Pre tracking pre regulator

[dmta]

Hi all,
How do I use the LM2678 as a tracking pre regulator?
regards

 

[ChrisP58]

More information about what you want to do please.

 

[dmta]

I want to make the dropout voltage of a linear regulator to be about 3V. I need a circuit for that using lm2678

 

[ronsimpson]

Using the adjustable version of the LM2678 where the FB (ADJ) pin is at 1.25 volts:
The goal is to keep approximately 3 volts across the linear regulator. The exact value is not critical.
The voltage across the linear regulator is across the 2200 resistor at the top. (-0.6v)
The voltage across the 1000 resistor at the bottom is approximately 1/2 that voltage.
The LM2678 will try to keep this voltage at 1.25 volts if you are using the adjustable version.

 

[MrAl]

Hello there,

Without looking into this too deeply yet, if you want to make a pre-regulator that keep the output 3v above the output of the post regulator, then what you basically do is provide a feedback voltage that is 3v above the output of the post regulator. So if your post regulator puts out 5v, then your feedback signal has to be around 8v. This would be for a fixed voltage voltage regulator, so the 8v would actually be coming from the output of the pre-regulator.

If the output voltage is variable, then you could make the output of the pre-regulator vary the same way just 3v higher. If you want to do this with feedback from the post regulator, then you have to make a fast voltage comparison using a device that is faster than the regulators themselves, like a transistor.

Here is a schematic showing how the transistor is connected. Ignore everything except the two regulator IC's and the PNP transistor. The PNP transistor and zener do the voltage subtraction and provide the proper feedback signal. It is also possible to replace the low voltage zener with several small signal diodes in series.
Also ignore the web ID and other related info. This was originally a group project and that information provided a means for people to go to a web site and order all the parts for the project in one shot. It's probably long gone by now though as this was from almost 10 years ago.