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

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Flintrock
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Updated cart in post, I think 236.   I think it's all set now.  That one-click BOM tool is really neat.  I really recommend giving it a try.  It's worth 3 minutes to read the instructions.  The authors todo list says they are working to support arrow too.

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Alright, last last version before release unless someone spots an issue (better now then later).

Gonna check over the parts list and put together a direct shopping cart link.

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Post 236 is my own, where is the latest parts list? Might want to copy and paste it here anyways so we can see it and the latest PCB side by side.

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oops... 326:

Flintrock wrote:
Here goes.  Hopefully I didn't mess anything up.  Your risk, not mine.  Only buy and build what YOU want.  I have nothing to do with it. Smile Parts list for 16.8V input, 12V nominal output, driving 4p xhp-35 at 2.5A per LED, 576 khz, 10A total (can go higher, but it’s an experimental driver) The main parts:MCU: Attiny85 the normal one, attiny85-20SUR I believe is usually fine.  V version is also fine I think. (However if driving with 4.2V an attiny85V should probably be used because this driver presently requires an LDO  for the mcu even for 1S batteries and the LDO will need to operate below the lowest battery level, needs more thought though and wil require some resistor value changes) LDO: MIC5235  3.0V (other voltages CAN work, but require some resistor value changes)Buck IC: LM3409  10-MSOP-PowerPad (or EP)Inductor, must be 17.15*17.15mm or very close. 10uH absolute minimum inductance.  30mohm max resistance. Vishay IHLP6767GZER150M11 is a good choice since resistance matters most.  It's very expensive though, $6!!, but other alternatives are about the same. Diode, "8PowerTDFN" (might be compatible with similarly named footprints?)  STPS30M60DJF-TR has a low 0.5V Vf and low leakage current when hot.  Mosfet:Powerflat 5x6 p-channel FET (PFET), Infineon BSC084P03NS3 G  Balances low Rdson with low gate capacitance. Gate charge divided by voltage should be less than ~12nF at worst. Half that is better. Rds on should probably be less than 20mOhm at worst.  Low Rdson is good for high power.  Low gate charge is good for low-current efficiency.  It's usually some tradeoff. Caps: C2 10.uF 0805, Vcc bypass 4V minimum, 6.3V or more highly preferred,  10%  Cf 1uF 0603, Gate drive bypass, 16V 10%C1 10uF 1206 , LDO input bypass, 35V 10% Cf2 10uF 1206, Iadj voltage filter,  4V minimum (6.3 preferred). 10%, x7r preferred  but 125C rated should do.        Alternative: 220uF for soft mode transitions. Tantalum and high ESR (ohms even) are OK here.Cin  10uF 1206 or 1210,  Input cap,  35V 10%, two minimum, 3 x 1206’s should fit.  4 might fit and is even better. <20% dissipation factor at 1Mhz, 10% prefered.Co  10uF 1206 or 1210, Output cap,  35V 10% One is enough but two is insurance.  <10% dissipation factor (5 is better), with good performance up to 1Mhz.            Just buy ten 1206 uF caps and you’re set.  Cin and Co should be high quality caps. Coff   470pf 0603 35V  Buck off-time cap.All caps should be ceramic unless stated, X7R or better strongly preferred (125C or higher rated) .  Resistors: JMP 2010 0-ohm 1/10 W is fine. R1 220k 0603 for e-switch 22k for clicky maybe.   (with OTC mods now under development, the clicky issue will dissappear)R2 12k 0603 for e-switch, 1.2k for clicky maybe.R5 4.7 ohm 0603 1/8 to 1/4 watt might be wise here, but 1/10 should be ok I think.BR  for lighted tailcap  470-630 ohm so I’ve read, optional.Rf2 3.9K 0603  Rf3 2.7K   0603 (Could use 4.7k and 3.3k also for a little softer mode transitions with the 220uF cap)Roff  4.7k 0603 with the 470pf Cf, this will give very roughly 580 khz, which seems ok.Rsense, 2512, current sense resistors, X 3. 0.074 ohm each, 2W rated, adjusted for max current, see below.All resistors should be 1% tolerance, rated for high temperature, 155C preferred, 1/10 W or more unless specified RSenseMax current occurs  at 0.248V drop across Rsense.So Rsense_total=0.248/Imax.But there are three resistors, so if they are the same, they should each be:Rses1=3*0.248/Imax.  