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LM140L: LM140LAH-12 oscillating

Joao Ferreira41
Prodigy 40 points
Part Number: LM140L

Dear All

I am using the 12V fixed regulator LM140LAH-12 in a simple circuit as in attached file.

The goal is to remove "all" DC-DC ripple for a very sensitive application, extremelly low noise required.

The problem is that the regulator is oscillating, its output is a sawtooth signal on top of the regulated voltage.

With a test load current of 20mA, the p-p amplitude is a few tens of milivolt, in KHz range.

Note this is not an LDO.

Is there any recomendation about the capacitive load of this particular regulator? Removing the 4.7uF capacitor it no longer oscillates!

Looking forward to your suggestions.

Best regards,

           Joao Ferreira

  • 0
    TI__Mastermind 22610 points
    " ... Removing the 4.7uF capacitor it no longer oscillates! ..."

    Schematic shows two (2) 4.7uF capacitors.
    I will presume that: you removed Cout (C22) and it no longer oscillates, and that C22 is a low ESR ceramic, and that the oscillation does not appear at the input.
    The LM140 family is very stable, and generally is not sensitive to output capacitance value or capacitance type.
    Best guess, with oscillation 'in KHz range' : Circuitry ~might~ be ringing.
    I would suggest changing C22 to Tantalum.
  • 0 in reply to Donald Jones
    Prodigy 40 points

    Hi Donald

    I appreciate your comment.

    In fact I was refering to the output capacitor (C22). It is a low ESR ceramic (X7R).

    Your comment confirms what I thought.

    My post was intended to hear from other people's opinions and also to find out if there is any technical note regarding this device.

    The data sheet for the LM140 is doesn't say much about capacitive load and the behaviour of the LM140 with it.

    I wonder if TI recommends a replacement for LM140LAH-12 with a more recent device.

    I am also testing the TPS7A4901 for this application, but I would rather use a fixed voltage regulator.

    Thanks Donald.

    Regards, Joao