Best Boost Led driver to design around?

[andersonEE]

I'm going to be building an LED Flashlight with some standard white LEDs (3.4V, 20mA, 75,000mcd). I'll probably use between 4 and 6 LEDs in series.

Anyways, I wanted to get your input on a good driver to design around. (I want to be able to use 2-3 AA alkaline batteries).

Here are the ones I'm considering (feel free to suggest others):
LM3410 - National Semiconductor
ZXSC100 - Zetex
AMC7135 - Micro Bridge
AMC7150 - Micro Bridge

Thanks for any input!

 

[qwertyydude]

3 AA alkalines won't need a boost driver, a buck regulator is all you'd need but right at 3 AA it would actually make more sense to just direct drive them with a resistor in between.

 

[andersonEE]

Well, not if I want to have the LEDs in series for constant current (which I do). Also, using a resistor like that is much less efficient, and since the battery voltage begins to drop almost immediately, so will the light output of the LEDs.

 

[MorePower]

I'm not an EE, but I thought the AMC7135 was a current regulator, not a boost chip.

I'd previously looked at the LM3410 to see if it was easy enough that I could hack together a driver, but never got the necessary bits to try it out.

 

[yellow]

better use any high power led (XR-E P4) instead.
Will be brighter at the current of the "between 4 and 6 led in series",
smaller
and cheaper because only one focusing device is needed

 

[mpf]

You might like to look at LTC3813
will handle upto 100V out i.e. 6xMC-E all dies in series.
Has both variable current control and PWM control. Also has voltage limiting when open circuit
Linear Technology has a good circuit simulator to test out your design.
I am planning to use this chip for my next high powered torch.
The only downside is that you will need a professional PCB made to get the vias to the underside of the chip for it GND and heat dissipation

matthew

 

[snarfer]

I built an LTC3813 driver myself. It is definitely a sophisticated IC and I think total overkill. I was not so happy with the result actually. Definitely must use a high side current sense amp, and it ends up being extremely expensive and uses a lot of board space. It is a project I will probably abandon. It certainly would not fit in anything but the most gigantic of flashlights, and there were some stability problems.

Also it would not handle 12xMC-E in series. The MC-E in series configuration has Vf 14v, so the limit would be 7x MC-E.

Of course for the OP, who only mentions a desire to drive 4 to 6 white LEDs in series at 30mA each, it is a little unexpected to suggest an IC that could probably handle 200 watts. His maximum output would be less than 1/1000 of that.

 

[andersonEE]

Thanks for the replies!

@mpf and snarfer -- Yeah, I think the LTC3813 is definitely overkill for what I need. I wont need more than ~20Volts and I don't need PWM either.

@yellow -- I wanted to use these standard LEDs because I have alot of them (and they are really bright).

@morepower -- yes, you are right. Forget about the AMC7135.


To be honest, I was actually more interested in feedback on the LM3410 and ZXSC100 (because these are the ones I was leaning towards). Anyone have suggestions, feedback, experience on either of these two? My biggest concern is efficiency. Remember, all I need is 20mA, since these are standard LEDs.

 

[qwertyydude]

Boosting 4.5 volts to about 20 volts and you're concerned with efficiency? A voltage quadrupler would be very inefficient and unless you have horribly mismatched led's the currents in parallel should be close if the vF's are reasonably close but to boost voltage almost 4 times you're looking at more losses in the boost circuitry than a resistor with led's in parallel. 3 batteries in series is almost perfect voltage so the resistor would be very low, maybe just 5-10 ohms, this would drop very little power. That's why 3 AAA is used so often because direct drive 3 cell is one of the most efficient circuits you can get. In fact you're probably gonna lose more in the battery than the resistor.

 

[VegasF6]

National's web bench suggested the LM2703-adj. Other's to look at might be the LM2794. I think it can be done with the Zetex C300 as well, but I am not real clear on that one.

 

[likeguymontag]

I think it can be done with the Zetex C300 as well, but I am not real clear on that one.

The Zetex C300/C310 work in boost configuration, and I've definitely powered two leds (2s) from 3V using a circuit from a gutted flashlight. I'm not sure whether it can be set up to boost 4.5v to ~20V as I have zero EE skills.

 

[mpf]

I built an LTC3813 driver myself. It is definitely a sophisticated IC and I think total overkill. I was not so happy with the result actually. Definitely must use a high side current sense amp, and it ends up being extremely expensive and uses a lot of board space. It is a project I will probably abandon. It certainly would not fit in anything but the most gigantic of flashlights, and there were some stability problems.

Also it would not handle 12xMC-E in series. The MC-E in series configuration has Vf 14v, so the limit would be 7x MC-E.
.....

My bad, I ment to say 6xMC-E. I am just about to start on this project so I would be interested in your experience. Not sure what you mean by "high side current sense amp", the LTC3813 already provides this. What sort of stability problems did you have. PM if you like.

matthew

 

[andersonEE]

Thanks again everyone.

@qwertyydude -- that's a good point about 3 cell direct drive. I'll have to benchtest them against my boost circuit to see which performs better. The only thing I'm concerned with regarding direct drive is voltage drop over time. I don't want my led's dimming, and alkaline batteries drop their voltage pretty fast.

 

[snarfer]

mpf:

Regarding the LTC3813, I suspect you are thinking of a different part entirely. The LTC3813 is a synchronous boost controller. It is not an LED controller. It features neither PWM control, nor high side current sense amp. Perhaps you are thinking of the LT3756?

 

[mpf]

Yes you are correct again LT3756 it is.
matthew

 

[VidPro]

Thanks again everyone.

@qwertyydude -- that's a good point about 3 cell direct drive. I'll have to benchtest them against my boost circuit to see which performs better. The only thing I'm concerned with regarding direct drive is voltage drop over time. I don't want my led's dimming, and alkaline batteries drop their voltage pretty fast.

and wont they be dropping even faster when the amp load on them continually goes UP trying to keep the led voltage and current similar?
(i am just saying
it might be much better to lighten up the load on the things as they cant handle it, so they run for time, you did say you want it to not DIM, and the only way to do that indeed is to keep loading them harder and harder when they just cant take anymore.
but there has to be some relativity involved here wherin the battery will be wasted, and you still wont have runtime , and it will still dim , you dont want a dwindling output, then dont use alkalines Beat them when they are down, and they wont deliver anymore anyways
(i am just saying