The Texas Buck driver series, Q8 / Skyray King 2S/4S buck driver RELEASED!

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Texas_Ace
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I don’t have time to read all of that but your summery is making sense. Using another small footprint FET to PWM the IC is an option, although it does add more components to the cost/build and complexity.

If we selected the right FET we could hopefully adjust the footprint to allow for a jumper to be used if someone did want moon mode lower then what the IC can do on it’s own (I still think it will be enough).

22uh is the best bang for the space I have come across, lower I don’t think will work at all. Higher is great but they quickly get too large to be practical. even at 22uh we are looking at close to 1mhz switching frequency according to the spreadsheet (I didn’t make it BTW, just found it online, no idea if it is correct).

Now for total current, I think we need to aim for 15 amps to start out with, it can always be lowered easily later but higher is much harder. Plus if it is stable at 15 amps then we know it will work fine at 10 amps.

For the Q8 we are going to be stuck with parallel LED’s on the stock MCPCB unless a drastic change was made and this driver became the stock driver for it (don’t see that happening). So high amps is a must. with 15 amps that is enough to drive 4x xhp35’s at around 3 amps + a bit of headroom. Or 4x xhp50’s at 3.75 amps (pretty low for them).

Like I said we can always go lower but higher is not so easy. For example with a custom 4S LED setup you would only need ~5-8 amps per LED which would be easy.

I would much rather design it for overkill and work backwards. Tis how I do things. Honestly if I thought it was possible I would be aiming for 20 amps on the Q8 driver. Just don’t think that we have enough room for that though.

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Ripple kills LEDs so it’s a good idea to think about dealing with it otherwise you have to set current max lower.

Three Tanna leaves to give him life, nine to give him movement. But what if he eats the whole bag?

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Rufusbduck wrote:
Ripple kills LEDs so it’s a good idea to think about dealing with it otherwise you have to set current max lower.

Yeah, it is something to be dealt with. It is mostly an issue of parts selection though as this particular IC allows for basically whatever ripple you want if you select the correct parts.

That said space quickly becomes an issue, as to eliminate ripple you need large components and/or very high switching frequencies. both of which provide their own challenges.

With the numbers I am working with right now, it looks like ripple should be able to be kept under ~250 mA, which is less then 2%. What it will do in the real world is yet to be seen though.

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Texas_Ace wrote:
I don't have time to read all of that but your summery is making sense. Using another small footprint FET to PWM the IC is an option, although it does add more components to the cost/build and complexity. If we selected the right FET we could hopefully adjust the footprint to allow for a jumper to be used if someone did want moon mode lower then what the IC can do on it's own (I still think it will be enough). 22uh is the best bang for the space I have come across, lower I don't think will work at all. Higher is great but they quickly get too large to be practical. even at 22uh we are looking at close to 1mhz switching frequency according to the spreadsheet (I didn't make it BTW, just found it online, no idea if it is correct). Now for total current, I think we need to aim for 15 amps to start out with, it can always be lowered easily later but higher is much harder. Plus if it is stable at 15 amps then we know it will work fine at 10 amps. For the Q8 we are going to be stuck with parallel LED's on the stock MCPCB unless a drastic change was made and this driver became the stock driver for it (don't see that happening). So high amps is a must. with 15 amps that is enough to drive 4x xhp35's at around 3 amps + a bit of headroom. Or 4x xhp50's at 3.75 amps (pretty low for them). Like I said we can always go lower but higher is not so easy. For example with a custom 4S LED setup you would only need ~5-8 amps per LED which would be easy. I would much rather design it for overkill and work backwards. Tis how I do things. Honestly if I thought it was possible I would be aiming for 20 amps on the Q8 driver. Just don't think that we have enough room for that though.

 

I think we can get one thing fixed (and that helps).

So I have a full caclulator setup now using that reference I posted.  I did make it, but I didn't make the equations.  I did check some of them and looking into others. 

 

I'm not sure I agree now that 22uH is a must.  I actually cannot find anything in 22uh above 12A rated (you can find higher saturation, so maybe good cooling can push one beyond spec) smaller than 22mm.  I did find this comprimise: http://www.digikey.com/product-detail/en/vishay-dale/IHLP6767GZER150M11/541-1287-6-ND/  at 15uH 14A rated 17.15mm 12A saturation shielded inductor.  I'm not worried about the saturation.  At high current you can live with less inductance.  This might be the part.  (darn expensive though)

 

But I think we need smaller than 22mm.  Basically everything above 10uH above 12A, shielded, below 22mm is 17.15 *17.15 or close enough to not matter.  So I would say that is the footprint.  What do you think?  There are options here.  You can get a 19A 10Uh if you want high power or a 12A 22uH if you want better low power efficiency, so the end user can decide. 

