Help! Killed three converters already

[goldserve]

I finally got the new boards and have the converter buit as follows:

**broken link removed**
**broken link removed**

My problem is in high mode, R2 is set at 3.66K which is supposed to give me 850ma but i'm only getting around 500mA!!

Does anyone have an explaination for this? 20K works at 150ma and if I set it to 8.66K, I get 350ma as calculated. 850mA is not achievable. I am wondering if the inductor @ 4.7uh is not enough. Thanks for your input!

 

[hjames]

Aah, I was when I initially saw the layout, I didn't realize that those 8 pads surrounding the part were for a PIC.

What kind of LED setup do you have? The data sheet lists the max Vout at ~5V. I think the white Luxeon parts can(if you get a crappy one) exceed this at .5A.

 

[goldserve]

I am using very nice low voltage LuxIII ..TWOJ bin code. They can do 1A at around 4.2V so the output is not a problem. I am however using a 4.7uh and not 5uh inductor...

 

[goldserve]

Yupp, A pic sits ontop controlling the two enable pins and other functions like strobe, etc. The two 1K resistors on the back are for pulldown. The holes on the board are for probes to do programming on the PIC.

 

[hjames]

I can't think of anything obviously wrong... After doing a couple sanity checks, I'll try beefing up some of the components - the inductor followed by some of the capacitors.

I do remember having some issue with Coilcrafts' rating system a while back - if you go look at the inductor, it's "rated" for a couple amps, but if you look at the L vs. current plot, you can see the inductance take a sharp dive at ~1.5A or so.

 

[goldserve]

That could be a possible reason for the limited current. The board gets pretty warm in such small area, the coil gets warm and the current rating falls another 20-30%. I've got on order some higher rated inductors, around 2.1a @ 4.7uh. I'll give those a try while I send you a converter as well. Cheers!