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

Well I guess an SRK with the SRK tailcap (more money) is the best. I don't know what TA has done with the battery contacts here but I guess it should work with the tailcap board to be configurable in 1S 2S or 4S battery input. Of course the BLF Q8 will be the best.

lol………yes. It has begun

I’m not going to pretend that I understand all of this thread, although I did read it all. But if you aim for 3A to 4 XHP-35’s you’re likely to be popping emitters. It’d be best to aim for 2.8A. 3 is right on the edge with virtually no tolerance consideration.

Ya’ll were busy last night.

I don’t have time to see what those high side drivers actually do, mind giving a quick readers digest version? That said we need to keep in mine that if these work out it would be nice to shrink them down to smaller drivers, so less components = good.

I thought about flipping the circuit around but I think at least for the prototype we should try to stick as close to the baseline diagram as possible the more we dig into this, that way we can see what works and what doesn’t and work out the issues from there.

The current sensing mofset being that large (and expensive) puts it into the keep it in mind if the sense resistors don’t work catogy IMHO. Fitting that on the board would be quite difficult and we would have to eliminate it for any smaller drivers anyways, might as well stick with things that are more easily scaled down IMHO.

Well this is being designed for the Q8 but since that will be awhile the next best option is a 4x SRK. Then obviously a custom shelf to handle the heat. A bit of work for sure until the Q8 comes out.

I have not even worked on the bottom side of the driver yet, but it will be like the other multicell TA drivers I have.

We are just designing it for max power right now, the power can be stepped down easily but going up is another story.

Although this driver should have MUCH less ripple then existing buck drivers (largely because of the new IC and the larger size) so it will not be as hard as some. Either way I figure finding the max amps for the XHP35 is something to be figured out later. I actually plan to order an XHP35 with intentions of finding it’s limits when I have some extra funds.

I've been thinking in some detail about flipping it around. There are three interface points:

a) coff.. This is trivial to deal with. Really I don't like how it is now anyway. You can just charge it straight off Vin, it won't be a problem.

b) the gate driver, that's your first question.. cliffs in a minute

c) The voltage sense. Will the LM3409 mind if the sense resistor is biased very near ground? It can certainly handle it at near 4 volts, within spec.

There's no other problem. Solve these and it all just works.

I've been working on b but first the reason.

That mosfet I showed you has an adjustable low-loss current sense, but it's an n-channel mosfet (better anyway).

So, b then. Need to convert off at 16.8V and on at 10 to off at zero and on at well 5 or more.

That's what those drivers can do (maybe not in the end), but actually all you need is a very small pfet. That's it. You drive the nfet from the drain of the pfet. You'll be driving the gate at 16.8V which isn't ideal (more switching loss) but WAAY better than doubling the heat through those fat sense resistors. Done. solved.

c then is my only problem. Will there be any issue sensing across a resistor with near zero offset voltage. I'm not sure yet. One way is to try it.

I don't think this anything to sneeze at. Going from 1 caps and pfet to 3 or 4 caps and nfet reduces heat from 9 watts to 1.5. That all adds up in a big way especially for performance. Sure there are some details, but I'm working through them. Nothing sinister or hidden. It's not complicated really.

It sounds reasonable, assuming the LM3409 does have any catches that would cause issues with this internally. Although how big is that gate driver? If it is too large to fit on anything smaller then the 46mm SRK then we would be doing all this work for this one single driver and have to start over for smaller versions. Seems like it would be wiser to find a scalable design that works for all of them (be it with different current capability ect).

Regardig the cin thing (an issue by itself anyway). Now coff is charged off vout. Vout is just a constant voltage though, or close enough for a give led configuration that changes don't matter at that level in our application. The coff time is just the cap charging time from that voltage. You can as well use the cin voltage and a different RC which we have to pick anyway.