 

Now as for 10uH,  Plugging some numbers through my calculator, at 10uH you get 2 to 4% inductor ripple current at high power (15A) at 1MHz or 8 to 15% at 300 khz, but lowside cap of 10 or 20uF easily shields the output voltage from this.   The problem is in low power, 0.25A per LED  you start to get in the the neighborhood of 100 to 300% ripple current or more in 2S depending on frequency (200 means entering discontinuous mode I believe), 3 times less still in 1S.  But that's not the end of the world.  Ok, so at low power you enter discontinuous mode. We know that and have discussed how to deal with it.  I'm still only getting 7% total power loss (my gut says it will be twice that from realities not calculated, edit: gut was right, had an error in inductor loss equation.)  at the threshold to DCM.    The question is how low and at what frequency?  From the equations there I'm not seeing big switching losses, but I want to review that more.  1Mhz makes me concerned we'll see unpredictable losses from many parasitics we can't easily predict, but this probably isn't a footprint issue anyway.

 

As for the diode it does look like it will be the biggest loss (edit: in some situations), but it's ok and hardly an issue at all in 4S to 2S.

I'm not worried about 4p personally.  Rewiring the LED board is simple, but I see your point.

 

 

 

 

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Updating this post for the records:

Calculations are here: http://budgetlightforum.com/comment/1017335#comment-1017335 (post 124)

Inductors: (done but not decided)

The three inductors to beat, all 17.15*17.15mm:  SELECTED FOOTPRINT

 http://www.digikey.com/short/39nz84.  IHLP-6767GZ-11 IHLP6767GZER150M11   15uH 14A rated, 12A saturation, price $5.95 each   SELECTED  (but the other two in this size are viable options and fit)

http://www.digikey.com/short/39nz8h  PA4344NLT Series Datasheet                         22uH 12A rated, 18A saturation, 26.5mOhm,  price $4.55

http://www.digikey.com/short/39nz8b  IHLP-6767GZ-11 IHLP6767GZER100M11      10uH 19A rated,  17A saturation, 9.3mOhm ,price $5.95 each 

 

If you can fit 22mm on the Q8 I definitely would.  You can bring 26mOhm down to 7 (BIG efficiency savings, few watts in 3.5V output ) and get a full 15A rating!

http://www.digikey.com/short/39nz8r 

 

Caps: (done)   see post: http://budgetlightforum.com/comment/1017741#comment-1017741 (post 132)

1210  10uF,  for input and output.  Maybe include 2 or three in parallel for input, (to save about three percent loss in 14V output)

 

I can find some cheap ($.30)

http://www.digikey.com/short/39nzjt  SELECTED.  (for input and output)

 

and some individually specced (rare, reading through these cap sheets is a pain) up to high frequency, even if not amazingly low ESR/tan delta/dissipation factor/high Q (average).

http://www.digikey.com/short/39nzj3

 

Diodes: (in progress, not quite given up yet)

Things to consider, primarily Vf, and max reverse voltage, forward current, but thermal performance is also important, including reverse current at high temperature.

A number of diodes in exist in POWERDI5060-8 like this:   SELECTED FOOTPRINT

http://www.digikey.com/short/39n297 SBRT20U50SLPQ

http://www.digikey.com/short/39nzj8 SBRT20U50SLPQ

http://www.digikey.com/short/39nzjz STPS30L30DJF

 

This one has a particularly low leakage even when very hot (Vf might be a little worse though)*:

http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeySTPS30M60DJF-TR

 

But there seems to be more selection in TO-263 (D2PAK) like this:

http://www.digikey.com/short/39n299

There are also TO-220 through hole packages like this:

http://www.digikey.com/short/39n299  (mouser has many)

Which may provide different heat sinking options.

 

Diode heatsinks (impact solder footprint and space):

http://www.digikey.com/en/product-highlight/a/aavid-thermalloy/surface-m...

 

The POWERDI5060 and D2PAK can proabably fit on the same pads if made a little cleverly and extra large pads are documented to help cooling anyway.  The 2-wire To-220-2’s like the one above might be able to span the same footprint too if we really want a do-it all pad with through holes. Most of the TO-220’s are three wire though with the anode in the middle, which becomes a different issue.

