LM2585-ADJ as 5W driver?

Anyone try an National LM2585-adj using 7.2V in to run a 5W luxeon? Not the dual winding application, the one that uses a single 10uH inductor (using National's WebSim). It's a little large, but there's plenty of room in a 2D Mag for this part with a few caps and resistors (and NO drive transistor). I've got a few samples coming, just wondering if there is a better IC to use. I know my favorite ZLT would probably go up in flames!
Thanks,
Rob

With a switching frequency of 100KHz, the output capacitor is going to be a fairly large component. I see in the datasheet some examples of ~500uF to 1000uF capacitor values for the output capacitor. I would make sure the inductor and capacitors you plan on using will meet your physical constraints.

If I'm not mistaken, a single inductor design according to the datasheet is a standard "Boost" converter or Step up Converter. You mention 7.2V as the input voltage and a typical Luxeon 5W is around 6.8V. I believe you would be in direct drive mode till the batteries dropped below the Vf of the Led and then the converter would start operating.

(IMHO) Considering that you have an output diode, you are very close (Vbatt + Vdiode = Vf LED) and that direct drive would not result in a severe overdriven situation. In fact one could argue this is almost an ideal situation for a converter design since the maximum efficiency is best for a boost converter when Vin = Vout. Of course in good engineering this isn't allowed and you didn't hear me say this...

I would think that you would like to find a 300KHz version of the same or similar IC so that the inductor, input capacitor and output capacitor would be smaller in size. I believe National makes Simple Switchers in the 300KHz range as well. This would make the design a little smaller and still would be fairly forgiving of physical layout.

Good luck.

Wayne

The output capacitor wouldn't have to be very large in this case, since in an LED application, ripple isn't much of an issue (esp. 100 kHz ripple).

You could probably even get away without the output capacitor (although that might screw with the regulation.)

Other than that - This is designed to be a constant-voltage regulator, which makes it not as well suited to driving LEDs as a regulator designed for constant-current operation. (Feedback voltage of 110 mV or so rather than 1.2 V)

But thank you SO much for pointing this chip out to me - It's perfect for driving op-amp circuits which require both + and - supplies! It's just what I need to escape dual 9V batteries in my noise cancelling headphone design.

*goes and orders a sample*