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

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:

Nice, I definitely want to give some of the details a very thorough once over still, not about major layout changes in this round, but just focussing on the details like the iadj voltage, input caps, etc. But I don't want to trickle bits out now. I'm pretty well along on a rework of some numbers, but I might need a couple or even three weeks, because I have almost no spare time for awhile now.

One hint.. I'd only looked at input cap current (and had some formula copy error, did say I needed to review those) but I hadn't looked at input cap voltage at all yet. Some news there. I'll also give a perspective on the issue of where to charge coff.

Anyway, do reread my last post. A few little tweaks there that I think need considering even in a by-the-book design, and this iadj usage isn't quite by the book.

What does the jumper do? Does that bypass the uvlo resistor? Rather it looks like it just disconnects it.

Opps, I noticed I mistakenly deleted a trace in the schematic and it led to it being erased in the PCB. The jumper traces you noted. I replaced them so it should be complete now.

In answer to your question, the jumper is simply to allow the ground pane to access the bottom of the buck IC for both heat sinking and EMF interference reasons. This is what they recommend in the Datasheet and it makes sense.

You will also note that I have ground/copper pours between as many traces as possible to keep EM interference to a minimum hopefully.

Also what kind of size / spacing should we aim for with the inductor throw holes? I suppose I can mount them offset to where the springs would be, not perfect but should work in a pinch.

I added an LDO as well, kept meaning to do this but never got around to it, things are getting crowded in a hurry!

The bottom side has now been roughed out, obviously I have not solder masked the area but you get the idea.

I will be placing a fairly large order with Arrow in the next week with a discount code. If we can get the components sorted by then it is possible I could order a set of them and if Leroy orders a set of boards maybe I can buy one of them from him and build one of these for testing.

It all depends on budget though, got a lot of money tied up in various projects right now and need to free up some more for a Group but for 219C 90+ CRI LED’s I will be handling before long.

I need people to buy my stuff! but that means I need to get an official listing up for the skyray kings lol.

Well, no problem, we're still polishing things off. I have/had a couple of bugs too.

Updated:

I hadn't thought much about the diode in the iadj input. I first wrote this and I thought I'd overlooked something there but not really.

If the cap never gets over 1.24V the diode is never an issue in the circuit and can be ignored. As it is now, that's the case. As it is now we saturate way below 100% PWM too (not necessarily ideal, but just lose mode resolution). And as it is now, the LDO is needed. What thought does come is that maybe there is a way to leverage the 1.24V reference where the LDO isn't needed. I'm not seeing it though without other parts anyway.

Of course one alternative to the LDO could be a zener diode on the mcu iadj output, but the LDO works fine.

Ok, I see what you mean about the jumper now. Well for now that will work. One can always choose Ruv2/Ruv1 small so the low voltage protection never kicks in. It just seems like if you're doing low votlage protection in the mcu, not point having it both places, and could remove two resistors.

The jump seems like it should be avoidable, but then I realized it probably helps heat sink the resistors anyway.

That is true, we could just use the MCU for the LVP, although since the pads are in place that is easy enough to simply not populate one of them later as well.

Yeah, I guess leaving off Ruv1 does the trick. Still need an Ruv2, but not a big deal. It provides an option I suppose if the mcu thing doesn't work well for some reason.

On the other hand, it only needs 1.24V as I reall to be high. If there's a way to route Vcc Mcu to it from the other side without cutting off other grounds, that could eliminate the jumper as well as the two resistors. Or it least turn the jumper into just an optional heat sink lol, just thinking about possibilities to remove parts, but cheap parts in this case.

You're worried about size of ground trace on the left of U2 (there is a thin trace through Ruv1 and Ruv2 even without the jumper) but on the upper right of U2 you've pinched the ground without much reason. Hmm, maybe I'm getting it, you expect that side to actually deliver heat instead of removing it?