First, I doubt I had this idea first. Not claiming I did.
We all know linear regulation blows off I^2R losses to produce IR voltage drops.
It's also true that PWM runs the LED at a fixed high current where it's not very efficient, even though the average current is lower when dimmed.
It turns out that linear seems to beat PWM:
(in part because PWM doesn't do as bad as some non-thermally corrected estimates might have you think)
And it's also true that when the number of cells in series equals the number of 3V led's in series you probably can't do much better.
But if we start with say 2s batteries and drive 1s LED's then we get a new tool, buck regulation of course, but does it really help? (Ok, that's not really the right question, but anyway, if we do it wrong it certainly will be worse than above and worse than it needs to be)
I've a number of people tell me how a) PWM is just buck regulator. False. or how Buck drivers are more efficient than linear (can be, but are any?, I mean compared to a linear driver from 8.4 down to 3.5V of course, but for dimming? Probably not, stay tuned.)
RMM's fabulous buck drivers:
use the MAX16820
"a dedicated PWM input (DIM) enables a wide range of pulsed dimming."
and page 4 shows that the current is really pulsed to the LED.
Maybe RMM did something different, but I suspect not.
So what's happening here is we buck down to some pre-determined 100% current level. This could almost as well be a fixed voltage level give or tak LED variation and temperature fluctuations. Then we're basically back being just like we have a 1S battery. There are some differences:
1)voltage doesn't drop over time
2)You choose a more suitable voltage for a more sane 100% level and this DOES gain some efficiency (but why be forced to choose, we can have it all)
3)You already lost some efficiency because bucks still aren't perfect, so if you want that 6A 100% level, you'll be worse off now.
We haven't fundamentally avoided the issues of PWM at all. We're just doing it the same as before and with the same general inefficiencies (and they're pretty big at low or probably even medium levels, and I care more at medium really where tipple drivers like the texas avenger don't fix it)
But what if we could actually control the analog current output of the buck driver. Then you keep that high 90+% buck driver efficiency, and it never changes it is not reduced by other inefficiencies! *
But. I think you can do this (well you can, hobby regulators do it, but can it be simple and small). Here's the latest proposed buck chip that RMM himself apparently recommended recently (according to TA):
And you can see that it has input 2, IADJ : "Analog LED current adjust. Apply a voltage from 0 to 1.24 V, connect a resistor to GND, or leave open to set the current sense threshold voltage."
That's not the same as the sense input. The sense input senses the current, this analog voltage adjusts the set point.
Problem, atiny13 doesn't have a voltage out. Oh, but it does have a PWM output. And if you filter a PWM out you can make it into a controlled voltage.
Why reinvent the wheel though, so I googled it and got this:
Presto PWM to voltage converter. And the great thing is, all the old firmware should still work!
So, buck.. and nothing else. Reuse PWM software, but no real PWM needed.
Now, I didn't really look in detail at how to setup the D to A converter, if the parts can fit well, if there are some side effects, so I'm open to comments there. It seems to me that shouldn't be so hard though.
Of course I have to ask, does this already exist? I have been actually a little surprised to learn that it's not the norm. I would have suspected it.
Edit:Kiri baru correctly points out that buck isn't necessarily more efficient than other methods because buck efficiency is tuned for a certain power output. For this or other reasons he can certainly be right, but while buck may become innefficient for instance at low output, it will only be more innefficient at low output with PWM.
We need buck sometimes, and it has some nice properties, and this is a better way to do it than buck +PWM I think.
Oh.. and does this mean we can get rid of an extra FET? I think so.
kiba-ru correctly points out that if PWM is done well (hopefully the bucks with PWM input do it well), it should be possible to keep the buck always operating in the same current range (during the pulse) where it has optimal efficiency. Loosing that possibility might come at a cost which counters the gains in LED efficiency. Overcomming this depends then on how well one can get the buck to operate over the needed current ranges.
(udpate) after doing much detailed calculations on buck losses, I'm less convinced by this concern. Basically several types of I^2R losses go up at high power. The freewheel diode loss is simply proportional to power (for a fixed voltage output, like driving a diode as we're doing). Two things happen at low power. FET switching losses don't budge and so become a bigger fraction. Also at some point inductor ripple becomes big enough to send the buck into discontinuous mode. I don't think discontinuous is necessarily a big problem but I haven't looked at it closely. But you can stay above DCM at a few percent power level (limited by space for inductor). The limiting factor then is switching losses. Again, looks like even for a p-type fet this can be better than 10% probably (rough unproven estimate) at a few percent power level. That did come at a very slight cost to I^2R losses at the highest power levels, in compromising FET selection. The trick will be using PWM from an appropriately low and efficient mode (optimizing driver efficiency and LED efficiency) to then go down to moonlight. But this will PWM down from maybe 10% not 100%. By far the biggest LED inefficiencies have been recovered already at that point. This can all probably be optimized slightly better yet with an n-channel buck, if we find a good (simple) one with analog control.
On the other hand nickleflipper points to data that shows that a particular buck with PWM control gets less efficient when PWM'd, so then you're loosing from PWM in both the buck AND the LED. The devil, and reality here strongly depend on details of a particular implementation clearly.