Ok, I had a few mins to knock out some updates.
The D2pak footprint is HUGE, almost as large as the inductor! It will not fit easily at all. I was able to fit 3x 2512 resistors in just over half the space of a D2pak.
I added more input and output caps in 1210 sizes. Obviously traces are still way undersized, I will fix that last. On the input caps, should they be before the sense reisstor like the datasheet shows or after so it is closer to the mosfet?
very neat:
http://www.nxp.com/documents/data_sheet/BUK7105-40AIE.pdf
current sensing mosfet. It puts out a tiny proportional current that you pass through a sense resistor. The method is described here among other places:
Very unfortunately, I haven't found one in a p-type. There might be some options for using it in n-type, constructing one etc, but probably not one-stop solutions :(.
Nice,
The one in the data sheet is right. I was confused on this briefly, but this sense resistor can't be shielded from the switching ripple. It needs to sense the ripple. Looks like you have it as compact as it can get there.
This sense resistor thing is a pretty big deal too. Looks like you followed that based on those three big sense resistors. That's another 4 watts and another big heat load to clear. Those d2paks are big for a reason (somehow I was confused they are bigger than I though). Big heat in small space though, we'll need some clever sinking.
Maybe you can make the inductor pads super huge at some point with some through holes in the middle to test unshielded inductors. I guess that's a 17mm inductor yes? I'll have a look at it all more closely soon.
The input caps right now are before the sense resistors, it seems like that would shield the resistors from the ripple?
Yeah, I thought about some through hole for the inductor (yes, 17mm, with all the new parts 22mm ain’t gonna fit) but got to remember that in a SRK the other side of the board is the battery positive so can’t really have anything sticking though the PCB for fear of shorts or interference.
I wondered about the battery interference,ok. Well would just have to bend the pins and solder it on the surface.
The resistor is supposed to be between the cap+ and FET input as in your drawing in OP. That will put it directly in series with the FET with nothing between the two. I think you've about got it. I'd really like to see the sense resistor gone even more than the diode, but it might not be practical with this IC. This seems like a pretty valid baseline implementation of this IC for crazy high power.
It's going to be a bit tricky to layout part selection rules for people. This constant off time means frequency changes with voltage for a given set of parts, which isn't necessarily a good thing as far as I can tell from my numbers, so may want different coff RC for different voltage output application. Also will want different sense resitor values depending on max current capability of your setup and priorities (use full sensing range for best control, but go lower to reduce heat and aim for extended xph35 turbo).
Getting these things wrong in hardware and/or software will destroy led's. This isn't like a 4.2 V direct driver, it's easy for it to kill pretty much anything we hook it up to (that should be a selling point around here :) ), possibly even xhps. This is something to keep in mind testing it. Having some good high power diodes (zener or whatever) for testing would be useful. It needs testing at the right voltage output, not just current.
oh, do you have a wire from the MCU to the EN pin? I think it it's good to throw it it for PWM. It's funny the doc talks about using an extr fet for PWM without an output capacitor on the buck, for higher pwm speed than EN allows, had me worried, then I realized they're talking about 20khz using EN and 100khz using the FET. So we're ok with just the EN pin.
Another thing, I'd rather have 3 input caps and one output, but it looks like a bit of a rearrangement. Looks like you had room for the output caps anyway, and two on input is still ok.
I forgot about the EN pin but that will be easy, I already see how I will do that.
The sense resistor I was not worried about, it was the input caps that I am not too sure on. Right now they are before the sense resistors per the datasheet but it sounded like you wanted them after the sense resistors as close to the FET as possible.
As it is, it should shield the sense resistors from the ripple a fair amount. It is easy to swap around, simply change the schematic in diptrace but was not sure what it was supposed to be.
I think it's right the way it is. The caps shield them from ripple voltage (it shields everything from ripple voltage) but not from the FET current. When the fet is closed, there's no current in that resistor. When it's open, there is, so it's getting that pulsing, and the sloping on current, which it should. It's in series with FET with nothing in between so it's got FET current. The battery side of the cap has current that's maybe more constant, as it recharges the cap during off time. Don't let me steer you off the baseline design too easily. I like making improvement, but changes from the baseline need a couple of days to digest. In this case it was just a momentary confusion as I didn't realize how this IC does current sensing.
I do want to review all my cap math in one sitting at some point. It's been strung out with parts selection in between and everything and a some changes, so yeah. Actually I don't think I've really thought through the input cap math at all yet, mostly just refined the output cap math. If you want to pull the trigger on a board order once you get the PWM wire in, it will probably work, but I will probably ponder it all one more time and generate a new set of numbers including sense losses, and higher power scenarios too.
Yeah, I won’t be ordering it for a little bit yet. I plan to get one when I make an order from a china house as it is WAY cheaper for large boards but I won’t have the funds for that for a little while (yeah, $10 is hard to come by lol).
I am just trying to get done what I can, when I can.
You could post it up though as highly experimental anytime. Some wildcatter might take an early stab at frying some leds with it.
I guess the right thing to test it on at first is 4S xp-l. I don't know if that's a reasonable use case, but they should handle full voltage without dying... maybe.
Yeah, once we are happy with the design and we do not think there is anything else we can do without actually trying it out I will release everything like normal.
The cost is a turn off for me, I just don’t see me building these for lights that cost as much as the driver itself. I like modding but I am also practical.
That said I want to see this come full circle, I hate unfinished projects.
More exploratory stuff:
http://www.linear.com/product/LT1910
Can drive the n-channel current sensing mosfet, but needs 8 V. The sense resistor there seems uneeded. It's just an overcurrent protection, redundant.
A bit complicated, but those three sense resistors take a bunch of real estate. I'll keep looking for better ways.
http://datasheets.maximintegrated.com/en/ds/MAX1614.pdf
takes 5V, need to look at the details more though. There seem to be several of these available.
Another option is just to turn the whole buck loop upside down and low side switch it. Hang the diode and the cap off Vin, put the inductor and the fet on the ground return, (some Ti drivers work this way by default), have to think about how to charge coff.. still have to flip the gate output though I think, so still need a small fet for that, but no charge-pump controller, big re-arrangement.
I'm kind of liking this though. Why wouldn't this work? In terms of components, just replacing three giant resistors with a level-reversing transistor on the gate drive, and using the current-sensing n-channel fet. Seems like a win. What am I missing? Maybe the level reversing isn't so easy. It's not like an attiny pwm, we don't get to drop the whole controller reference voltage.
I have been known to fry a thing or two. I’m game when ready.
How do you want to run it? Can you deliver 4S battery to it and wire 4S xp-l/xm-l led's? I'd run it 4S battery and 2S led personally for xp-l/xm-l, but it's more dangerous while the bugs are being worked out (because if it opens up too much you're putting up to 16.8V on 7V worth of leds). 1S to 1S might be reasonable too. Some of the components should probably be tuned to your application, especially the sense resistance and maybe RoffCoff.
I just used my quad xpl hi but will order another one. Will PM TA for host advice.
Just saw that current sensing MOSFET is 16mm x 10mm. Still, it would replace the existing fet and the three big resistors, but needs a couple of small components to go with it too. They're also $3 !
Well I guess an SRK with the SRK tailcap (more money) is the best. I don't know what TA has done with the battery contacts here but I guess it should work with the tailcap board to be configurable in 1S 2S or 4S battery input. Of course the BLF Q8 will be the best.