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LM337-N: Is there any tips in using LM337 to make it more stable?

user4382324
Prodigy 50 points
Part Number: LM337-N
Other Parts Discussed in Thread: LM337, LM317, LM317-N

I am using an LM337-N in sot-223 package. The circuit is simple, but I find the output of the circuit is ringing at a frequency about 17kHz. (I am sorry I have put a wrong image at first)

It's so annoying. I have replaced the output capacitor to a larger one, about 4.7uF, But it is still ringing. at a frequency about 6kHz.

Is there any tips in using LM337 to make it more stable?

  • 0
    Intellectual 515 points

    Please note that LM337-N is a Adjustable Negative Regulator, while the LM317 is a positive voltage regulator. Two different devices.

    I will presume that your question is regarding the LM317, as shown in your schematic.

    For the LM317-N : A 1-μF solid tantalum, or 25-μF aluminum electrolytic, on the output insures stability. Any increase of the load capacitance larger than 10-μF will merely improve the loop stability and output impedance.

    See Section '8.4.1 External Capacitors' in the LM317-N datasheet.

    http://www.ti.com/lit/ds/symlink/lm317-n.pdf

    Low ESR ceramic type capacitors are generally not recommended for LM317 (or LM337) Cout.

  • 0 in reply to Daddio
    Prodigy 50 points

    I am sorry I put a wrong image. I have put two new graphs in it.

  • 0
    Prodigy 50 points
    I have changed the output capacitor from ceramic capacitor to aluminum capacitor, and the ringing effect disappears. Maybe the ESR of ceramic capacitor is too low for the frequency compensation.
  • 0 in reply to user4382324
    Intellectual 515 points

    Yes, the ESR is the key.

    You can still use ceramic for Cout, you must add a low value resistor in series to simulate the ESR.

    I would suggest 1.ohm value for the series resistor. I would also suggest increasing the 1uF ceramic value to 2.2uF, or 3.3uF, to compensate for the loss of capacitance due to applied DC voltage.

    See : "Ceramic Capacitors Replace Tantalum Capacitors in LDOs " ; Document # SLVA214A

    www.ti.com/.../slva214a.pdf