NEGATIVE VOLTAGE CONVERTER

You can use a converter that bucks 12V to 7V and connect its ground to -12V, Vin to GND, and generate a Vout of 7V.  The TPS62125 can do this for you.

Hello,

I would like to do something similar to what you describe. In my case I have two inputs (+15V and -15V), and a common ground. I was going to use a LM3102 (or something similar) to produce +5V from the +15V input line. In theory, I can use a slightly modified circuit as you describe to produce -5V from the -15V line. Basically connect the LM3102's Vin to Ground, and LM3102's GND to -15V, and then configure it to produce 10V relative to the -15V line.

However, my problem is that I would like to limit the peak to peak ripple voltage to some extent. Unfortunately, the 15V lines are not regulated that well. For the +5V output this is not an issue, as the LM3102 FB pin is referenced to ground. But, using your suggested setup, the -5V pin is referenced to -15V. As such, if the -15V line swings by a volt or two (or more), the regulation of the -5V line will also swing relative to ground (even with the feedback compensation circuit shown) .

So my question is, there a way I can make the FB only relative to ground? Or is there a better way for me to produce the -5V from the -15V line? (Note, a negative linear voltage regulator is not an option due to power issues, nor is it an option to turn the +15V into -5 with an inverting buck as I need the current from both sources).

Thanks,

Matthew

I am not aware of a device that does what you want, so I would suggest a buck stage first, followed by a linear regulator operating with 1-2V of dropout.  This gives you very good noise performance as well as decent efficiency.