Even in the 10uF, selection is much more limited in 0603 than 0805.
Here is the component footprint list as it sits right now.
Cin is 1210, it could be swapped out for 3x 1206 it looks like but it is already too close to the edge for my liking. I actually want to pull it back some more in possible.
Rses is 2512 simply because it was the largest resistor I could find and they had the highest wattage ratings It could be shrunk to a small size obviously and I might need to anyways to get the edge clearance I want.
Here is an updated CF2 to 1206 (not reflected in the list). It works but is the very limit of what those component sizes can be
looks good. I was thinking you could move D1 a little down and then right (but not much) and then shove Q1 right and Cin over to the right a little for clearance. I agree on Rsense when I looked at what's available. Using two would be VERY tight. Might end up with some burned resistors. Anyway, Cin is ok with 2 by 1210. I suppose if you really needed the space you could cut it back to 2 by 1206. If left as is though, I'll try to pile a heap of caps in there with a wad of solder and see what sticks. I'll write up the baseline as 2 though.
I can connected the Cin pads easy enough if you want to try that.
I guess it doesn't matter much. They're almost connected anyway. I'm going to be worried about them bridging to Rsense though. That looks a bit tight, but it is how it is. If things are bad enough there, we can go the other direction and back off to two by 1206.
Bridging to rses should not be a problem unless you use way too much solder paste.
I guess I'm a little surprised you oriented Cf2 that direction. If there's a short to the case isn't ok the other way, but not this way? Or there's enough clearance it won't be a concern? I realize now that when I said there was room there I was looking at the old version without rf3 in it.
OK, I guess the other way takes a mm more space. I see.
I tried my best to have it flipped but it was just a bit too close. This is the best I could come up with. A 0805 pad would be much nicer. As it is, the RF3 is closer to the edge so it should hit first.
OK well this little 0805 220uF Ta cap
http://www.digikey.com/short/3bbc1d
is only 4V rated, but maybe it does better than a 6.3V ceramic. All the ceramics have C vs DCV curves that drop off as DCV goes up. The Ta's don't as far as I know. The spec sheet calls for derating it above 85C, down to 2.7V at 125C. Is the board going to be getting over 85C? I kind of doubt it while a person is holding the light at least, but I'm not sure. Anyway, we can go with 0805 and consider this cap experimental/at one's own risk. This is the only one though, on digikey at least. You know you're pushing the limits when you select all your criteria, as loosely as you can possibly tolerate, and the search box says: "1 remaining"
Ouch, at $1.75 each sticking with the 1206 seems like the better idea.
It works as it is setup now, just a bit tighter then I prefer. Once we finalize the component list I will go back over things one last time to try and get back ever mm I can of edge clearance.
Yep. Ok, I've got a mental deadline of this weekend to write up a 4p xhp35 application. It doesn't take very long now, but I should put aside a bit for other things probably until then (we'll see). Should I aim for 4*2.5=10A? Or maybe 4*2.8=11.2, living on the edge slightly? Of course that part's not a big deal, but I want to choose everything down to Rsns for one reasonable setup at least. Actually even finding some combinations of Rsns might not be so easy, so that's part of my reasoning there. Let's get at least one full suitable example down to the last resistor. After that I'll probably target a 2S xm-l2/xp-l setup, maybe 11 or 12A. I think those are the two best use cases for the Q8 version.
One thing I like on the 1206 is you can buy the same 10uF cap as for Cin and Cout if you want. You can use 5 maybe 6 of them then and it becomes almost free to go ahead and order 10. The $0.30 I like has a one week lead time but there are others for 60 or 70 cents I think available instantly too.
So, if you want to save a good bit of space, I keep coming back to that jumper and I had an idea. You can connect any steady voltage over 1.24 to pin 1 of the the IC to enable it. Which means any battery plane other than the ground plane is fine. I'm pretty sure the one directly behind it on the board is not a ground. So stick a via through to connect pin 1 to the battery pad on the other side and you remove two resistors and a big jumper, save 3 components to spec, purchase, install, and you can shove the IC waaaay out to the left and gain all kinds of space. Cf can slide out around rsense then, the Rsense can come down, the IC can come down or up a bit, a ton of flexibility opens up.
I like not needing extra resistor values anyway, ones that will need to change for 2S vs 4S battery configurations (or 1S), add more permutations to everything.
Then I might ask C1 to get bigger too... why? Just because we'll have about 5 or 6 10uF 1206 caps already, might as well buy 10 for $2 and use 7. I'm trying to simplify the parts list, but not a big deal either way obviously.
Question, what's the deal with the switch? I thought the Q8 has a switch, so shouldn't it connect somehwere? The switch pads have a cap specced, not a switch. Related then are we using 220K and 47k or 22k and 4.7k?
