Understanding the difference between Linear, Buck, Boost and Direct Drive drivers

@Islisis, i think you are right, the size is the problem, nothing else.

I don’t think I ever posted in this thread to thank you lagman, but I do point people over here every little while. One of these days I’ll make some suggestions of my own for sprucing things up, but I think you’ve done a good job!

As my post starts by pointing out. It is an opinion.

I always accepted the inevitable: My batteries run out.

How does the new Cree MT-G2 and XHP 6v+ emitters affect things

I notice that the article calls out single cell and single emitters a lot but I think it might be more accurate to limit the ideas to Voltage over and under the emitter requirements. Point being that with the MT-G2 emitters there are lots of standard designs that are exceptions to the single cell nomenclature.

$.02 worth

A long time ago I said that I’d like to contribute. Sorry for the wait folks. Today I took an hour or two and did some editing.

While I think more than half of lagman’s text has been rewritten, the significance of the edits varies. Some editing was rather heavy, some just minor reformatting. One or two things were incorrect in minor ways or simply misleading. It’s still not perfect, and it might have gotten slightly more technical but I think it’s an improvement. The overall structure is still lagman’s!

Much of my editing was to make the guide more compatible with discussing drivers for 6v and 12v LEDs. I removed a lot of mentions of LED voltage being ~3v. I also added context in other places where 3v or a single li-ion is used as an example. I also added detail and context where I thought it most appropriate.

I did my best to keep it short and sweet. I left out plenty of things. Unfortunately I was unable to avoid increasing the word count by about 75%.

I welcome any feedback on my edits or the overall state of the guide…

Thank you wight! I've read through the wiki but this is better.

Thank lagman. The initial writeup took the most time and effort.

I see that my thread is not dead yet. :slight_smile:
Thanks to you all for the support!
You did a good job wight. It is true that I didn’t take into consideration multiple LED dies like the new Cree XPH. That being said the theory remains the same.

I have one minor point on which I disagree though:

I think it is wiser to consider the “recommended operating conditions” which is 6V instead of the “absolute maximum rating” which is 7V. But in any case two lithium batteries in series will have a max voltage of 7.4V which is sure to damage the chip.

Thanks lagman.

Let me make sure you have a good handle on actual 7135 operation / implementation: There are two voltage specifications for the 7135. “Input Voltage (Vin)” and “Output Voltage (Vout)”.

  • Input voltage is measured in the normal way, Vdd - GND = Vin. [Since GND is zero we may simply say that Vdd = Vin.] When we are powering this from an MCU you may expect for “Vin” for the 7135 to be in the 2.0v to 5.5v range. We typically regulate the voltage to our MCU at less than 5.5v using an LDO or Zener and the Vdd pin is powered by the MCU. For this reason we may generally ignore the Vin specification as we are always in the “normal” range.
  • Output voltage is measured in this way: Out - GND = Vout. [Again, since GND is zero we may simply say that the voltage at the Out pin = Vout.] Vout is the voltage left over after the LED (Vf), so in order to get Vout we do Vbat - Vf = Vout. In an 2s Li-Ion (2x 4.2v) powered 8.4v MT-G2 light the input voltage will be no higher than 8.4v and at 3A output we may expect an LED Vf around 6.4v. So Vbat - Vf = 2.0v. Again, no problem.

So operating the 7135 in high voltage lights is not a problem. The key (as I’ve tried to express) is the amount of “extra” voltage beyond LED Vf you have. Running an XM-L on 2s Li-Ion is actually within spec for the 7135 in terms of voltage! The problem remains that the wattage would be far too high in that scenario.

Ok, I agree with you. I wasn’t thinking that the VDD of the 7135 chip is powered by the MCU… :slight_smile:

:slight_smile:

Great read you guys!

great read. if only I had read this a few months ago. would have saved me a lot of time :bigsmile:

What a great wealth of information. Thanks for taking the time to compile it and share it with us who are light challenged. :slight_smile:

Carl

Excellent resource. :beer:

Thank you lagman for your great explanation!! I’m new to this forum and have been searching for a completed guide about flashlight driver but I haven’t found yet.
Could you draw simple circuit diagrams of some popular drivers and explain its components/ how it’s work…?
I have some knowledge in electrical physics but not in electronic components, the driver boards are too complicated to me :person_facepalming:

So will a linear regulator have better efficiency when using a IFR (LiFePO4) cell than ICR,IMR or INR since it is about 3.2V nominal?

A boost converter (step-up converter) is a DC-to-DC power converter that steps up voltage (while stepping down current) from its input (supply) to its output (load). Step-up switching converters, also called boost switching regulators, provide a higher voltage output than the input voltage. The output voltage is regulated, as long as the power draw is within the output power specification of the circuit. Many of our step-up switching regulators are designed for driving strings of LEDs.

If your step-up/boost application is not power intensive (IOUT < 0.5A), you may prefer a different power conversion technology for boost applications.

I would like to build a quad for solarforce with an extension tube. What driver can I use? Is it possible? Planning on buying my parts from mtn. Thank you.