Err on the high side of resistance values to be safe.But no software adjustment can ever make the current higher than the max value set here. Ex:  2.5A for xph35*4= 10A total.Rses= 3*0.248/10=0.074 , closest match, 0.075 ok. Roff:To be written later. Iadj:Rf2, Rf3, Cf2, LDO voltage, and a pwm duty cycle all determine the current set point.  Only duty cycle should really need to be adjusted though (through software modes) for current control.  Max current is adjusted with the sense resistor, not with this.  Of course software could limit the duty cycle, but that’s not the safest approach to limiting current. Soft mode transitions:To achieve slow (~0.33 s RC) you can use a Cf2 of 220uF and adjust speed by increasing or decreasing Rf2 and Rf3.  Some options are:Rf2: 3.9K,  Rf3: 2.7K (flintrock's favorite)  ~0.35s RC timeRf2 4.7K,  Rf3 3.3K  ~.43s RC time  requires one less new value.  Much larger and the current control circuit gets an offset, but it should be < 0.7% offset with these values still.  Twice as high could still be ok but I think these RC values should feel nice.  PWM will be used to get lowest modes anyway so a small control offset is ok.These are chosen to prioritize resistors, but either matches the max control voltage to better than 1.2% after bias current correction.   For best precision you can use a 1.02K and a 715ohm, but won’t get very soft mode transitions and probably won’t notice the precision improvement.  If you’re using a 10uF cap anyway, then you can as well use these lower resistor values. LM3409 low voltage shutoffRuv1, Ruv2 are the voltage divider for buck IC shutoff.  This is redundant with MCU shutoff.  You can leave off Ruv1 entirely and use pretty much any value at all for Ruv2 then. This will disable this feature. You can also use Ruv2 36k and Ruv1 4.7k for a 2.7V/cell shutoff and a 2.9V turn on. The cart: Updated with one improved resistor pairing.  Digikey carts will aparently expire and they only work on digikey. This is better: References Quantity digikey Description U1 1 ATTINY85-20SURCT-ND IC MCU 8BIT 8KB FLASH 8SOIC LDO 1 576-2783-1-ND IC REG LDO 3V 0.15A SOT23-5 U2 1 LM3409MY/NOPBCT-ND IC LED DRIVER CTRLR DIM 10MSOP L1 1 541-1287-1-ND FIXED IND 15UH 14A 14.4 MOHM SMD D1 1 497-12421-1-ND DIODE SCHOTTKY 60V 30A POWERFLAT Q1 1 BSC084P03NS3 GCT-ND  MOSFET P-CH 30V 14.9A TDSON-8 C2 10 1276-2872-1-ND CAP CER 10UF 16V X7R 0805 Cf 1 311-1446-1-ND CAP CER 1UF 16V X7R 0603 Cf2,Cin,Co,C1 10 1276-6767-1-ND CAP CER 10UF 50V X7R 1206 Cf2 alternate 1 490-13970-1-ND CAP CER 220UF 6.3V X5R 1206 Coff 10 1276-1094-1-ND CAP CER 470PF 50V X7R 0603 JMP 1 YAG3380CT-ND RES SMD 0.0 OHM JUMPER 3/4W 2010 R1 10 311-220KHRCT-ND RES SMD 220K OHM 1% 1/10W 0603 R2 10 311-12.0KHRCT-ND RES SMD 12K OHM 1% 1/10W 0603 Roff 10 311-4.70KHRCT-ND RES SMD 4.7K OHM 1% 1/10W 0603 R5 10 RNCP0603FTD4R70CT-ND RES SMD 4.7 OHM 1% 1/8W 060 Rf2 10 311-3.90KHRCT-ND RES SMD 3.9K OHM 1% 1/10W 0603 Rf3 10 311-2.70KHRCT-ND RES SMD 2.7K OHM 1% 1/10W 0603 Rsns 3 696-1670-1-ND RES SMD 0.075 OHM 1% 2W 2512  The 10 uf 1206 Ci Co caps keep going on backorder.  That's because they are cheap and seem to be really nice caps.These are an alternative, at twice the price, and slightly worse ESR, but still good and maybe available much faster: 1276-3103-1-ND, or 1276-3102-1-ND USING THE SHOPPING LIST:You can installhttps://1clickbom.com/  in firefox (doesn't work as well in chrome)And cut the table above and hit paste in the browser tool, and "complete" (hit complete several times actually, waiting for it to update between each, seems to help) to get shopping carts at all digikey, mouser, Newark, Farnell, (no arrow :() .. Click the add to cart icon on each to automatically create a shopping cart on any of them.  Click "copy" and you get a table with part numbers from all of them.  It's pretty neat actually.  Of course a few parts are missing at mouser etc because these were chosen at digikey. Anyone clever might let this simmer a couple of days.  TA please give it a good looking over. Also anyone else with expertise on the MCU side. Updates, changed C2 to 10uf  and now 0805, as needed for LDO.  Chanted R1 and R2 to work with 1.1V reference as now standard in bistro.