 

*0.1A leakage current is 1.6W times duty cycle so 0.8W at 2:1 or 1.6W in 4:1)  Some of these get up to that level when very hot!  Not a big deal when using 84W output but keeping leakage to 30mA is better.  Low reverse current seems worth probably 0.1V in Vf to put them in some balance.

 

  

MOSFET: 

The trick here is rdson and gate charge divided by voltage it's measured at.   Every 30 nC/10V   (3 nF) produces about 0.11W of minimum switching power at 1Mhz even in low modes.  Keeping this below about 6nF is very helpful, 3 is better

 

30mohm of Rdson however produces in the ballpark of 9W of power at 15A in 14V output, so getting this below 10mohm is very helpful, 5 would be better.

 

TA found this:

http://www.digikey.com/short/39nh5h

Which I think is probably fine, and the footprint seems easy to find things in, but I'l look a bit more later.

Update: gate charge is way to high.

 

This seems much better, 

http://www.digikey.com/short/3bj1wp   STL30P3LLH6     

 

and this is probably a better compromise yet:

 

 http://www.digikey.com/short/3bpr78  BSC084P03NS3 G (I'm liking this) SELECTED   8.4 mohm Rdson, 6nF gate capacitance.

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Something seemed, wrong, was missing the square in I^2R in my inductor power loss..  Now this matters, getting up to 18% power loss total in worst case (high power, 1S, so highest output current)  seems more right.. still working bugs out.   This is actually a pretty big deal though and makes the other options more attractive than that particular 22uH inductor at least.

 

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Percent power loss for the 22 and 15 uh inductors and the percent inductor ripple current:

      2S 1S
    1 MHz 300khz 1 MHz 300khz
  Value High P Low P High P Low P High P Low P High P Low P
15 uH 14.4mOhm P_L % 2.67 0.40 3.17 1.82 6.44 0.72 7.10 1.79
  delta I_L  % 2.84 68.06 9.45 226.85 1.54 23.09 5.13 76.97
                   
22 uH 26.5 mOhm P_L % 4.79 0.52 5.41 1.92 11.70 1.13 12.51 2.30
  delta I_L % 1.93 46.40 6.44 154.67 1.05 15.74 3.50 52.48

 


Diode and switching losses bring that closer to 20% for 1S high power, plus whatever losses I haven't considered. 

High power there means 15A, ~53W for the 1S and is 12A ~84W for 2S.  Low power is 3.5W total in all cases.

 

1S is tough.  In non-shielded inductors or in 22mm footprints, there are considerably lower resistance options available though.

 

3.5W is discontinuous mode at 15uH 2S, but 1Mhz keeps it alive, and maybe this fancy IC will anyway.

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All very good info. It is possible to fit a 22mm inductor onto the Q8 driver but obviously any smaller drivers will not handle that.

While I want it able to handle 15A I am also realistic that we will most likely be overdriving something to reach that.

I am not sure what you mean when you say that rewiring the Q8 MCPCB would be easy? More like impossible? Now you could use individual stars as a replacement but that kinda of ruins the point of the nice mcpcb. It is an option for the extreme among us though.

Honestly my biggest goal is as an XHP35 driver with 4s input and 4s output and able to provide the ~3 amps per LED that they can handle with a low enough ripple that they don’t die.

If we can reach those goals with a bit of headroom it should handle anything else we may desire. XHP50’s would be best run with an FET+2 for the amount of power they need, same for the 3V emitter options. This is why I made the Texas Avengers for those setups.

Now I do plan to make smaller versions of this driver for other lights but it will obviously have much lower current needs as well. Just not sure what lights would be able to use them besides things like the L6.

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First Cap footprints:  Main cap, 35 to 50V rating, requires 1210 (or 1206) specifically this:

 http://www.digikey.com/product-detail/en/samsung-electro-mechanics-america-inc/CL31B106KBHNNNE/1276-6767-1-ND

Maybe use the same cap before the FET.

 

If you can fit 22mm on the Q8 I definitely would.  You can bring 26mOhm down to 7 (BIG efficiency savings) and get a full 15A rating!

 http://www.digikey.com/product-detail/en/wurth-electronics-inc/744355822...

 

Worry about smaller ones for smaller drivers. We’ve found a few, easy to come back to them.   Further I would add through holes to allow optionally mounting unshielded toroid inductors unless there’s really no place for them to exist.  It will give options for testing/learning/improving and that might help down the road.

