A more efficient driver (digital to analog controlled buck)

OK, I'm not sure exactly what kind of PWM you were doing. The proposal I was making in that quote and the OP, is that any PWM used at all is only as control, a communication channel to direct the driver as to what output current it should target. My statement was in contrast to actually turning the driver on and off. I think the PWM you are referring to, is enabling and disabling the buck regulator. I think your observations are actually in support of this idea. Turning bucks on and off equals bad, right?

Linear discourages PWM’ing the EN (able) pins with their drivers, as it can cause irregular operation. So yes, you don’t want to control the LTC3454 this way. The PWM I mentioned went to the Iset2 pin, which operates the internal current feedback loop.

Flintrock, my guess is that once you are down to some, let's say, “peak drive current/power efficiency”, it would be perfectly 0K to use some PWM on/off on the driver current feedback loop (thx nickelflipper) to get even lower outputs without significant negative impact on overall efficiency. Of course, a high enough frequency PWM as to avoid disturbing or causing discomfort to those more sensitive (this may be meat for another discussion).

Cheers ^:)

Ok. Interesting. I'll have to read the manual.

@Barkuit, right, I think FPC said the same. This seems right. At some point the LED is driven low enough that you've already gained efficiency there and for most people probably once the battery lasts a day, that's good enough anyway.

So.. reading the manual:

Iset2: "LED Current Programming Pin. A resistor to ground programs the current through the LED to ILED = 3850(0.8V/RISET2)."

So this is analog input. It doesn't convert PWM to a level, right? It will just try to track up and down with the pulses, no? You were sending it pulses... or you were generating pulses, filtering them into an DC level, and inputting those into ISet1? Sorry if I'm too slow, don't worry about it. I was just trying to put the idea out there anyway (even if it already was).

If we look at the LTC3454 page 11 fig. 3d, that is PWM with a low pass filter that I used for dimming. You mentioned in the very first post this link as to what that looks like http://provideyourown.com/2011/analogwrite-convert-pwm-to-voltage/ so not quite a dc voltage, but close enough. You also see in figure 3 of the LTC3454, the other dimming methods that FPC mentioned, which is using a DAC (which is just a digitally controlled resistor divider network), or a pot (digital pot works too).

Most info posted in this thread is far away from topicstarter question.

  1. All chips that are named as special dc-dc buck convertors/drivers dont have any secrets and solve 2 problems only: they can work without mcu and they are smaller size that same circuits which consist from more known parts.
    So, using their datasheets to understand what is buck driver and why it is not so high efficincy is not a good idea. Choose most simple circuit and test how does it works (without mcu, with analog management)
  2. Input pwm and output ripples are two different parameters. You can manage some chip with pwm and have no ripples at some modes and manage chip with variable resistor and have big output ripples.
  3. Most of you have discovered russian buck drivers from e-bay. They are very suitable to make such tests and explore buck drivers. I can check output ripples, but cant measure their efficincy. We need to find a person who can do this. Current can be managed with variable resistor, so there are no input pwm what all are saying about (buck is not fet, it has other problems).

Please note I only skimmed this thread but I don’t see what the thread is even about if I am honest.

A buck converter works by sending a PWM signal to an FET, that then sends the pulse to the inductor (this is the key piece that I didn’t see mentioned in the OP), from there it goes to the LED.

The inductor is what causes the buck to work (and boost for that matter) without it you are indeed doing nothing but PWM like we normally do (although at much higher frequencies).

The key here is the inductor, an inductor is kinda like a capacitor except it resistors change and stores it’s energy as a magnetic field instead of electricity (also why it plays havoc on the circuit at high currents).

Since the inductor stores the energy from the pulse, when the pulse stops it then releases it to the LED. The controller monitors this and when current falls to a given level (usually a few % below the target) it sends another pulse that charges the inductor to a few % over the target. It repeats this process up to 2 million times a second.

The end result is a constant voltage / current (most converters can do both) with a “ripple” voltage from slight over and undercharging of the inductor.

With this setup you get efficiency’s of up to 90-95% based on the data sheet for what we are doing and it can dim down to moon mode just fine.

So I am not seeing what needs to be improved here? Sure the ripple is not great but it is a heck of a lot better then PWM and it can be minimized with circut design or the use of better buck converters.

All of that said from what I could make out of the OP, it looks like what you really want is a linear regulator, something like the OP-amp used int he LD-2. It gives a constant current that is quite stable but it is horribly inefficient as it has to burn off all the excess voltage as heat.

Improving both the output capacitance and its ESR equals to less ripple on the load.

Cheers ^:)

Posted on: Fri, 09/23/2016 - 14:42; edited for a little fixup.

