Pushing the Zetex over 400mA ?

I have been working on getting the ZLT+ over 400mA. But evey time I get close to or over 400mA the inductor will begin going into saturation intermitently. If I try to push the circuit anywhere near 500mA the inductor will saturate every time. I would really like to be able to push this circuit to around the 500mA mark. I have tried several different wire gauges, and number of windings, for the inductor. But all the results are the same.

Is this typical of this circuit? Or is there a solution that I have not found?

Do you have another core arround? I'd expect saturation was mostly a characteristic of the core. If you have 2 of the beads, try a coil thru both with about half the turns.

Boy, I've got all the materials here to play with that circuit myself, but there is always something else that seems to need doing NOW and the electronics sits patiently on the shelf.

I am using the core that is suggested at the 5thcolumn. 11mmx5mm. I will give the 2 beads end to end a try though, as I am currently only trying to fit it into a 2D Mag.

I do have to say that it runs great under 400mA. But I would like to get alittle more out of it.

69Hawkeye,
I had the same problem with my circuit, but I was able to run it up to about 550 mA before it would start saturating. There was a post about this some time ago, probably over six months ago, but I think if you change the inductor your efficiency would probably go down somewhat.

i also had this problem, although i wanted 500mA output. it actually would work sometimes and not others (would apply power but no noticable output) which was even scarier IMHO. so i dropped the zetex and tried a maxim circuit based on the 1674 chip. even in a very crude, simple layout the 1674 chip performed better, it puts out 500mA in it's 5V preset output (and presumably would in it's 3.3V output preset as well, but i didn't try this) which is probably close to max because it's transistor is rated at 1A, although dat2zip has gotten more out of it (i think his madmax and badboy stuff is all 1674 based).

papasan- I think that might be the direction that I have to start heading if I want to get more output from the LS.

Thanks for the info guys.

OK I have just spent the last 5 evenings trying to get some reliable performance from this Zetex circuit. But no matter what I do I can't keep the inductor from going into saturation and taking out the transistor.

Time to move on and try a 1674

Check your data sheets out. There should be a graph giving peak inductor current versus load current. You will need to find a inductor that is rated for that current and a frequency in excess of your switching frequency. I am working with the NS lm2621 and the peak inductor current should be 1.25 amps for 350 mA out. The evaluation board I bought came with a Coilcraft #DT1608C-682 if that is any help to you. You might also check www.digikey.com They carry a series of J.W.Miller surface mount power inductors, the PM105S series might be what you are looking for.