positive regulator in "negative configuration"

[Dimitree]

Hello
I've been looking at the 7805 datasheet from Texas Instruments, and notice the figure "Positive regulator in negative configuration".
So, I was wondering, would something like this work?
I'm not asking if it makes sense to use it in practice, maybe it doesn't    I just want to know how that works, and what are pros and cons of this approach.

[Rob Strand]

As drawn, there's a problem with the negative part; there's no high current path through the GND of the regulator.
The positive part looks OK.

An obvious way to get dual supplies with positive regulators is to connect them in series at the outputs but this requires two completely isolated input supplies.  The two regulators are otherwise wired identically.

This type of thing:
3.bp.blogspot.com/-Vom1XWebrUw/Uj7tev5ugNI/AAAAAAAAC1Y/6A0jDB4HM_A/s400/Build+a+Efficient+Negative+Voltage+Regulator+Circuit+Diagram.png

[davent]

Page 13 of this National Semiconductor datasheet has a schematic for a bipolar supply using two 78xx's.

http://cva.stanford.edu/classes/cs99s/datasheets/LM340.pdf

dave

[PRR]

http://www.ti.com/lit/ds/symlink/ua7808.pdf

page 13

Figure 6. Positive Regulator in Negative Configuration (VI Must Float)

This generally means the raw supply must NOT be connected to ground.

Here's TI's quick sketch and National's equivalent but fully drawn out +/- plan.



Note the TWO transformer windings, rectifiers, and raw caps.

Essentially we make two completely "floating" supplies but define the "+" on one to be "ground/common".

There ARE reasons negative regulators exist.

[Dimitree]

thank you,
I was confused by some app.notes and articles about using a positive step down regulator (and not a proper inverting DC-DC converter) to produce a negative voltage.
But I guess a linear 78xx and switching regulator are different beasts in this approach,
so I guess you can do that with step down regulator even if the input ground is not floating.

This is an example using a non-isolated Recom R-78xx switching regulator (do not confuse with linear 78xx series)



and also:
http://cds.linear.com/docs/en/design-note/dn1021fa.pdf
http://www.digikey.com/en/articles/techzone/2012/oct/generating-negative-output-from-positive-input-voltage

[Transmogrifox]

Charge pumps seem the simplest for low-power applications.

If using an inductor switcher then it seems worthwhile to use a transformer or coupled inductor so you can also provide ground reference isolation to break ground loops.  A simple unregulated forward converter can be realized with a square wave oscillator and transformer with 2 output windings.