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

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.

Ah, that would explain why the 2.56v is used and I think we should keep using it for that reason. We don’t need more accuracy then that honestly.

Far as resistance the parasitic drain is much lower the higher the resistance goes. I think the 200k+ range would be a good goal.

Well sure it is, exactly inverse proportional. Ok, I think 11k and 220k was good when I checked. No time to run the numbers now, but something very close to that is good.

We missed one. According to your LDO TA series docs, C2 should be 10uF when using an LDO, which makes sense. This one at least is low voltage, so really doesn't need to be physically larger. This is why we waited a few days. Updates coming soon.