The reason they use Vout is because they want the same ripple current at every Vout. But this doesn't actually help us. In fact it kind of hurts. All this and the silly comparator flipping (that actually significantly increases ripple current and does cause problems), is not designed for better efficiency or such. It seems designed to give very predictable power output in different applications, so a 36W ceiling light is not 40W I suppose. The reason they want constant ripple is that average current is peak minus half of ripple, and so that makes calculating average power easier and more linear, especially for high inductor ripple, which is popular because it's cheaper. But for us, if output voltage is changing it's because we've drifted to 100% duty cycle are no longer able to support the same Vf, and power is going to fall off regardless. Otherwise, it just makes the power settings slightly not linear. (level 8 might be 2.1 times higher than level 4) So what. Heat and part variation also make them slightly off. For us it will be pretty close at high power where it matters. (That's not even true, this is the same for either, it doesn't impact dependence on load only dependance on output voltage, and the output voltage change vs load is relatively small).

This does NOT make performance better or more uniform across different LED configurations. It actually makes it significantly worse (I have numbers) and I would absolutely target a build to a particular led configuration anyway and just calculate COff to get the frequency I want at that voltage. You can do that if anything easier and more consistently if Coff is related always to the same charging voltage, Vin. A fixed frequency at all output voltages for a generic setup is better than their frequency curve for our purposes anyway.

Those are the only three interfaces and once dealing with those three things, the internals don't know any different. Forget the gate driver. That was an early idea. They won't work well, or not those anyway. You just need a small pfet to drive the nfet. But yeah the one "internal" thing is if it handles voltage sensing at low bias offsets well. That's not entirely clear. I might buy one and try to test. It's not simple. Need a scope, and well, maybe need to build a buck, probably not though, just a scope and some RC. I'll stare at their pseudo circuit more.

Ok, so the other thing is switching delay, but the IC needs some accomodation for this anyway, and as I've read in detail how this works, I'm very convinced that's fine. As switching delay gets too long you'll get a minimum duty cycle at high frequency since it will take time for the on cycle to shut off, which raises the voltage minimum. 10% would be doing really bad (100ns delay at 1Mhz) and that's a 1.7V increase, and even then you can just cut back the frequency to reduce that for 1S drivers.

The thing that's big is that current sensing n-fet, but it's probably not too big.

EDIT: now I almost forgot what one of those drivers does. One of them actually can drive an n-channel mosfet on the high side, generating the extra voltage needed to drive it. That actually is cool because it doesn't require flipping the buck and might even get around worries about the sense resistor, but I think I decided that one was too large and a bit slow. I'll have to look again. I kind of like flipping the buck better if it will work.

Anyway, I wouldn't abandon this simple version. It's almost done. But if you want we could maybe work out the other one too. I might though argue to run coff from vin even on the simple one. I'll post numbers later. Oh, and if you did iadj RC from the manual, I'm not sure how great that is. They aren't expecting a PWM input. Probablu multiplying C by 10 would about cover it, just as a guess, but we should run numbers. Probably 0603 covers any of it though.

Well we can play with the layout but the final version my goal is to have it as universal as possible. Meaning the fewest possible parts changes between setups to make it simpler for new guys to build and use these successfully. AKA, we need to keep the parts list simple as we can, so even if we give up a little efficiency, if it covers a wide range of uses, it is worth it.

The layout I am using now is 100% by the example in the datasheet.

I think that's important for having a baseline to go to smaller sizes. It maybe won't be a big deal for an extra model at this size. We're talking about 1 extra small fet but three less unusual resistors, so from an assymbly perspective, it's about the same, or actually fewer parts (but as one of them is more specialized, potentially harder to get). The basic design is a little better for downsizing. The current sensing nfet won't shrink (there aren't many models made), although you could replace it with a regular nfet and small resistor but still stuck with the small pfet to drive it, so for small models may as well have this (I mean your present) circuit.

It's fine either way. And if you aren't tempted by it, it gives me a project to pretend I may one day tackle to learn diptrace, and who knows, maybe I even will. I was kind of thinking about it before you restarted this, so I might still.

Oh, I never said we can’t do it, I just wanted to keep this in mind.

The whole idea behind my driver projects is standardization. When I first started researching drivers on here it took me WAY too much effort to figure out what the difference between drivers was, what parts you needed ect. That is what led to me making any of these drivers and I don’t want to stray from that.

So Like the Texas Avenger drivers I want the Texas buck series to be the same basic setup for all the sizes, just with different components for the current/voltage but once again those are standard for all the sizes.