 

It’s not quite true that 2S is a slam dunk after doing 4p.  Yes I^2R and IVf loss/heat get easier, but inductor ripple is maximum at 50% duty factor, about 3x worse than for 25% duty factor, so you enter discontinuous mode earlier and either need to start pwm earlier (not so bad), or you need more inductance or higher frequency, either probably at the expense of some losses for the same footprint, but cheaper than the same game at 1S.  Probably PWM earlier is as good of a solution.

Like you, I don’t see output ripple being a big issue.  As I interpret the capactor dissipation factors, I’m getting values of next to nothing (1 mOhm) for ESR at 300khz.  I’m inclined to stick 20mOhm in the math just in case, which still leaves output ripple under 2%, actually a tiny fraction of that at high power.  The hard part here is likely good connections and traces. 

Anyway, Rufus, ripple kills LED’s? and PWM doesn’t?  I don’t believe it.  6A modes with 10% current ripple kills LEDs. That’s not exactly the same as “ripple kills led’s”.  Of course we're talking about ripple voltage though, and that translates to more in current, still it's overvoltage/current that kills it.

 

As far as PWM, actually it probably doesn’t need another FET at all, just use the PWM port on the IC as it was designed.  You just need one trace and a spare mcu pin, no component.  Then it's up to software.

 

Ok, making 2S is probably somewhere between easy and impossible, I haven't looked.  You should be able to cut traces (no?) and solder onto the top connections of the LED’s (after scraping the coating).  Apparently that’s a pretty big pain after they are already on a heat sink.  Or just sand off the trace coverlays and solder to the board.  This is really too bad though.  Why not have jumpers/solder pads on the LED boards?  Is it really too late to do that?  I doubt there can be a more capable driver with present tech, than a 4s to 2s buck. You can get melt-down modes, and suck the life out of the batteries to maintain intermediate modes, although moonlight will probably end up slightly more efficient on the 1S. so like 500 hours instead of 350.

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I just started searching for schottkys.  I way underestimated Vf.  At 0.55V (ok 0.45 at 15A, less if it warms up well) ... schottky losses are looking very big in 1S output, about 9.5% vs 3.1 in 2S. I did find a synchronous IC that could work (bypassing this loss), but it was constant voltage, divider style, only.  Might be dimmable with some hack.

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There are 10uf caps available in 0805 as well, I have some that I ordered for C1 at some point.

22mm should be able to fit but would leave very little extra space for other components so we need to make sure they are all in order first.

Far as 1s vs 2s vs 4s. In the Q8 (what this driver is really meant for), it comes stock with 4× 3535 LED’s in parallel and another 4× 5050 led pads as optional, also in parallel (assuming that Thorfire uses our design).

The MCPCB design is setup for parallel only and switching it would be much harder then simply installing new DTP stars.

So with the setup it comes with the 1S 3V emitters it comes with are best driven by an FET as they will be needing 16-20 amps of current, while this buck driver could do that it would be a lot of extra cost and effort for very little gains unless the LED’s were rewired in series with individual stars, that is not worth it except to the extreme modders.

The 5050 pads are basically for XHP50’s, they are once again best driven by an FET driver due to the fact they could easily use 20+ amps. Once again this buck driver would work but if max light output is your goal we all know that the FET will win.

Now the last major mod idea is XHP35’s, they are obviously 4S voltage only and can not be driven with an FET due to the fact they like to fry like that. So that is where this buck driver comes in, it is to allow XPH35’s to be dropped into a Q8 for a high CRI 8000+ lumen monster of a light without a donut hole issue.

So 4s input to 4s output is the primary concern, the others are great to play with later and if it works with 4s to 4s it should work with any of them given component changes. The XHP35 is the only setup that requires a buck driver, the others would just be for the extreme modders that want the very best, even if they are spending as much on the driver as the light itself. Although most of them will want an FET anyways.

I had thought the PWM port on the IC should work but figured I had missed something.

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I'm sorry, yeah, I misread (maybe a few times) as 4p being primary concern I see (didn't really know about xhp35's and hadn't looked yet).  Well I still think 4s to 1s is nice even if 2s might be "impossible".  Max output is not all that matters.  The buck will give you controlled output much longer (in anything other than 4s to 4s at least).   Can that light really sustain more than 50W or power without burning your hand?   Turbo shine is neat, but for long use at say 30 to 50 W without battery sag getting in the way, a buck still seems useful.  I'll have to get my hands on the light.  It's hard for me to imagine how it could be so hard.  Worst case you resolder the LED's with their bottom pads grounded down and use the top contacts to connect wires, unless their is really no space for a wire.  Mildly complex mod but so is assembling one of these drivers.

