netgative rail switching power supply

[millwood]

Hi, there,

I am working on a switching power supply (to power a couple of 12v fans in an Antex HD cooler that will be the home for my gainclones). I plan on using LM2576 for the positive rails and the hook-up is a 4-terminal ATX connector. so I thought I would just throuhg in a -12V source there just in case. The LM2576 is much like 317 in that it generates positive rail voltage.

To my surprise, I couldn't find any negative rail swtiching regulators (the equivalent of 337). What gives?

[Claude Abraham]

You can use a 2576 for a negative converter (buck-boost) as well. Read the application notes from Natl Semi. The "ground" pin of the 2576 gets connected to the negative output voltage. The input voltage capability gets derated. For example, if the chip is rated at 40 volts maximum input, and you desire a -12 volt output, you must limit the input to 28 volts max.

Or you can use a Tex Instr UC2572, specifically dedicated for negative output. It can withstand 35 volts input, regardless of the output voltage, no derating needed. I hope this helps.