I have a question. I want to make 30V/4A hybrid power supply with stepdown regulator LM22679 as an preregulator and LM338 as an linear regulator to smooth voltage. LM22679 would be powered by Mean-Well power supply with output voltage of 36 V. When I want let's say 10V output voltage, from 36V input I make about 14V with buck regulator and then there would be lower voltage drop on LM338 when making 10V from 14V instead of making 10V from 36V. My question is how to simply control output voltage? If I can somehow use LM338 as Vin=Vout regulator, I can simply set only output voltage of preregulator with one potentiometer but I am afraid that i cannot do that. I hope you can understand what i mean. Thank you.

  • \$\begingroup\$ Welcome to EE.SE! A schematic is worth more than a thousand words. \$\endgroup\$
    – winny
    Apr 9, 2019 at 21:36

1 Answer 1


You could try a circuit like the one below. R1 provides bias for the zener, while R2 and C form an RC filter to reduce the unwanted ripple from the switched power supply (if you have no filtering, you defeat the purpose of using a linear regulator because the LM338's output voltage would carry all the ripple from the switched regulator). The LM338 output voltage should follow its input voltage minus ~3.9V, what provides an appropriate dropout voltage for this regulator. The output voltage could be controlled by a potentiometer connected to the feedback line of the LM22679, as you suggested.


  • I haven't tested or simulated this circuit
  • the choice of values for the discrete components may be a little tricky

Other comments:

  • I'm not sure how clean you need your output to be. Keep in mind that you can reduce the ripple of the switched power supply by simply increasing the inductor and capacitor values. Depending on the load, you could also have a simple RC filter in front of it to further reduce the ripple from the switched power supply


simulate this circuit – Schematic created using CircuitLab


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