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

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?

As far as through holes. On digikey bourns is the main brand that pops for less than 10mOhm greater than 22uH > 15A. They have some in about 30mm spacing (yikes) and some in about 12mm spacing. For the most part 12mm is a vertical inductor mount. 1.7mm pin diameter. I didn't realize how big the pin spacing is on the flat ones. Probably would need one pin southeast of LED+ and one between Q1 and D1, with whatever interference that causes on the other side.

Here's one of the flat kind:

http://www.digikey.com/short/3bcd55

However there is also this:

http://www.digikey.com/short/3bcd55

http://media.digikey.com/pdf/Data%20Sheets/Wurth%20Electronics%20PDFs/S14100034.pdf

2mm pin diameter, 11mm spacing (close enough to 12, I'm sure they'll bend) and it's a horizontal off-center mount. That actually looks pretty good for us. But you have to look at the picture to understand hole placement. I'm not sure about that 5db attenuation at 1Mhz. Seems to be related to the self resonant peak. I can't pretend I've understood that issue well, which is why I've preferred inductors that just don't get funny until a few MHz, but these big ones just don't have the detail spec sheets.

Not clear if the bourns inductors are better. They just don't show as much detail.

One could I suppose solder some pins to the sides of one of these low resistance 22mm's we were looking at:

http://www.digikey.com/short/3bcdfz

but that's not very user friendly.

IADJ is licked

I started this PWM the iadj pin thing, so I'm finishing it. I worked out by hand and time slice integration/simulation, the situation with iadj, with the two agreeing. Bacgkround, the iadj pin can only take up to 1.24 V input to control current output of the light. Any more gets cut to 1.24 anyway by an internal diode.

Now, imagine a resistor R1 on the left in series with a capacitor C1 on the right, and they meet at Vout in the middle. A resistor R2 is in parallel with the cap, both connecting to ground on the right. R1 connects to a PWM voltage V1 on the left.

This happens to be just like your circuit except R2 is infinity, no problem. (my R1 is your Rf2)

The result is the maybe not surprising (I'd call it less obvious than it appears, when you start proving): Vout= V1* D * [R2/(R1+R2)]

If R2 is infinity this is just Vout=V1*D * 1

D is duty cycle.

Great so far. So we can make 100% max range 1.24 V if Vin is 5V by making R2=0.33*R1 in the usual voltage divider way (requires adding pads for R2, and I think it's best to add it, can always not use it)

Or we can just leave it alone and just use a duty factor of 25% for max output.

But what about ripple voltage? Ripple is obviously zero at duty cyle of 1. So if we set the max to 1.24, we get zero ripple at max power. That's kind of nice.

On the other hand, ripple (p2p) as a fraction is, (don't read the equation, just skip to the words past it)

dVout/Vout = (1-D)*(R1+R2)/(R1*R2) *1/(fC)

This is a max at (near) 0 voltage output, and if R2 is infinite max fractional ripple is just:

1/R1 * 1/(fC)

The ratio of max fractional ripple as R2 is decreased from infinity is just the inverse of the ratio of the max output voltage.

So for 1.24V output range, the max ripple (which is at minimum output) is 4 times higher than if setup for a 5V output range! (that's too bad)

So for 1.24V output range, we get better control ripple (0) at the max light ouput, but 4 times worse ripple at lowest output (near zero), compared to using the full 5V output range. I'm not exactly sure where they cross. Easy enough to work out, just didn't do it.

This is a little significant because for 1.24V output range, and R1 10kohm and cap of 1uF, the max ripple is 2% at 20khz PWM, not soooo tiny, and it's 1% at 50% outpu and .5% at 75% output etc.

Of course we care most about ripple at max output so I still like setting max to 1.24 by adding another 3.3kOhm resistor.

I like it anyway, because it just seems more sensible and probably more matched with existing direct drive software to have 100% PWM actually be the max output, not 25% PWM being the max output.

So let's add another resistor.

Correct, I wanted it electrically connected there to help remove any EM interference but I except the diode, FET and inductor to be significantly hotter then the IC thus no reason to feed the heat that way. Plus I want the heat to radiate out towards the flashlight body, not in to the center.

So basically add another voltage divider, thats not too bad. Got that done.

I tried adding in some through holes but they are just not going to work without risking a short with the springs on the bottom. The pads on the SMD inductor are the correct distance apart, you could always just solder it directly to the pads and epoxy it into place if you really needed to but I highly doubt anyone building this will try to cheap out on that when the rest of the driver is still so expensive. Anything besides an SMD inductor will not fit in the Q8 anyways.

Latest version with added Rf3:

Great. The only things I have left for this model are reviewing the input caps again and Coff charging. My input cap loss numbers were wrong (just the percentage form, but that's what I was looking at) but voltage swing is now a bit of an issue, especially at 4:1 and a little at 2:1. With more wiggle room on ESR now, I'll want to review again if I can find bigger caps that are suitable. Ti has a document about this and they use 82uF for a 10A supply.

