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The target of our journey into building is the Zetex ZXSC300, a step-up driver IC designed to power LED backlights, etc. off a 3-volt power source. It's a little thing, a 5-pin SOT-23 package, but can it handle the kind of current a Luxeon Star needs?

Yes! Not just yes, mind you, but HELL yes!

Above, we have the quartet of parts that will go onto the driver board - the ZXSC300 driver IC (botton left) that runs the show, the FMMT1617 15-volt 3 amp transistor (left center) that acts as a switching device, the ZHCS2000 40VDC 2A Schottky diode (left top) that acts as a high-speed pulse rectifier, and the 150uF 10VDC Kemet tantalum capacitor (right) that smooths the output voltage. Not shown is an inductor made from a ferrite bead and #24 magnet wire, and a short piece of #36 magnet wire that is used as a current sense resistor.

As it turns out, the hardcore psychoes at the CandlePower Forums had already found this IC and designed a few variations of it that were cultivated from hundreds of experiments with the same basic circuit. The end results they came up with can be found here. It is from this that I start my work, so you might want to read it as well so you'll know what's going on with the hand-made inductor, for example.

The first thing to do is design a suitable circuit board. Using the designs the CPF folks came up with, I threw my own spin on things and the result can be seen below.

This is my PC board pattern. Note that it needs to be scaled to exactly 0.5 inch in diameter, so scale it as needed if you're making them from this pattern. Also, aim for a trace gap width of 0.25 to 0.5 mm, so that you don't accidentally get solder bridges across traces. (If your gaps are too wide, you'll not get enough solder on the parts' pins.)

Note the comparative sizes of the board patterns (there are FIVE driver boards there, ready to etch) and the parts - the driver boards are each 1/2 an inch in diameter!

The boards were etched and hot-tinned with a soldering iron and a generous coating of flux, to produce the above.

Next, the boards were cut apart and caregully ground down to the final size, which is shown above. Note the small size compared with the Stars - one driver board could literally be glued to the back of a Star!

Here's how the parts are positioned. The parts aren't numbered but it's easy to tell what's what - the IC has five pins, the transistor has three, and the Schottky doide has six. And the capacitor is the monster of the small-outline world so its location is obvious. :-D

Note the text on the black areas - this denotes what connects where: L1 is the inductor, Rs is the #36 wire sense resistor that is trimmed to increase the output current, +V is where the battery voltages comes in, LED+ is to the anode terminal of the LED, and LED- is the circuit ground to both LED cathode and the battery's negative terminal.

The boards were then populated as per the CPF information on the subject, and an inductor was made, again as per the CPF info. The above picture shows the result before the inductor is attached. Then, with a Star connected and a DMM in the cicuit to measure output current, that piece of #36 wire was trimmed until the output current was 400 mA (when the circuit was powered by a brand-new 123 Lithium battery) and then the free end soldered into place

This is the result. It took about three hours to make, etching and fabrication time included, and can pump 400mA into a Star even on not-so-new batteries.

"But how well does it work?" you may ask. The answer: "VERY well!" It's too soon for runtime graphs, but these are going to be added to all sorts of upcoming projects of mine since they're so small they can slip into tight spaces.

Now if I can shrink the inductor size down a bit I can stuff one of these into my MiniMag... But that is a story for a new article...