I’m already going to be ordering a couple of these to do my own circuit-only mockups before I start trying to cram it all into into the physical casing, but I was wondering how strict the 2 amp input limit is on these, and how efficient they are when making comparatively small boosts (~30%).
Parameters are thus: Power supply is a single Li-Ion cell (3.7V nominal, actual 3.0 to 4.2V), 1200mAh rating. I want a voltage stabilized 4.2V supply to a 3.3ohm load (load is fixed, I can’t change it) for 5-10 second periods. At the rated input capacity (2A) and efficiency (80%) of the boost regulator, this means I am operating at ~85% of rated capacity at nominal supply voltage. Supply being a Li-Ion cell, however, means that loaded voltage will actually drop to about 3.2-3.3V before nosediving and triggering the undervoltage protection in the cell. 3.2V works out to a needed supply of 2.09A, or 104.5% of rated max. It doesn’t help that to ensure Vin never exceeds Vout (which seems to be bad for this regulator), I really should set the output to 4.3V (since the Li-Ion may be as high as 4.25 when freshly charged).
What happens when I hit a supplied 2A current because of dropping supply voltage? Is output voltage dropped to compensate? Does the circuit cut off? Does the whole thing catch fire? 2A is kind of a weird threshold, most switching booster IC’s are 3A output (if not significantly smaller or larger), is the 2A threshold a deliberate safety fudge factor? If it is, can I change/disable it (by replacing other components, maybe)? What happens if Vin briefly (~2-5 seconds) exceeds Vout?
Most DC-DC power converters tend to be much more efficient when Vin and Vout are fairly close (>90% when output is within a factor of 2 of input), and less efficient at high differentials (~80%). At these voltages, can I hope for a bit better efficiency from the Pololu board?
I have a few other options, but all have their own drawbacks (bigger form factor, requiring a separate low-amp 5V input, significantly more expensive, pick 2). I’m really hoping the Pololu PCB can do what I need.
2A is pretty high for this small of a part; can you share some part numbers for 3A output? That kind of board is on our list of things to make, but everything I’ve seen in that output class requires external transistors and typically can’t operate from that low of an input voltage. I’m also skeptical about the >90% efficiency claim; maybe that’s for higher voltages or fancier designs (I haven’t seen synchronous rectification for this class of boost regulator; again, I’d love to hear of such parts if you know of them). With 4.2V out, even a 0.4V drop on the diode would already represent almost a 10% loss.
Your application is close to the limit, so some units might work and others might not. The current limit is built into the chip, so it’s not something we can adjust or disable. If you need many of these units, we could be convinced to move faster on a higher-power unit (especially if you have some good part recommendations); if you need just a few, we can hope that the units we have now will cut it. If you have a decent power supply, you cold test them at a little lower voltage, like 3V, to give yourself a bit of a margin in your real application.
Power-One ZY1207HG (7A rating, requires an external 5V 50mA supply if Vin is under 4.75V, I plan on using your 200mA 5V PCB), can get single units for $17-18, and the 5V PCB adds another $8. Dimensions are 22.2 x 12.5, plus the other PCB will bring the long dimension up to about 31-32mm, right at my practical limit before fitting gets dicey and labor-intensive. There’s no doubt in my mind I can make this one work, but it’s half again the cost of your PCB and twice that of my other option:
Lineage Power AXH005A0X-SRZ (5A rating). No need for another PCB, but I will have to abuse the output trim functions of what is really a 3.3V regulator (others in the same line, including the SIP version of this board, have a 3.63V max rating) to get anywhere near my target voltage, but I have plenty of current margin to play with and this outlier in the series shows there’s give, so I think it can take it. If I can do it without feeling like I’m overstressing it, this is the primary contender against your board, as it’s the same form factor and cheaper (~$12 for singles).
There’s also an Emerson power supply I’m considering, but I’m ambivalent (17x17mm footprint would require a rework of the physical design, and it’s normally used for embedded systems and hard to get in quantities of less than a few hundred).
Another factor I’m worried about is heat, even passive ventilation will be difficult, and active fans are out of the question. I can play around with adding heat exchangers or using wire runs to wick some of the heat to a separate radiator, but then we’re back to fiddly labor-intensive assembly issues. That alone may force the use of the 7A Power-One board (finding a few more millimeters is easier than trying to mess with complicated cooling).
EDIT: I should add, both of those have slightly higher-current versions in the same form factor, but higher leached power and more crowded and harder to cool PCB’s. So I have fallbacks, I’m just prototyping right now.
In terms of my quantities: Initially on the order of single digits to 10’s of units/month, if the product catches on that could scale to hundreds (would be nice, but not counting on it). I can’t be the only person wanting to produce significant voltage off Li-Ion without using multiple cells, though most probably don’t have my combination of physical and electrical fixed parameters.
EDIT: And to answer one of your questions I missed: None of the 3A boards I’ve found will operate on less than 4.5Vin, the next chipset down in the Lineage (Austin MicroLynx?) line is the basis for most of them. But many of them are considerably smaller, 12.5x12.5mm being typical.
Unless I’m missing, everything you linked to is a buck (step down) regulator, not a boost regulator, like our unit. That means they won’t give you a higher voltage than you put in, which it sounded like you needed.
Well, that ruins my whole day, I shouldn’t look for PCB’s at 3am (I forgot that just because some boards are labeled “Buck/Boost” doesn’t mean those are the same thing, and it didn’t register when I started looking at buck regulators). Anyway, the best option I see now (if your board won’t work) is to switch to using a pair of smaller cells in series (giving up most of my capacity) and use the step-down regulators to get my 4.2V.
Can your board be run in parallel?
Anyway, if you’re looking for IC’s for a higher-output step-up, try the MAX1709 or MAX669. If you can fit either one into a PCB no more than 14x30x8mm, you’ve got a buyer.
The parallel option might work, but I’m not sure since there is no specific provision we’ve made for that sort of thing to work. (Some devices designed to work in parallel can be set up to be synchronized with appropriate phase differences to minimize noise, and you wouldn’t have that kind of option with our units.)
Those Maxim parts don’t look all that compelling, so I guess I’ll keep up my search for a higher-current boost solution.
There’s also some IC’s designed for driving high-power LED’s off single Li-Ions that I’m looking at, some of them have a fair amount of logic for intermittent power or time limits. None rated higher than 1A output I can find, but the lower amperage versions are frequently paralleled and the 1A probably could be as well. They will suck an unprotected cell right into a flameout, so you have to be careful with them.
I don’t care about output noise as long as the boards themselves won’t have a problem with a potential negative voltage situation (one board transiently higher than the other).
EDIT: This TI TPS61050 is the most promising for my purposes, the 1.2A max is only 70mA short of my target, and close enough for my needs. The example board claims you can cram it and the ancillary components into 4.7x4.7mm, although you’d need to add some heatsinks for sustained output I can probably breadboard it together to start with, and look at getting some custom PCB’s made up when I’m ready for production. Since you’d want higher max output voltage, it probably isn’t going to help you, but the 61500 3A model might fill your needs.
