While developing a nanoSTACK-based robot recently, I had a need for a source of 15V. I had these terrific Maxon gearhead motors that just cried out to be used; only, they were surplus, and therefore an irritating voltage (someday I'll write about all the 48V Maxon's that roll my way). Now, the robot is powered from a 6-cell NiMH pack with a nominal 7.2V.
I immediately thought of the MAX660 (and clones), those wonderful voltage doubling and/or inverting integrated charge pumps, so useful for op-amp circuits. No so fast, though - 5.5Vin and 100mAout, max. My motors had a stall current of around 250mA. I could parallel a few... but still, only 10Vout or so. I thought about discrete charge pumps, and maybe should have thought harder, but nothing obvious presented itself. Eventually, I figured National Semiconductor could come to the rescue, and, indeed, they did.
I read an article once by their inimitable Bob Pease extolling the virtues of their Simple Switcher line, and figured I'd give them a try. Not well-versed in switching regulators (but: now see the bonus links here at the bottom), I used their Power Design web applet to generate my design - couldn't have been easier (hey, wait, I'm giving away secrets here...!). It came back with a pretty simple design using their LM2585-ADJ; I was almost there!
With some funky TO-220 samples from National (thanks!) and inductors from Coilcraft (thanks also!) I was in business. There are a few gotchas about constructing the circuit, like placing that 0.1uF cap right near (or on) the ICs pins, but nothing dramatic. We've got 100kHz here, so the perf-board construction is passably ok. I had no low-ESR (equivalent series resistance) electrolytic caps handy, and tantalums in an experimental power supply make me nervous, so I paralled several standard electrolytic caps - it seems to have worked out.
A heat sink on the LM2528 seemed wise, and I found a small aluminum disk to attack to the top of the inductor as well (with heat sink grease on both). The system efficiency is over 75%, and the ripple is low (I'll get some actual, you know, numbers, one of these days). It even works well, outside the specified input range (with lower efficiency), so I'm pretty happy. A good first switching supply experience. I'd say.
This schematic is modified from a figure found in the datasheet (only components values and specs have been changed):
And the circuit, as built (I love the rows of capacitors):
More Switcher (Simple and otherwise) info from National: an online seminar, transcript available.
|Last updated April 29, 2005||
Original content copyright © Christian Weagle unless otherwise indicated.