LM117: Using LM117 to generate a negative voltage

Hey Rajan,

Is there a reason you can't use the LM137?

This would be much simpler.

Thanks,
Daniel

Hi Daniel,

I do not have any other negative voltage in the design, I just need to generate a single -ve rail.

the lm137 requires an unregulated -ve input to produce a regulated -ve output

Hey Rajan,

The problem I forsee with doing what you describe is that the LM117, unlike switching regulators, doesn't generate a certain voltage.
The linear regulator will simply continue decreasing resistance/increasing resistance until it's reference voltage is satisfied.

Furthermore, I am unsure without a negative voltage how you could create the feedback to be attempting to regulate to a negative voltage using the LM117. The LM117 output will always try to regulate to Vref above the voltage at ADJ. Thus you would need a negative voltage at ADJ.

Thanks,
Daniel

Daniel,

I think you are right and I believe the same argument applies to all linear regulators. There are other techniques, e.g. using a charge pump, a transformer or a switching regulator, but I want to create the -ve rail easily and need about -0.5A of current!

kind regards,
Rajan.

Daniel,

There are a number of techniques which one can use to generate a -ve rail from a +ve one, charge pump, inverting buck-boost etc ...., the former are limited in source current while the latter are more complex and expensive.

My conclusion is to use something like a LM137 and the use the isolating DC-DC which comes beforehand to also generate a -ve rail, which should be as simple as adding another winding.

What do you think?

Rajan.

Hey Rajan,

That seems appropriate for what is needed.

Thanks,
Daniel