 

Anyway, I will add 4s to my scenarios.  I put in component specs and get out performance specs for as many use cases as I want.  Still, all these scenarios have different difficulties.  I don't think any one makes all the rest optimized.  In 4S the diode loss won't matter at all for example, in 4:1  it's a big deal.

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As for the caps, yes, I've found many in 0805.  I didn't find any with high voltage rating. I guess they should be 30V minimum, but I definitely could have missed them.

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This is the proposed MCPCB for the Q8, as you can see it would be very difficult to convert to series, although not impossible. That said the work to convert would be more then simply dropping in new stars, which is what I would do if converting but that is something I would only do for a show light.

The buck does not have to run at 4s, it can also be run at 2s2p levels.

Naturally the light will not handle that level of output long term, heck even stock it won’t handle turbo long term. We don’t care about that here at BLF, we want usable modes for using the light and then turbo modes for play time. lol

Driving 1s or 2s LED’s is nice but honestly unless they are in series you won’t gain a whole lot over the Texas Avenger driver I already have for the Q8 with the bank of 7135’s. Not nearly enough to be worth the price or effort IMHO.

Now if you convert the LED’s to series as well, then things get more interesting but that is far too much work unless going for max possible lumens (not something I chase much anymore).

Now the smaller versions of this driver will only need to worry about the ~6 amp range which is much more reasonable.

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I have to find a place to post pics.  My first idea goes like this.  Isolate the cathode with a vertical cut to their left.  If cutting from top to bottom, stop when you hit anode trace and turn right, cutting across the two through holes and stopping above led 3.

 

You can mount led 1 and 3 backwards and jumper the cathode to the area north of led pad 7.

 

That requires reflow.  With just a bit more cutting (basically continue the horizontal cut out to both edges) and two jumpers you can do it without reflow.  All this could easily be accomplished with a few cuts in the trace, zero ohm resitors and/or an extra solder pad prior that would allow reconfiguration.  It's really unfortunate in a light intended to be moddable.

 

2S isn't just about max lumens.  It's about keeping say 2A per led (or whatever it can sustain) from being affected fairly quickly by voltage sag. It gives you a setup where light performance has nothing to do with the battery voltage sag/depeleting, so long as it's capable of putting out the power required.  But yes, you get max lumens too, so everyobody should love it.  Anyway, I didn't understand about xph35's and I should have stopped to understand what you were saying there.  I will try to post a pic later.  

 

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That's probably not even the most clever way (almost the least) just first things I saw.  I think can get a solution with either no jumpers, only cutting, or certainly can get one with no reversing of led's and probably still only requiring one jumper. I'll try post pics later.

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Bettter yet, you can use the extra led pads for the jumper(s).  It might be possible to make this fully reversible/reconfigurable just by re-configuring 0-ohm leds on thee extra pads. Splitting the wire pads gives even more options.  Now it's like a game.  Is there a way to cut traces so that xml's can then be configured in 4p, 2s or 4s only by jumping large led pads, and changing wire connection points, without even resoldering leds?  The thermal sensors might get in the way, not quite sure how they are wired.

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Oh, you can mod the MCPCB to work in series, anything CAN be done. I said it was easier to install individual stars and I still say that is the case. With stars you simply install them, line them up and glue them down. All bolt in parts with no custom work needed. Even for me I prefer bolt in whenever possible. Why makes things harder then they have to be.

Personally I would have preferred the light be setup for 4s LED’s run by 4S cells from the start (maybe 2s2p LED’s). The overwhelming majority said they wanted 4s cells and LEDs though so that is what we have at this point.

Got to work with what we have.

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To me this all seems MUCH easier than assembling a driver (which you still have to do but...) and it's quite a bit cheaper and faster than ordering new stars.  If a few cuts in traces, and existing jumper points (the extra led pads, or just add proper jumper pads) can make this reconfigurable is it really too late to push it through?  

 

(are they heat sink pads case grounded?  do they need to be avoided  electrically?)

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Yes, the MCPCB is DTP. Like I said it could be converted but I personally hate cutting traces, I have always had massive issues getting a clean cut that doesn’t short out.

The $4 for new stars would be well worth the cost for me but to each their own.

