or not. My goal is to make something that is relatively simple, which I know is a bit hard with the switching converters. I just want to use minimal components, but still have an effective driver. I want to make it easy enough for the average person do DIY, even though that QFN package will make it a bit tough.

I have all the ideas in my head... I just need some guidance I guess ;D
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Old 06-05-2008, 02:43 PM #4
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Default Re: Boost driver ideas...

If you are willing to etch your own boards, and solder SMD components, look at the LM3410. It's a great constant current driver, rock steady and perfect for lasers, with some filtering.


If size is not important, there are several versions of the SEPIC design, which can do CC from a voltage higher or lower, than the load needs (buck or boost, depending on what is needed), and if you want it small, check out the boost only version Woop made, that actually fits into an AixiZ module. With the boost only version, you just need to make sure, that the input voltage is ALWAYS lower, than the output. Otherwise, it goes into "direct drive". It also requires you to have the negative output isolated from the ground, but for the Blu Rays, this is fine, as they don't use the pin at the base of the diode.


But there is also a complete micro SEPIC version with a current sensing IC, that allows grounding the load, and adjusting the current with a pot. That one could drive any diode from a single Li-Ion, just like LavaDrive, and still fits into an AixiZ module!

It's just not tested yet, but i'm making it next week, as soon as i have some time on my hands.


Check out these two threads on this subject:
http://www.laserpointerforums.com/fo...num=1194375801 - three versions of the LM3410 SEPIC circuit towards the end, together with Phenols op-amp modification, for grounding the load and adjusting current with a pot.

http://www.laserpointerforums.com/fo...num=1209911140 - Woop's boost only version, and a complete micro SEPIC version with a current sensing IC, allowing grounding the load and adjusting current with a pot.


For the Blu Rays, the boost only version, and a Li-Ion are the perfect combination. I would however suggest getting the LM3410, that has some legs.. It would be MUCH easier to hand solder..

I'm using the LLP package, with only pads beneath. It might require some redesigning of the PCB, but it could make your life easier.
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Old 06-05-2008, 08:05 PM #5
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Default Re: Boost driver ideas...

lots of good info there *

also, if anyone's interested in doing some prototype boards, I could possibly help you out on the etched boards. *With all the boards I make, I'm getting pretty good at it, LOL. *If you supply me an ExpressPCB layout, I could be inclined to etch a 4x6" board with as many of the circuits which will fit, if you tempt me with some goodies (money works as well *;D). I'd obviously need to make sure the sizes and everything will work alright, but I'm pretty confident I can do tiny stuff pretty well.
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Old 06-05-2008, 09:10 PM #6
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Default Re: Boost driver ideas...

That current limiting resistor it has needs to be pretty darn small though, on the order of 1 ohm or so, ruling out using a pot to regulate the current. If I were to use some other means of current regulation, I wonder if I could just tie it to a 0.3 ohm resistor so it produces current at the max level.
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Old 06-05-2008, 09:32 PM #7
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Default Re: Boost driver ideas...

Quote:
Originally Posted by Bionic-Badger
That current limiting resistor it has needs to be pretty darn small though, on the order of 1 ohm or so, ruling out using a pot to regulate the current. If I were to use some other means of current regulation, I wonder if I could just tie it to a 0.3 ohm resistor so it produces current at the max level.
You mean the sense resistor? Yeah, the reference voltage is only 0.193V, so the resistor IS very small. For Blu Rays, i use 2 - 2.15 Ohm resistors. For the red i had to combine three resistors, two in series and one in parallel with them, to reach the odd value i needed.

You can't just put a pot in there, even if it was of a small enough value. That's why an op amp or a current sensing IC is used to achieve pot adjustability.


But i actually prefer using resistors. Pots can be dangerous, especially if someone gets an idea to &quot;pot mod&quot; a laser...
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Old 06-05-2008, 11:32 PM #8
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Default Re: Boost driver ideas...

What if you were to just use a 0.2 ohm or some small resistor for the FB, but not connect it to the line powering the laser diode and then regulated the current yourself with another resistor instead?
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Old 06-06-2008, 12:02 AM #9
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Default Re: Boost driver ideas...

Quote:
Originally Posted by Bionic-Badger
What if you were to just use a 0.2 ohm or some small resistor for the FB, but not connect it to the line powering the laser diode and then regulated the current yourself with another resistor instead?
I'm not sure i understand what you're saying.. And if i do, why would you wanna do that?!?

If you used a 0.2 Ohm resistor for the Rsense, the current would be 965mA (if the regulator could handle it at all) and this current would flow through the load, if you put a resistor in series with it or not.,

On a constant current driver, a resistor in series with the load does not change the current. And even if it did, why?!?


A constant current boost (or buck/boost) driver is all you need for safe driving of laser diodes. The 3410 keeps the current rock steady untill the voltage is too low for it to operate, and it shuts down. What you're suggesting would not work. And the driver could not take a current that high. And another resistor (in series with the load?) would not regulate the current in any way, it would just give the driver even more load, as it would have to produce a higher voltage for the current through the load to stay the same.


