boost circuit design

[bfromcolo]

I have spent a good bit of time searching and just not finding what I am looking for, sorry if this has been asked many times...

I am looking for a circuit layout for a driver that can drive 3 series connect K2 LEDs from a 4.8V NIMH battery pack.

Thanks

 

[wquiles]

I have spent a good bit of time searching and just not finding what I am looking for, sorry if this has been asked many times...

I am looking for a circuit layout for a driver that can drive 3 series connect K2 LEDs from a 4.8V NIMH battery pack.

Thanks

I don't know which sites you have looked at, nor what size restrictions you have, but George has a nice comparison table showing the various LED drivers he sells:
www.taskled.com

 

[DIWdiver]

I have spent a good bit of time searching and just not finding what I am looking for, sorry if this has been asked many times...

I am looking for a circuit layout for a driver that can drive 3 series connect K2 LEDs from a 4.8V NIMH battery pack.

Thanks

You say you want a 'circuit layout'. That sounds to me like you want to make your own circuit boards from a design that is complete and proven. Is that right, or am I misunderstanding your request? I design and build circuit boards for a living, and to me 'layout' has a very specific meaning, so I want to make sure I understand what you want.

If I'm right about what you want, you may have a hard time finding that. By the time they've done the layout, most people are looking to sell some product and aren't willing to share their work. Your best bet would be Linear Technologies. They make the chips to do what you are looking for and they build a lot of demo boards for their products. The layout files are often available for the demo boards they make.

 

[bfromcolo]

You say you want a 'circuit layout'. That sounds to me like you want to make your own circuit boards from a design that is complete and proven.

I am looking for a schematic for a simple LED boost driver that I can play with, not a PCB layout, sorry if I wasn't clear.

Thanks

 

[georges80]

Visit www.linear.com and check out their various LED driver IC's.

What you want to do is non-trivial. You're looking at 10 to 11V total Vf with K2's and presumably you want to drive at 1A (or more) from 4 nimh cells at say 4.5V typical running voltage. That means you're up at around 2.5A input current.

So, for that you need a pretty decent switcher IC, decent inductor, schottky, ceramic caps etc and a GOOD PCB layout. Components will be generally all surface mount, so unless you have experience in laying out and assembling those kind of designs, you're up for some learning fun.... Hope you have some decent test equipment etc.

cheers,
george.

 

[DIWdiver]

I am looking for a schematic for a simple LED boost driver that I can play with, not a PCB layout, sorry if I wasn't clear.

Thanks

As George points out, that's a tall order. The chips specifically designed for LEDs are all (AFAIK) surface mount, high frequency parts not suitable to 'play with'. The ones not specific for LEDs aren't constant current...

These days it seems you have to put in a lot of design work before you ever touch a part or a soldering iron:sigh:.

Still, you might have some luck with an LM2577 (www.national.com). It's designed as a voltage regulator, but you could use it to generate say 12V, then use a resistor to control the LED current. That design isn't super efficient, but is more amenable to playing with than most other circuits. Operating at only 52 kHz, it's more tolerant of poor layout, but short wires and solid connections are still a must.

An improvement to the current setting resistor would be to use the adjustable voltage version of the chip, and (looking at the typical application on page 3 of the data sheet) replace R1 with the LEDs, and select R2 to adjust the output current. This would give you a true regulated current. Since the regulator wants the voltage across R2 to be 1.25V, you can select the current by:
R2 = 1.25/I, where I is the desired current in amps.

You could improve the efficiency by using a lower value of R2 and an op-amp to amplify the feedback voltage to the IC, but that's a longer discussion.

Hope this helps.

Don