If these new components can scale to the smaller drivers just as easily as the stock setup, then I am all for them. I honestly just don’t have the time to research the details on the components right now.

The existing pfet version should scale down a little better. So it's better for that. I think of the nfet version as a Q8 special, non-series version. I edited my reply above about the driver too. It's still possible maybe to consider a minimal layout change with an nfet. I'd momentarily forgotten that indeed on of those drivers allows an nfet to be placed high-side. I think it was huge or had some other issue, but I didn't search hard for other options for it yet either. This wouldn't be kept in a scale down either though.

Unfortunatley though, probably many of the big components will need to change in any scaled down version. The inductor takes too much real estate. And the diode could probably be significantly smaller. The sense resistors, well maybe can just use fewer, we'll see, but probably will want to tune the values better than just that. The pfet might be able to stay the same. It won't quite be like you just buy 5 of everything and make different drivers though.

Yeah, those will have to change but that is also due to not needing as much current, so it will still follow a simple chart where people can figure out what they need by voltage and current.

It is mostly simplicity of design, aka the design is the same and uses the same type of components, only the values (and footprints in some cases) change.

It is not iron clad either way, it is simply the goal I am going for.

Sure.. but I thought buck flipping is like the national sport of Texas :) It's more dangerous than cow tipping for sure, but more fun?

... So I found the problem with the sensefet. Oh well. It has a current output of sorts for sensing, but it can't support that current with more than the drain to source voltage drop, typically half of that or so.

It's nothing more than a parallel current path effectively though a parallel fet. No amps or anything built in.Aparently this is more stable than just measuring the Rdson voltage drop directly, but it's still a tiny drop, and needs an amp.

At that point, it seems might as well just use tiny sense resistors and an amp. Sure that adds more loss than this, but still maybe 10 times less loss than what we have and much simpler to implement. Which is to say, maybe still not so exciting, I don't know it's the next thing I'll look at. There are aparently off the shelf IC's designed for that too, "current sense amplifiers" so next I'll find out what's wrong with those. That at least is a directly removable solution when downsizing, just increase the resistor size and remove the amp.

All this though is just for ideas to make what might already be the best driver here (depending on use anyway), even better.

I did find a very nice and very expensive current sense amplifier designed for almost exactly this purpose.

But it's not quiiiiite drop in. After staring at the lm3409 sense circuit I confirmed that yes, it needs a Vsns- to be at least 1.25V above ground. That can probably be done by referencing the amp ground to the MCU Vcc for instance, but let's come back to it after finishing up the baseline design.

So, thinking about iadj a little. A couple of things. Right now, the iadj voltage is 4.2 V times the iadj PWM duty cycle. Except I think that 4.2V is actual battery voltage? You are using a single battery Vcc design here right? This is unfortunate. It's going to make output fall off as voltage sags, just as it would in a direct driver!... because the battery is directly driving the iadj pin. That's pretty obnoxious for a buck driver. So you might want to bring back out the ldo.

Edit: I got confused on PWM RC optimization. I'll come back to it.

Still.. an RC of 1 would have a nice touch that all mode switches will be soft.

Another point though, right now full scale is at a duty factor of about 25%. If we use a voltage divider we could make 100% be full scale. But is 50% still 50%? I have to scratch my head on that.

Finally, did you actually hook up uvlo? Do we want voltage protection there or in the attiny? Having it redundant seems unnecessary and could cause confusion. Plugging Vin straight to uvlo will just override it, and you get rid of two resistors too.

I had a few mins to update the design some. Getting closer to a final layout, thick traces all around and everything is moved in to allow for 3mm edge clearance (4mm would be better but just not going to happen without cramming things too close to get thick traces.

The pads for LVP are there but do not have to be used. Still got some work on the bottom side to figure out along with some other items of intrest but getting it worked out as I have time.

If you can come up with a better option for basically any of it that is scalable down to the smaller drivers using the same design and offers notable improvements without a ton of extra cost ect I am fully open to it.

I dont have much time to digest the rest of what you said right now, maybe later this weekend.

Here is the latest layout: