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I am somewhat new to electronics and am trying to get the output of an LM350T_NOPB voltage regulator to have less than 50mV line noise at all loads <= 3A. Please refer to the schematic, oscilloscope screen shot, and datasheet link below for the following discussion.

The problem is there is a 4.1MHz oscillation on the output of the LM350T ranging in excess of 600mVpp that starts at just a few mA output. I can reduce the amplitude of this oscillation to about 80-210mVpp (depending on load) with a 22uF capacitor from output to ground.

I do not understand the origins of this oscillation. I would also like to reduce it to <=50mVpp or even eliminate it altogether.

On page 6 of the data sheet, under "External Capacitors", they mention "like any feedback circuit, certain values of external capacitance can cause excessive ringing" and that a capacitor placed from output to ground can "swamp" the effect. They recommend a 25uF electrolytic capacitor.

Is this feedback circuit ringing that I am observing?

I am also having trouble determining which specification applies to this oscillation in terms of output voltage tolerance.

I have found a specification on page 3 of the datasheet entitled "RMS Output Noise, % of Vout" which lists a specification of "0.001% of Vout" for "10Hz-10KHz" but am unsure if this applies to this oscillation because the frequency and amplitude this troublesome oscillation are far higher than that listed in the specification.

I would very much appreciate any help in understanding this issue and any suggestions on how I may reduce further or eliminate this oscillation.

Schematic:

enter image description here

Screen shot:

enter image description here

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  • \$\begingroup\$ Where are your input and output capacitor decouplers on the 7905? \$\endgroup\$
    – Andy aka
    Commented Jan 6, 2023 at 12:49
  • \$\begingroup\$ I do not have any right now but thought about a 0.1uF for input and have experimented with a 220uF for the output (which did help reduce the 7905 Vpp output noise). Do you adding them might help reduce the LM350T oscillation issue? \$\endgroup\$
    – Pat
    Commented Jan 6, 2023 at 12:58
  • \$\begingroup\$ @pat: What kind of capacitor did you use for C5 (the output capacitor of the LM35T?) That part was made back in days when ceramica capacitors with high capacities weren't available - the datasheet mentions using a tantalum capacitor. A ceramic capacitor might cause the problem, and the datasheet won't warn against it because that wasn't a possibility back when the LM350 was developed. \$\endgroup\$
    – JRE
    Commented Jan 6, 2023 at 14:32
  • \$\begingroup\$ I would avoid high current linear regulators these days as decent off the shelf DCDC converters are better and far more efficient (get far less hot!!). But, sticking with linear, I'd question your inputs to the feedback ADJ pin. If you have any capacitance or inductance here (read long wires or big variable resistors) then you will introduce a delay which will cause the oscillation you see. \$\endgroup\$
    – Jay M
    Commented Jan 6, 2023 at 14:55
  • \$\begingroup\$ @JRE - I am using an electrolytic capacitor on C5. I could check out tantalum capacitors. Yes, the author of the datasheet really seems to like tantalum tantalum capacitors because, according to them, they have low impedance even at high frequencies. I'll look into getting tantalum capacitors. Thanks for the info. \$\endgroup\$
    – Pat
    Commented Jan 6, 2023 at 16:04

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This is a very delayed post concerning the solution to this oscillation issue but I wanted to close this discussion out with the solution.

Basically Andy aka and Jay M were correct. I checked the output of the LM7905 and it had a 6 MHz oscillation on it. This fed to the Adj terminal of the LM350 which then found it's way to the LM350 output. I followed Andy aka suggestion and put a 22 uF electrolytic capacitor on the output of the LM7905 and the oscillation was gone and the problem was solved.

Thanks to Andy aka and Jay M and everyone else who helped out!

EDIT: 6 MHz, not GHz.

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