 

but I don't want multiple places to maintain it.  If I make changes, I will make them there. 

Here's the shopping cart as generated from copying that table into one-click BOM.

http://www.digikey.com/short/3bvw7p

 

edit:so the quoting is terrible.  Just see post 326.

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I was just comparing prices at Arrow and Arrow is a fair amount cheaper on everything. The only issue is they only sell the FET in 5,000 increments. Too bad since it would knock off at least $5-8 if we could get everything from them.

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Could get this one then:

https://www.arrow.com/en/products/search?q=+TPN4R712MD

 

It will have twice worse low power efficiency (maybe that comes to 0.4W minimum power if I remember right) and half the high power loss.  

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Flintrock wrote:

Could get this one then:


https://www.arrow.com/en/products/search?q=+TPN4R712MD


 


It will have twice worse low power efficiency (maybe that comes to 0.4W minimum power if I remember right) and half the high power loss.  

Sadly that one also appears to only be sold by in lots of 5000.

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ah, missed that.  

I updated post 326 with more clear instructions on Ruv1/Ruv2, assuming LVP in the IC won't be used for now. I've realize that if used, this needs to be setup for very high hysteresis, so if it turns off, it stays off until a power cycle, and furthermore you would need to have low to high mode order to have any hope of getting it to come back on.  I guess the same is true for the mcu LVP though. 

 

I also added some words about Coff, but I'm kind of thinking now any attempt to give math or explanation for tuning any of these things is probably more information than most people want to even see.   Edit: Created an "adjustments" section for Rsense and soft transitions, and "expert tuning" section for everything else.

 

I will probably soon work up a calculation sheet with Vf correction (because it matters near direct drive) specifically for this build, just for fun.  It could be good to start adjusting the OP, but wouldn't hurt if I had a second post.  I don't know if it's possible for mods to insert a post.  I doubt it (would change all post numbers).

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Question:

How does the bleeder resistor situation work for this board?  For a typical board, it bleeds current from batt+ to case "ground" and that case current returns to batt- through a tailcap led.  However on this board the bleeder resistor connects directly from the batt+ contact to the batt- contact.  It's just a battery short.  There is no case path in the middle.  I haven't looked yet, but I wonder how this works on the Texas Avenger too.   

 

I haven't looked at how a lighted e-switch would work.  It's a momentary so it won't short out an led like a tail cap switch will.  I guess they're either always on or they'd need software control, but just speculating.

  

One thing I realized when working out new values for th Ruv1 and 2 is that the pad the via comes from not only never goes to batt-, but it's always at 8.4V above batt- in both 4S and 2S.   Because of that the voltage divider (if ever used at all) would never need to be reconfigured. I never noticed because I wasn't considering to leave a voltage divider on anyway, and I'm still not sure I like the idea of pulling current (voltage is fine) off a middle voltage.  It could unbalance cells over enough months.  It could be something to have in mind though, maybe even for R1 for other SRK boards.

 

 

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The bleeder in this case is not needed and most likely would not be used but there was room for it so why not. It could also be used for an indicator as well.

Although in this case you are correct, it is simply a short since the tailcap is not connected to ground. You could still wire it up but there is no need as you could directly tap the positive and ground at the tailcap. In fact I might add this to my tailcap MCPCB come to think of it.

In this particular case it’s most likely use would be an indicator LED installed near the switch for when the light is off. Something that could easily show you the button location. In fact I really like this idea and will have to see if it could be implemented on the BLF GT. Finding the side switch to turn a light on is always the hardest part.