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Solved all that:

http://imgur.com/gallery/ebS7c  (well solved in theory, I'd have to learn dip trace to actually solve it)

 

4p as shown, positive is always to the left (or negative if you prefer).

Can do 4p, 2s, or 4s.  No cutting, only jumpers (4s requires jumpering two of the wire pads, although there is room to route a trace for a zero-ohm resistor instead for that too, but jumping the wire pads seems ok).  LED's do not need to be reversed.  This should work as a stock board.  (I deteted the thermistors because they were in my way at the time, but this is just a concept version obviously)  

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I had considered this, actually had a design half made up for something like this but decided against it as people would complain that the traces were not big enough.

If Miller says he is ok with trying to change it at this point I can finish my first design and see what the Q8 guys think of it.

Either way it doesn’t effect this driver much. The goal is to design it for overkill and then work backwards for smaller versions. I figure I would like to work down to at least a 26mm and maybe get a 20mm if possible since the mtnmax is not available in that size. Although that would be limited to the tiny13 which is not a great option but thats for a later design.

Flintrock
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What's wrong with the one above?   The places where traces are narrow, it's obvious how to improve it, just didn't for the concept drawing.  Random addressing is fine, but that plus laying it out for short jumpers for the most used paths without ever needing LED reversal is a bit more of a trick.  I did try to keep the central pad/hole design the way it was because I didn't know the constraints.  If I could put pads anywhere I want, traces get a little better yet, but I'd still do it like this basically.

 

Anyway, if I inspired movement on it, that's great to me.

Texas_Ace
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There is nothing wrong with it, but if going for a fully customizable board, might as well go all out IMHO. I tossed together another version and posted it in the Q8 thread that allows any setup you could want using jumpers.

The issue is practicality of thorfire actually using it or any of our designs.

leroycp
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Get it to production. There are a number of us that wish it……………..when the Q8 is ready. Hope we get an update from TH soon!

Flintrock
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Edit.. moved my comments there.  This is was buck driver thread. 

Flintrock
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Ok, I felt if the board had a chance it had to be now.  Done all we can on that, so back to the buck.

 

So I'm mostly finding power diodes with about 500mV Vf.  I'll get a summary of something soon.  That produces 10% resistive power loss in the diode alone for all 1S configurations, regardless of current level.  I don't like that.  

 

But I did find a couple of gems like this:

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=2088505...

Which is really slow (and can't find just how slow) and just not meant for this.  It's half tempting to put one in parallel with a faster diode though, so it can kick in when it catches up, if it ever even does though. 

 

I think without finding a current regulated synchrounous IC though, that's just reality. We could try to parallel two diodes (or more), but it's not guaraunteed to help much since whichever takes more power, gets hotter.. lowers Vf, and takes even more power.  So I'll try to round up the best of the bunch, mostly already done probably.

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I was also thinking about parallel diodes.

Keep in mind that the primary usage of this driver will be for 4S setups or 2s setups. For example if you were running 1s 3V LED’s then you would run the driver as 2s voltage. If using 2s LED’s then at 4s voltage and obviously 4s voltage for 4s LED’s.

I don’t see a reason to use 4S input to 1S output in any case. Although having the option is great naturally.

Flintrock
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I tend to agree.  There's not much reason for 4:1.   2:1 is showing about  3.4% loss for the diode, and 1:1 (close to it anyway) well I haven't run the numbers yet but it will be next to nothing, so it's not so bad.  

Flintrock
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This is neat:

http://www.linear.com/product/LTC4412

It can turn an asynchrounous buck into a syncronous one as a drop in replacement for the diode, without support from the buck IC.

It uses a MOSFET (of our choosing) to do switching in place of the diode, but instead of being controlled by the main IC, it has a voltage sense.  When there's voltage on the cathode, it closes the switch.  The body diode of the MOSFET is used to cover the slight delay from sensing.  This won't work for a 17mm version I guess.  No room for the extra IC.

 

Anyway. I said before, and it's written many places,  that max ripple current is at 50% duty cycle.  As with many things technical that statement has to be taken in (a fairly unfair in this case) context.  It turns out that's true for equal current.  For equal power, the ripple as a percentage of average current is a bit worse or anyway pretty similar at high duty cycle/high output voltage compared to 2:1 operation.   Most things though are easiest at high voltage output and get harder/worse at lower voltages.

 

I've been thinking about total voltage drop issues a little for 1:1 (now included in my tables), and adding some tweaks to the math.  It seems to me on first scratch like it won't be an issue.

 

 

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