The pad behind the IC is a ground pad sadly and there is not an easy way to get to a positive pad without a jumper somewhere or running around the ground ring. Plus if I did have it connect to anything besides where it is now the layout would have to change for 2s2p and 4s1p setups.
The switch pads are just a pad, It is easier to set it as a resistor in the schematic so that it doesn’t fight you having it in place plus it makes sure you don’t forget it.
The MCU side of things is just like the rest of the TA drivers, it could even be used with a clicky switch if wanted. So for an e-switch it would be 220k/47k. For a clicky I actually like 19.1k/4.7k a bit better, it makes the OTC a bit more reliable.
Well it would only have to change if where it connected to became a ground. There's at least one pad that's always hot right? Looked possible to get a trace from there. Ok, well I'm probably missing something. Sure seems like a bunch of space just for a voltage.
Just noticing C1 can move left and jump the pwm trace downward. That might allow a bigger C1 (maybe not), but also opens a trace that you could run out through roff under the mcu. Loses some of the space gains.
We're already right at 3.0V for the voltage sense on the mcu for full battery, 3.06 for 4.35V batteries, so going less than 22k/4.7k cuts out voltage monitoring on the top end unless I go up to a 3.3V ldo. The 3V ldo helps the soft mode transition a little, but 3.3 isn't a big difference either.
I don't fully understand the off time clock issues/voodoo.
In 4s mode the pad directly under the buck IC is ground, in 2s mode it is hot. The only pad that is always hot is the pad behind the sense resistors. Although the jumper resistor really doesn’t take up much usable space. I still could not put anything on that side of the IC without cutting off the ground plane running under it, which is the entire reason for that jumper in the first place. It could be moved out but it would not free up as much space as you think.
C1 doesn’t need to be any larger, the normal 0805 10uf amps we use work fine for it. Although having it jump the trace is not a bad idea anyways, the ground path to it is kind thin as is (well it is actually non-existent right now due to a minor movement during the last edit).
I actually forgot about the multi-cell useage. The voltage divider will actually have to be completely recalculated for the higher voltage, I think people use 36k? for 4s? I forgot.
I meant to say bring the C1 trace out under the U1, not mcu. It's a way to get voltage there. C1 doesn't need to be bigger but we'll have 5 to 6 1206 10uf caps already so will be ordering 10 anyway. Can probably just squish one in though. I've already got 0805's so it's fine with me in any case, just thinking to simplify.
Yeah, I think R1 can be at minimum 22K but I don't know. It depends on
1) battery config
2) ldo voltage
3) software and mcu capability
I think I know mcu capability, but I don't know the software side of things as far as calibration.
For different configs we need to change:
Ruv1 (if it's used at all) , R1, Rsense, Roff.
This is what got me back on this track.
Not sure I understand, you are saying bring the trace from the via that feeds power to the MCU around the ground ring of the driver to the buck IC?
That is a not something I like doing unless there are no other options. If you scratch the masking it will short out. Plus it also doesn’t allow the ground pour to contact the body and thus transfer heat effectively.
The only other option is to run another trace on the bottom side of the driver to a via neat the buck IC. Not ideal but possible. Although I am still not sure how much usable space that would free up. That space used by the jumper is not really usable anyways, the only thing it does is allow the IC to be pushed a bit closer to the edge.
For battery side, I don't understand all the issues there with battery configs and you do so that's fine. I thought bringing it off the midline like the mcu via might work, but I don't know. I thought scratching is a risk on that side (because of bashing and spinning batteries).
The last one I was talking about was splitting it off from the hot side of C1 on the component side, and winding it between LED+ and the switch, under the around Roff and under u1 to pin 1, so not around the outside.
Ok, maybe it doesn't save as much as I think. I thought Cf could then push out behind the Rsense, minor, but all together then U1 can move out quite a bit, or out even more and also up a bit, or out a bit less and down a whole bunch (I'm removing both UV resistors too in my idea because we just don't need them, so that's two resistors, a jumper and cap gone from that space and more use of the curvature). Looked to me like in the U1 down-and-out option you have a ton of room to move rsesne down (opening some space or options top right), and probably to move roff and CF2 up then at the same time adding clearance there.
Anyway, just ideas in case they're helpful, just ignore them if they're not. If there's enough room as is, then maybe no need for any of it either.
For now though I am speccing Ruv1/2. So what voltage cuttoff do people like? 2.7V and 0.2V hysteresis ok?
I'm trying to find options to do everything with E24 series resistors, fewer values being bonus points. I'm not sure how important that is. We have to work out R1/R2 though. I have no idea how calibration works, and I'm not sure I know how the OTC works either. I don't know where standard values exist, or if they would be setup for the same mcu voltage as us anyway, which I think matters for battery sensing.