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

Worst case is early testers have to change resistors, not recompile software.

Yeah, there is firmware to support it out there but this will be running on either Narsil or Bistro. TK is MIA lately so we have to use whatever it is setup for since no one else can really edit this type of thing.

Narsil I have no idea on but it would be best to not need a separate firmware just for a voltage divider.

In the case of Bistro it uses a 4.7k/19.1K and ends up around 50% of the usable range.

Running quick math that lines up with a 1.1V reference but a 2.56v reference would have the divider be something like 4.7k / 5k. This suggests that the 1.1V reference is used and as such should be what we aim to use as well to keep things standardized.

I am a major fan of standardization, it is the entire reason I made the TA drivers to start with.

Mtn max reccommends the same for 1S. Hmm... yes that comes to about 0.86V max, 0.833 for normal cells. I'm not sure why they changed it for 4S. Must have been a reason, but I definitely get your point. This is why I wanted to really verify what's being used around here as far as software.

If we want to hit 0.833 that's (16.8-0.833)/0.833 =19.2 which gives 90K R1

If we want to bump it up to 1.0V that's 74k, call it 75 to be standard. A 68K, a bit more standard, would make it 1.08, pushing it, and over the top for full 4.35 V cells.

82k, also a bit more standard gives 0.91V, seems pretty good to me. I could go with that.

Of course standards are fine, but what standards? 36 seems to be a somewhat standard 4S resistor value. 1.1 seems to be a somewhat standard internal reference. Software standards then? Ok, nobody is around to change it now, but we will need new software for this driver. We'll want dual PWM modes. I guess you've been playing with modes, so if that's something you can handle then great.

Anyway, 82k is fine with me if you've got a way to make it run.

Indeed I thought mountain uses bistro, but not on that driver. Just guppydrv and their own software.

But what about calibration. Do we have people responsive who can deal with that? If not, we better go with a 91K to keep even the calibration the same.

another issue, so it's not 90, it's more like 900. At such high resistances I get worried about stray current through places it shouldn't go. 10Mohm becomes an important path that level. So I read the manual and it says ADC inputs are subject to the input leakage current of the pin... which elsewhere in the manual is specced at 1uA. So that's 6% of our divider current. Ok That actually should be ok. But it will reduce the voltage by 6%. So much for using existing calibrations. Enh, it might be less than this since it's only 1V on the pin, not 3 or 5.

Still I'm a bit tempted to lower R2 to around 1K. 18uA of current makes me slightly nervous.

Yeah, an 82k sounds a lot better.

Recalibrating it is easy, anyone that can compile the software can do that. You simply flash TK’s calibration firmware and read off the numbers it spits out at a few voltages, feed those to the script she made and boom, you have your new calibration. Takes a little time but not hard to do and no coding expertise needed.

Far as firmware for this driver, it will run on the standard Narsil / Bistro firmware no problem. You will simply have to play with the PWM and modes a bit to dial in everything but once again, just a matter of playing with numbers, no actual coding going on.

Ok, I spent the morning re-working the driver for what is hopefully the final version.

I was able to increase C1 to a 1206, removed the jumper and generally compacted everything for as much edge clearance as possible. Still not a ton though, only about 3mm at the closest points but that is just enough to clear most of the SRK’s I have laying around.

If this looks good to go then I will put together a completed package along with a shopping cart link and people can give it a try at their own risk.

So what kind of voltage/amperage ranges is this capable of?

Nice work, a good looking board. I'm a bit surprised you left the Ruv's in. It won't hurt though. I guess you're worried we won't ever figure out R1? It defeats a good bit of the space saving though. I will try to dig through source and code and attiny manual code and if I can get to the bottom of the R1 thing.

Might reach 15A for some seconds. 0.6V dropout should be achievable at that current level in theory at least, for a perfect battery of course. The driver is capable of opening up to 100% duty cycle, basically direct drive, but with a bit more resistance from parts. It can take 16.8V input.

So I'll work on the last changes to the shopping cart soon, but what about about heat sinking? There is no exposed copper on the edge. I don't have an SRK. If the copper there was exposed could it be made to contact the case? Maybe with some thermal grease at least?

I left then in because taking them out didn’t free up any edge space and I am always a fan of cramming every possible option into something if there are negatives.

Look it over carefully, I will give it a day or 3 to let us all figure out any possible issues and then I will put together an official release.

Yeah, I was debating this myself. Obiously we don’t want it grounded to the case and shorting out but as long as the tailcap side is not shorted to the case then it should be fine. It is easy enough to expose the copper on the edge like normal.

Can't the outer edge of the tailcap be ground? connected directly on tail-cap side to ground pad, and by vias to outer ring on component side? Ground touching the case should be fine, right?

Ground touching the case is fine, the issue is anything from the tailcap also touching the case will cause a short.

As long as the tailcap is isolated by using the matching multi-cell tailcap properly setup for this driver, it should be fine.

of course I meant tailcap side of this board. I think you got that. I think we want all the heat sinking we can get.

I agree, I will take care of that before releasing it.

I found this in bistro:

    // 1.1v reference, left-adjust, ADC3/PB3
    ADMUX  = (1 << V_REF) | (1 << ADLAR) | CAP_CHANNEL;

And 17.13.1 of the manual explains the bits clearly, so:

ADMUX =(1 << V_REF) | (1 << ADLAR) | CAP_CHANNEL | 1<< 4;

Would set the 2.56V reference for attiny 25 and attiny 8, but really should add a define, and might be different in attiny13, and needs changing in a couple of places and might interfere with temperature readings (but I don't think so, it's got separate code), etc... So doable, but not quite just flipping a switch.

Anyway that line and the details behind it confirms that the way things are setup now is for 1.1v, so I agree, that's what we should use for now at least.

The only question then is what total resistance to aim for for acceptable drain current. is 95k enough or should we double R2 and bring it all up to the 200k range? I don't like 900k.

And the reason it's setup for 1.1V is 2.56V cannot be used if Vcc drops below 3V. So 1.1V is more universal.