This only seems to have applied to old versions of the 45.
So what footprint are those small pads meant to be? They measure .11*.04 inch by my pixel count. Seems kind of a compromise of anything from 0603 to 1206. Anyway, there's certainly room to widen Cf2 out to .06 wide.
I forgot to redo my cap searches with lower voltage ratings for the control caps. That turns up 220uF caps. There's one that's a $1.74 on digikey in an 0805 but it's only 4 volt rated. For 3V low current, that would probably just about cut it. There are also a couple of in 1206 that are 6.3V rated for $1.26.
A 220uF Cf2 makes it possible to hit Iadj RC times of 0.33 s or a bit more, ie soft mode transitions, with only a small increase in resistance, leading to an 0.6% minimum current mode instead of 0.2. I'm going to squeeze this on mine. I'll might write it up as an alternative setup, because you can get 10uF caps for 0.30 or maybe less, and they work fine too. The dollars add up on these things.
(The 220uF also makes software frequency control with 0.3% offtime jitter possible)
I ran numbers on a 1S batt setup for fun, with a vf curve added since it matters there. The thing actually makes a nice 1S driver. > 93% efficient on paper down to 0.5A or less. In the middle modes, 0.5A to 1A, linear is only about 70% to 75% efficient, so that's not bad. 93%/75% = 1.24. That's a pretty big savings if it works out real. The reason linear has been considered ok, is because its only competition is PWM'd DD. And because, as you've pointed out, it makes light come out of the front end of the device, which is the main thing.
All of the small pads are standard 0603 pads, you can put 0805 components on them if you need to but better to have the correct pads.
If CF2 is the only one that would need a 1206 that could most likely fit but the rest are already as large as they can be and maintain any kind of proper clearances.
That's the only one. Well.. I'm aiming towards 1206 now for Cin and Co and I personally will squeeze as many 10uF caps in Cin as I can, certainly 3 will fit, just maybe 4. I'll write it up for the designed way, plus some comments. If you want to draw it as 3*1206 that's fine (not sure you can really draw it as 4). Or could just make 2 solid rails and be done with it.
The Rsens is what 2010? Updated: Ok, this appears to mostly be what's available. and yes, 3 seems right unless we want to push things very very close to burning up. So I'll write something up.
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.