Linear voltage regulator advanced

[vladtess]

Hello fellas. I am new here and maybe you can help me.

I'll get to the point:

I have an LM338, a 5A version of LM317. It works well, but I want to do a few more wild things with it. It appears that this regulator is capable to do much more than what is stated in the data sheet.

Here's my goal: An LM338 whose voltage will be controlled via a microcontroller. But I do not want to use a digital pedometer. Rather, I was thinking of using a DAC and feed the output voltage (1-5) from the DAC into the opamp voltage multiplier and then feed it into the adjust pin.

So far.... it works well. At the moment I have a voltage divider that feeds into the opamp and via the voltage follower, the ouput feeds into the adjust pin with 120 ohm connected to the output. The schematic is attached.


So far everything works as expected, except that the minimum voltage is ~2.5. Can anyone advise me how I can make this voltage lower, lets say down to 1.2V?? Thanks much!!!

 

[Grossel]

Na, you need to include it's output in the feedback loop to make it work.

But then again, it's really no point as I can see. I'll rather pick out an opanmp combined with a powerful output stage to achieve voltage regulation.

So yes you can get it to work as you want pretty easilly. Only thing you should be aware of is unwanted oscillation as you add a feedback loop to a ic regulator that I guess wasn't ment to be used that way.

 

[vladtess]

Na, you need to include it's output in the feedback loop to make it work.

But then again, it's really no point as I can see. I'll rather pick out an opanmp combined with a powerful output stage to achieve voltage regulation.

So yes you can get it to work as you want pretty easilly. Only thing you should be aware of is unwanted oscillation as you add a feedback loop to a ic regulator that I guess wasn't ment to be used that way.

Thanks for your reply. How would you redraw my schematic to make it work? Thanks!!!

 

[ronv]

Could be wrong but I think it is because the LM321 can only sink about 6 ma and the 120 ohm will ask for more than that making the output of the 321 go up to about a volt.

 

[bountyhunter]

Hello fellas. I am new here and maybe you can help me.

I'll get to the point:

I have an LM338, a 5A version of LM317. It works well, but I want to do a few more wild things with it. It appears that this regulator is capable to do much more than what is stated in the data sheet.

Here's my goal: An LM338 whose voltage will be controlled via a microcontroller. But I do not want to use a digital pedometer. Rather, I was thinking of using a DAC and feed the output voltage (1-5) from the DAC into the opamp voltage multiplier and then feed it into the adjust pin.

So far.... it works well. At the moment I have a voltage divider that feeds into the opamp and via the voltage follower, the ouput feeds into the adjust pin with 120 ohm connected to the output. The schematic is attached.


So far everything works as expected, except that the minimum voltage is ~2.5. Can anyone advise me how I can make this voltage lower, lets say down to 1.2V?? Thanks much!!!

Caution: the op-amp may not be able to sink enough current out of the GND pin of the 338 when it is cranking full current.

You could use a 2N3904 to sink the current and drive it's base from the op-amp... but you need to reverse the op amp inputs. You will also need feedback compensation.