In the last episode of RageBridge, I redesigned the logic regulator section of the board in order to make it compatible with a wider range of low input voltages. The current logic power architecture is a single LM2594 buck circuit that supplies the gate drive circuitry with 15 volts, and then a linear regulator to the 5v microcontroller & peripheral logic and receiver power. The issue was that this setup limited the minimum voltage of the input to be around 13 volts – too high for 3S lithium/12 volt systems which are still very common – because the gate drivers shut down at 10 volts. I determined that a better architecture would be to convert down to 5 volts first, then boost from that up to 12 volts. 5-to-12 converters are very common, as are switching buck regulators for 5 volt systems – hell, even the LM2594 is $2 cheaper in small quantity if I only want the fixed 5v form.
Another approach which I settled on after deciding it was at least worth trying is using an integrated buck-boost chip that could handle a very wide input voltage and still give me 15 volts out. This would have necessitated changing the power regulator chain the least, so it was the solution I favored first. After a long period of staring at manufacturer datasheets for relatively simple self-contained buck-boost regulators, I picked the LT3433 to experiment with. It was supposed to be this super magic one-chip solution to the issue, and would allow me to keep the 5v linear regulator (since it could, in principle, generate me 15v from anything between approximately 8 volts to 57).
That’s the board design I sent off to MyroPCB to fabricate. Only 4 this time, though even that was kind of overboard… I figured if it did work out, I could assemble more of them as needed.
2 weeks or so passed, and I received the boards:
Between the last board revision and now, I discovered how to make Eagle ‘pave over’ vias with solder mask. The technique is generally known as ‘tenting’ the via, and its main goal is to prevent exposed vias from wicking solder away from nearby pads (in a via-in-pad situation or via-kinda-awkwardly-hanging-out-near-pad situation if you’re me) or being another source of potential shorts. Turns out there’s a DRC setting for that. Hence, I basically set the Limit under the Masks setting to juuuust bigger than my standard signal via (16 mils). All of the little layer change signal vias were therefore tented, and the whole board just kinda looks better as a result.
I assembled two of the boards for starters. 3 is probably better to reduce the chances of assembly errors and flukes making it through, but I again ran into the trouble of running out of critical components. In this case, it was the 16mhz ceramic resonator and 30A current sensors.
The LT3433 was supposed to make this easy. And as far as I can tell through much testing and component replacing, the chip is working just fine. However, it is not as magic as I had hoped.
The symptom was that while the board by itself worked fine between voltage of about 7 to 55 volts, once I connected a receiver or fan to the thing, I could only get down to about 16 or 15 volts before the gate drives dropped out completely, and the 15v rail began dropping in voltage even up to 22 volts. This was worse than the previous configuration!
I used a Spektrum BR6000 receiver known for being very power-hungry for most of the testing. Maybe it’s not realistic of a receiver these days load-wise (this thing drew, by itself, upwards of 100+mA), but Spektrum receivers are still very common and I’m not sure if they’ve gotten any more efficient. Regardless, if the power supply is unstable supplying just a receiver or fan, it’s definitely not going to work for both at the same time. A Hobbyking receiver, which draws less current (50 or 60mA), fared better.
The picture shows a series resistor I added to the inductor in order to try and measure its current. The 3433′s datasheet is somewhat confusingly written, and I’m thinking I interpreted the load line graphs as a “minimum” load (i.e. load must be this amount or more to retain output stability, because boost converters need that kind of thing). The page of design math also pulled variables out of nowhere (or had other bad notation issues), and I actually skipped over it the first time figuring that the reference circuit would work just fine.
It turns out the current was indeed a maximum limit, meaning with my component choices I could only realistically get 160mA in “bridge mode”. Additionally, the reference design (8-60v to 12v converter) enters bridge mode at 18 volts – with my output voltage requirement being 3 volts higher, I indeed saw the transition happening at around 21 to 22 volts.
I tried other dumb inductor hacks like adding 2 of my 330uH discrete inductors in series (for more inductance), hoping to store a little more energy in that “bridge mode” . Unfortunately the results were not much better, and the reason is because small power inductors of such high inductance values also have very high DC resistance. Hence, the majority of what I gained was lost to I*R drop in the inductor itself. They also heated up quickly, indicating the same.
I ended up playing with the maximum output current design rules and found that yes, more inductance would help, but only if I could keep the DCR very low. In the size of inductor I use, the amount of space available for wire is very limited, and so all of the ones on DigiKey higher than 330uH were also much greater than 1 ohm – a few up to 5 or 6 ohms!
For instance, I can get output currents of 400mA if I had a 1mH (1000uH) inductor, but only if the DC resistance was still under 1 ohm. If I used a known part specification (6 ohms) in the design equations, then the result is actually not much better. I would be burning more watts in that inductor than what would be passed to the board.
“Bridge mode” on this converter is kind of strange. Basically it not only pushes current into one side of the inductor (standard buck converter behavior), but at some point it also starts pulling the other side to 0 volts (boost converter behavior). The input side voltage is the yellow trace and output-side voltage the blue trace – right now, the input is lower than the output, so it is operating solidly in bridge mode.
This is too weird for me, man. I’m not into discontinuous converters like that.
In conclusion: This version of the board is kind of fail. Let me be very clear that the LT3433 is not necessarily a bad thing – I just seem to need more current out of it than what my space-constrained layout, voltage demands, and component choices can sustain. It seems that I definitely need a solid 3 or 400mA+ converter serving the board for worst-case loads of a power-hungry receiver and a fan, and maybe some blinkenlichten.
It’s also not like the board doesn’t work – it functions just fine at voltages above 22 volts (topping out at 50+v with current part selection), or even down to 15-16 volts with suboptimal gate drive performance. But as a result it’s not really an improvement over the original LM2594 input-to-15v conversion. So I’m going to keep these two around anyway in case one of the higher-voltage devices needs one. For instance, Null Hypothesis can run this at the moment because it has a 25.6v nominal systemm.
Therefore, I’m going to quickly whip together a third board revision that retains the minor power and signal layout changes of this revision, but uses the LM2594 again, a circuit that I know and understand well.
But instead of converting to gate drive voltage (15v) first, I’m going to convert to logic level (5 volts) directly, then use a tinyboost like the LT3460, which seems nice and simple, to generate the 15 volt gate drive rail. The 15v will then be only for gate drive – no external tap will be available.
Bucking directly to 5 volts is a little riskier because switching converters will inevitably have more noise than an equivalent linear regulator, and if I am to spec 5v fans to be powered from that , it introduces yet another potential source of noise. I think massive bypass & bus capacitance spamming and 5v TVS diodes will make this a non-issue. I do trust the 2594 to be a stiff 5 volt rail, however. Using the fixed output version instead of the adjustable 2594 will also save on the space taken up by passives.
I don’t have this board designed yet, but hopefully will by the end of the weekend such that I can fire it off ASAP.