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The most important thing for the average person when it comes to the component list is an easy to follow list for 2s input and 1s or 2s output along with 4s input and outputs of 2s and 4s. Those are the most common uses I see.

The only real change most would make besides that is the sense resistors, which a simple to follow calculation or a simple list would do for that. Anyone building one of these should be able to figure out what they need with a few reference points or a basic calculation.

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Texas_Ace wrote:
. You could still wire it up but there is no need as you could directly tap the positive and ground at the tailcap. In fact I might add this to my tailcap MCPCB come to think of it.

 

That's actually the direction I was going with it in my head, in almost the same steps.. yeah, could wire to tiny 0603 pads, but, why not...

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The thing that seems slightly harder with e-switches  is, well, with a clicky the switch shorts the tailcap light and turns it off.  With a momentary, it doesn't.  So if you want it to turn off when the light's on, I guess you need another control mechanism?  Of course it could just stay on.

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Flintrock wrote:

The thing that seems slightly harder with e-switches  is, well, with a clicky the switch shorts the tailcap light and turns it off.  With a momentary, it doesn’t.  So if you want it to turn off when the light’s on, I guess you need another control mechanism?  Of course it could just stay on.

I figure the .05ma that the indicator LED would use pales in comparison to the 12,000 that is being shined out the front lol.

There is a spare pin available for a controlled indicator LED but why not have options? Plus it is easy accsess to both positive and ground for anything else that might be needed.

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Texas_Ace wrote:
Flintrock wrote:

The thing that seems slightly harder with e-switches  is, well, with a clicky the switch shorts the tailcap light and turns it off.  With a momentary, it doesn't.  So if you want it to turn off when the light's on, I guess you need another control mechanism?  Of course it could just stay on.

I figure the .05ma that the indicator LED would use pales in comparison to the 12,000 that is being shined out the front lol. There is a spare pin available for a controlled indicator LED but why not have options? Plus it is easy accsess to both positive and ground for anything else that might be needed.

 

I guess it just seems less slick, but it's not really.  Always on is fine.

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Texas_Ace wrote:
You could still wire it up but there is no need as you could directly tap the positive and ground at the tailcap. In fact I might add this to my tailcap MCPCB come to think of it. In this particular case it's most likely use would be an indicator LED installed near the switch for when the light is off. .

 

So actually I was thinking a + pad next to Rsense on the driver board. Slightly dangerous for case shorts.  Could put it after (right) of rsense, but it gets in the way of the star of Texas, or could make a hole. The tailcap won't actually have direct access to batt+, only to 4.2V in 2S and to 4.2V and 12.6V in 4S, and the quality of the ground connection depends how well the ground ring gets connected to the case.  Also ins't it easier to wire the switch light from the driver compartment than the tailcap?  

 

 

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I will be honest, I have no idea what you are talking about there. Talking about an extra + pad added?

Far as a lighted tailcap, it would only need 4.2v anyways.

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Yes, we were talking about where to get voltage from to wire an extra circuit for a light.    You can add a plus pad on one side or the other of Rsense and could piggy back on the LED - pad.  

 

I thought the SRK lights usually go at the side switch so you'd need wires there somewhere, and include your own resistance inline on your wires probably or at the light.

 

Right for a light at the back end 4.2V would do it.  It would unbalance cells though over the course of a some months, which could then result in cells getting over-depleted even with LVP engaged.  This is ultimately an issue with the 1-batt mcu idea on the avenger too.

 

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For the e-switch that is what the switch pads are for. For an LED output there is the pad next to the MCU for control an indicator LED and the bleeder resistor gives options for a uncontrolled LED. Not sure there is a need for anymore?

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I see.  I wondered if it could power through the switch contacts.  Didn't know if the mcu put out enough juice.    The BR pad is a bit small, but yes, it works. 

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Oh, power the LED through the switch wires? Hmm, not sure about that. Kind of doubt it. Generally you run the LED to another pin anyways on both the switch and PCB, which is what the pad next to the MCU is for.

Come to think of it, there is enough room to enlarge the BR, so I might as well do that, either 0805 or if it will fit 1206.

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Texas_Ace wrote:
Oh, power the LED through the switch wires? Hmm, not sure about that. Kind of doubt it. Generally you run the LED to another pin anyways on both the switch and PCB, which is what the pad next to the MCU is for. Come to think of it, there is enough room to enlarge the BR, so I might as well do that, either 0805 or if it will fit 1206.