While true it will not affect the ripple if the buck converter module itself has a ripple of 10. It will still wait for voltage to drop 5 below target and raise it back up to 5% over target no matter what you do to the circuit.

In order to improve that you would need a better buck converter IC that has less ripple and a suitably large enough inductor to be able to support it.

Unless of course you are saying to put a large capacitor after the sense resistor which would have a minor effect on ripple but due to limited space the size of the cap would be very limited and as such it would only mildly reduce ripple on high current driven LED’s. Still a good thing to add though if you have the space.

Although that in a simple addition to the end of the circuit and has nothing to do with the PWM?

You are inviting bicycle.
The only known way to improve buck efficincy is using IC with PSM mode.
But I suppose that 1% mode in real buck driver on 17-20mm board all the same will be less effective than 10% PWM to 1x7135.

https://www.startpage.com/do/search?q=%22inviting+bicycle%22 -> click/tap on images (lewdness warning ). O:)

Cheers ^:)

Ok, so nickleflipper that's interesting. SO that setup seems to actually be exactly what I described in the OP. If that's the best there is, then bad news. I'm optimistic that FPC is right and there are much better. I mean there is no reason there can't be better ones.

TA no, I didn't talk about the PWM inside the buck because it wasn't the point. I'll try to summarize the short, or at least more direct, version. A buck converter is fine, but if it's just regulating off one fixed sense resistor to either a fixed current or voltage, that doesn't give you modes. That just gives you your 100% level. Now how do you get modes? That's the question, and how to do it efficiently.

The MTN-MAX drivers actually turn the buck on and off to get low modes. This is nothing like the internal switching and it's at a much lower frequency. This results in square pulses to the LED, just like an FET driver and has the same inefficiencies as an FET driver due to driving the LED at higher peak current than needed.

You could instead just linear regulate down to the modes and you get the linear regulation inefficiency. All this on top of the buck losses, adds up.

So instead you could try to get the buck to output less current for lower modes, after all it's a buck converter. That's what we're talking about. The last use of PWM here is the one that still comes from the microcontroller, but doesn't operate an FET. It instead gets filtered into an approximately DC level to feed into the bucks feedback controls. This is shown in the image nickleflipper referenced above, but it seems it didn't work all that great in that regulator.

Since buck regulators can have trouble going to low very low currents it was proposed that the buck could bring say anywhere between 20% and 100% and to get to 5% you use buck throttling plus PWM.

I think you are misunderstanding what is happening. It is already doing what you describe internally in the buck IC. It outputs a constant current, it does NOT PWM the current past the 10% ripple.

The output from the MCU goes to the dimming pin on the buck IC. The buck IC sees this signal as a matter of duty cycle. Whatever duty cycle it sees in that signal, it then cuts the output accordingly. The MCU DOES NOT turn the buck IC off or on at any point.

So if you have it setup for a 1A output and input a 50% duty into the buck IC, then it will output a .5A constant current output.

The PWM out from the MCU is simply to tell the buck IC what current you want relative to the max. The buck you listed in the OP has a dimming ratio of 5000:1 IIRC. Which means if setup for a 1A output, it can dim all the way down to 0.2ma, far less then most moon modes.

Efficiency wise, it will be just as efficient at that mode as it will running 1A in a theoretical world (It will actually be slightly different due to the lower VF of the LED at that current but that is balanced somewhat by the larger losses due to higher current at 1A).

While it is good to think outside the box, in this case though the box already does what you are trying to do.

I don't know what buck implementation you're talking about. Some do that already. Nickleflippers buck setup did that. The MTN-MAX buck is PWM'd by the microcontroller through its enable pin. The max buck typical application diagram makes this clear and unless RMM badly misunderstood me, or I him, he made that clear. We can head back to that thread and ask again. He seems to monitor that one.

From all the looking at docs and discussion here, actually none of it has suggested than any of the IC's discussed so far can "interpret" or internalize an external PWM signal to modify the internal switching cycle. What nickleflippers setup did was pass the PWM to a cap and resistor attached to the voltage sense, effecitvely inputting a near DC control level. This is also what I discussed in the OP.

Well I did most of my research on buck IC’s for things other then LED driving so maybe these are different, it doesn’t make a lot of sense for that to be the case though. It destroys the entire point of using a dimmable buck driver in the first place. What is dimmable about it?

Looking at HKJ’s driver tests:

Most of the buck drivers use PWM to get lower modes, but some use constant current for lower modes.

I agree with that. edit: was responding to TA.

Let's be REAL clear about these wordings. It's difficult. PWM from the microntroller is still what nickelflipper was testing, but that produce constant current from the buck in that implementation. I'd prefer to refer to "LED PWM", as in the current through the LED is pulsed (to/from zero) to achieve an average LED output. I assume this is what you are saying.