All constant current drivers we use, have a sense resistor over which they keep a constant voltage drop, and the resistance determines the current. The only way to adjust the current is to change that sense resistor. And a constant current driver is the only way to drive any diode safely. If you set the current to 100mA it will be 100mA on any load, regardles of the load's forward voltage.


If you're thinking of this as a voltage booster, set to very high, and then limiting the current with a resistor in series with the load, well, the 3410 is a voltage booster (in boost config), yes, but it boosts the voltage in a way, that there is always just enough voltage across the load, to keep a certain current through the load, the current being set by the sense resistor.


I really don't understand what you mean. With this driver, you are setting the current with a resistor. Or if you use the op amp or current sensing ic, you can do it with a pot. What is the other resistor supposed to achieve?
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Old 06-06-2008, 01:07 AM #10
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Default Re: Boost driver ideas...

Hey GooeyGuss, Im making a circuit based off the same chip cdanjo is its much simpler though, not sure if mine works completely but pm me and i can send you the schematic for etching. I think the chip is the texas instrument tsp6102
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Old 06-06-2008, 01:17 AM #11
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Default Re: Boost driver ideas...

I've made a boost converter before; I need to find the schematics though. We used it for one of the building events in Science Olympiad, it can convert ~3.2-40v to 7-24v at 2a with 94% efficiency. It's not a terribly small package, but it is pretty nice, and all the components are readily available from digikey. It's an LM2588.
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Old 06-06-2008, 06:27 AM #12
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Default Re: Boost driver ideas...

Quote:
Originally Posted by IgorT
[quote author=Bionic-Badger link=1212653254/0#7 date=1212708729]What if you were to just use a 0.2 ohm or some small resistor for the FB, but not connect it to the line powering the laser diode and then regulated the current yourself with another resistor instead?
I'm not sure i understand what you're saying.. And if i do, why would you wanna do that?!?

If you used a 0.2 Ohm resistor for the Rsense, the current would be 965mA (if the regulator could handle it at all) and this current would flow through the load, if you put a resistor in series with it or not.,

On a constant current driver, a resistor in series with the load does not change the current. And even if it did, why?!?[/quote]


Hmm, that makes sense now. Mostly I was thinking that I'd use the boost circuit to provide me a high enough voltage for the diode, and then use some current regulation further down the line so that it could be run-time adjustable (since you can't really do that with the sense resistor). Maybe something with a mosfet doesn't really provide a load, but can still regulate the current readily.
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Old 06-06-2008, 09:03 AM #13
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Default Re: Boost driver ideas...

Quote:
Originally Posted by Bionic-Badger
Hmm, that makes sense now. Mostly I was thinking that I'd use the boost circuit to provide me a high enough voltage for the diode, and then use some current regulation further down the line so that it could be run-time adjustable (since you can't really do that with the sense resistor). Maybe something with a mosfet doesn't really provide a load, but can still regulate the current readily.
Yeah, i had a feeling, that was it, but that would work with a constant voltage boost circuit. It would really have to be very stable tho. A resistor could then limit the current somewhat, but the current would still change, as the internal resistance of the diode would change from heat. If you use constant voltage to power a laser, through a resistor, the current goes up, as the diode heats up and it's resistance drops.

This is why it is important to use constant current drivers. They don't allow the current to change in any direction. And the efficiency is much higher, if you do it directly.

If you boost higher than needed, and then use a linear regulator, the efficiency will be very low, and it might not work very good. With a resistor, it would work bad, and be dangerous for the diode (thermal runaway).


But a CC boost driver always gives the diode just the right voltage for a certain current. When the load changes, the driver adjusts the voltage and current stays the same. It's the only right way to do it.
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Old 06-06-2008, 05:20 PM #14
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Default Re: Boost driver ideas...

The LM2588 boost converter is run-time adjustable, you just need two trimpots. It provides a VERY constant voltage, with virtually zero ripple, and is high efficiency. The datasheet also has a few good examples, and it explains the usage well.
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Old 06-06-2008, 08:41 PM #15
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Default Re: Boost driver ideas...

That LM2588 is probably overkill for a laser diode. It's also quite large and expensive at $4.50 each.
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Old 06-06-2008, 10:04 PM #16
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Default Re: Boost driver ideas...

Not to mention, that it's a constant voltage boost circuit. The adjustable version could be adapted to regulate the current, but it would require even more components and it would have a lower efficiency.

As a constant voltage setup it's really not safe for laser diodes. At a certain constant voltage, every diode will draw a different current, even if they are of the same type. So it has to be adjusted for each of them separately and then it has to be set lower than max safe current, because the current will climb as the diode heats up. With constant current, every diode will draw the same current, even if they are completely different.


It's better to go with an IC, that is designed to do constant current. There are many options, but going with one, that is not meant for this and modifying it is not really useful. It would be more complicated to make it, and it would be bulkier in the end and it would not work as good as something meant for this purpose.
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