 

Yeah, but if that's the reason for enlarging it, there are probably easier places to make a big batt+ pad.  Either way though. On the other hand if you forget the ground pad on BR, there's even more room to just get a hot.  It's all good.

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Oh my shopping list still has a JMP in it I think or at least my text does.  At some point I thought I should remove it, then realized, one jumper is needed for the 4S battery connection anyway.  Not sure if that's still a good size, or if wire is better anyway.  Wire is surely fine, so it's not a big deal anyway you slice it, but we could include a few reasonably sized jumpers in the list.

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Flintrock wrote:

Oh my shopping list still has a JMP in it I think or at least my text does.  At some point I thought I should remove it, then realized, one jumper is needed for the 4S battery connection anyway.  Not sure if that’s still a good size, or if wire is better anyway.  Wire is surely fine, so it’s not a big deal anyway you slice it, but we could include a few reasonably sized jumpers in the list.

Yeah, I need to check and see what size is needed to make the jump effectively. Although basically anything will work, the ones you listed will surely be large enough and if cheap they should be fine.

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 $0.14 each and $1.20 for 10.  I wouldn't call that very cheap.  1206 are $0.24 for 10.

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Just checked it and the 1206 should work fine, or 1210.

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I bought one of these as a fairly random impulse, not necessarily greatest or cheapest:

https://www.amazon.com/GEREE-Continuously-Adjustable-Step-Down-Transform...

It's got the d2pak or something similar FET and diode. Those are 470uF electrolytic input and output caps, however the output is shorted with some sizable SMD cap, most likely ceramic. That seems to be a common practice power through output transients while maintaining low ESR of the ceramic.

The input cap is shorted by an "M4" diode (you can see it on the bottom of the board). That's interesting. It seems to be rated for 1A and basically shorts reverse polarity input to ground. However I don't how that wouldn't fry in a couple of ms, so I'm not sure it really serves as reverse polarity protection.

The whole thing is 26mm by 60mm including pots, terminals, and the giant caps, however it's a two sided board. Of course there's no mcu. Or I don't think so. There is an IC, that I'm fairly certain is a buck controller. I've tested it so far on a wire short, and it was horribly inneficient, (like 10% efficient) but that was waay into DCM and way below the diode Vf. The regulation control got very wonky down there too. I also tested it at 2A output to an XM-L2. It was 87% efficient, for input anywhere from about 4.5V up to 17V, and seemed to control very reasonably. The input power really didn't change. So it seems to work ok for that.

For the wire short I pushed it up to 10A for awhile and it handled it pretty well inspite of that huge inefficiency. I didn't have a chance to see how long it could keep it up or really measure how hot it got het. Of course that wasn't much actual power, I think 7W total power as recall, about 0.7 output. I don't have any good way to test it at high power at the moment. On the other hand most of the losses depend more on current than power.

It would kind of be interesting to see what other cheap similar sized bucks are capable of. Of course as for cheap, they have the advantage of mass production. I'm kind attracted to some of the simple analog controls and self oscillating designs just for the simplicity of it, but it's not clear how much cheaper any of it really is. You still need a good inductor, good fet, good diode, and for anything this big, of course the big board. There are some voltage regulated designs that drop the sense resistors.

It's hard to see how to make anything cheap on a one off DIY scale though. Of course the board is nearly half the price for one-off. It can be worth thinking about what it costs to make 100 though and maybe in some design MTN would do it eventually so it's not only one-off costs that matter. Boards get 3x cheaper automatically when you build more than three at a time, and many of the components get cheaper if you build 10. It might be possible to build 10 for the price of three. This is not necessarily meant to be correlated to my thoughts about simpler designs.

 

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Anyway, I'll update the jumpers in the cart shortly.

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Hmm.. I suppose with protected cells or a fused input supply, that input-shorting diode, rated to 30A for a couple of ms, would likely give enough time for the protection to kick in, and may prevent frying the IC in that case.

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I have a few buck converters like that. They usually have ripple and are not that great but work fine for what we do with an LED. How they are able to do that so simply is what I don’t know. Same for flashlight drivers, not sure how all these buck drivers you see work so well and are so cheap. I don’t see much we could really cut costs on except the inductor.

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