For Coff, it's a matter that setting it up for a good frequency at one voltage will probably make considerably unoptimal frequencies at other output configurations, but at worst it means you need to select new RC for different applications.

Both issues will work the way they are, but may be significantly improvable too. But I won't wrap up my notes on them for a few days at least.

Thats fine, I will start trying to find the large components from Arrow and we can work out the values for the caps and resistors later, those are easy.

Well yes and no.. values above 1 uf at moderate voltage have size constraints. I haven't looked beyond ceramic. The coff thing would also be a rewire, I don't expect you to be convinced on that until I convince you, and it can be a beta revision (this being alpha) if it ever happens. Again this all will work as is, and can be tested as is if it's go time.

Remind me what the coff thing was, just too busy lately to put my head into this like I should. You have been a big help, there is no way I would have had time this year to research parts selection enough to really get this project going. Still not sure how you find some of these things.

Heck I was just looking for a simple compact op-amp for a possible linear driver and was coming up with almost nothing.

It is not a major rush or anything, I am just making an order from arrow anyways and with the discount code I have it should make it reasonable to order a set of parts to give this a try. So I figured I would grab it while I can. Not worth making an order just for these parts if I have to pay shipping.

Same reason I was looking for an op-amp.

Well 95% of it has been useless lol. It's like the monkey with a keyboard thing, but I like to be satisfied that the options have been explored.

Coff is the offtime clock (buck cycle off-time, not the traditional OTC). Frequency depends on that and duty cycle (on time) which is Vout/Vin so it depends on Vo no matter what you do. But they also charge it off Vo which modifies that Vo dependence. I'm not sure that modification is useful for our purpose. Charging it off Vin or maybe now even off the more stable ldo, might give a more all-purpose setup for us. But I'll have to generate all the numbers, mostly done, but still, no time is no time.

It hits me that if we really want to nail the Iadj thing, we have have to consider the output resistance of the mcu (both high and low if different). I think it's specced at 5ma output (this might be entirely wrong/bad memory), but I don't know if it's limited to that by 1kohm ish internal resistance or if it just means we shouldn't draw more than that off of it. Using 10k ish output, of course that's a 10% correction on the voltage divider. That's not a design change though, just details of the setup to work out/refine either in tweaking iadj resistors and or rsense. Probably just things that will get fiddled with in field testing.

I guess this could be an argument for not using the second resistor (but we still don't have to use it).

Edit looks like output and input impedance are about 20 to 40ohms depening on Vcc, so no problem.

Ok, found the big components from arrow, they are actually a fair amount cheaper here as well:

Inductor: https://www.arrow.com/en/products/ihlp6767gzer150m11/vishay

Diode: https://www.arrow.com/en/products/stps30l30djf-tr/stmicroelectronics

Mofet: https://www.arrow.com/en/products/si7157dp-t1-ge3/vishay

Buck IC: https://www.arrow.com/en/products/lm3409mynopb/texas-instruments

First a minor thing: I have actually polished off some thoughts on Iadj, but no time to write it up. Basically the diode bias current is going to cause a voltage offset at high resistances, preventing to reach zero, but you get ripple using low resistances. None of it is terrible, but may we may need to live with some compromise between the two. It's mostly unrelated to the two resistor issue.

Now the bad news. For the first time I looked closer at the mosfet. That mosfet has 625nC of gate charge at 10V. I hadn't really noticed that before. That seemed a bit giant to me, and well, it kind of is. That's 63nf of gate capacitance!

Both the potential and charging energy is lost every cycle so just P=CV^2*f (no 1/2). Switching voltage is 6V, fixed in the IC (Vin-Vcc).

so 63E-9*6^2*1E6 = 2.268 W! (I hope I made a mistake here, but I don't think so)

That doesn't sound sooo aweful, but that's happening all the time. So at 2W power output, you've got 2.3W of switching loss. No fancy coff correction helps this. Coff, and frequency only are impacted by voltages (and are under our control anyway). This may finally be the reason PWM from full power doesn't work out so bad after all!

There are other mosfet losses that I've barely thought about. It seems very complicated actually and I likely won't ever find time or energy to deal with it all, but anyway this gives us a measure of at least part of it, and some relevant spec to improve.

I'll present the case about Coff when I get time to present it, but it's a triviality compared to this I think. Although the two will be very linked. This may drive the need to care more about how set the frequency.

Time to look carefully at what fets are available again. I read a doc, maybe Ti, exactly warning against overspecced large, low rdson fets, for this reason. We might need to look for something a little less beefy. However, I still probably don't much free time for a bit.

For comparison quickly the SIR404dp nfet has only 97 nC of gate charge. nfets are said to be better. Starting to see why. I might be giving that high side nfet driver another look, but not before looking for more suitable pfets.

Please let us know when these boards are available.

The PCB is basically done but it is useless without the parts list to fill it so waiting till we get that knocked out before releasing the PCB. I don’t want people spending money on something that doesn’t work, or at least have the possibility of working.

Hope we find out soon. Like!!