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LM4140: Stability issue

Dandanache
Prodigy 40 points
Part Number: LM4140

I'm using the LM4140 ACM -1.0 as a voltage reference for some current shunt amplifiers and the ADC converter.

Input voltage is 3.3V(from an TLV70033 LDO with a 10uF cap output and threw a ferried bead, this rail is clean), input cap is 100nF, output cap for the reference is a 1uF 16V X5R (GRM155R61C105KA12D) and with a 1k resistor to provide a constant load on the output of about 1mA.

With this setup I'm seeing a saw tooth waveform with a Vpp of 45mV and a freq of about 44.6kHz.(see below)

Removing the 1k helps but there still seems to be some oscillation present.(see below)

This feels wrong, such a device should be more stable IMO.

I've looked at the output caps suggested in the datasheet but they can't be sourced, and the part I am using falls withing the recommended ESR but only between 1 to 10kHz.

Am I doing something really wrong here?

  • 0
    Guru 140200 points
    Hi Dandanache,

    when using a ceramic cap at output of LM4140 put a small resistor in series to this cap to increase the ESR of cap. In your case this would be an 1R resistor in 0603 or 0805 package.

    And what is with the ferrite bead at input? This could also resonate. Can you show us a schematic?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 40 points
    Hi Kai,

    Thank you for the input. I'll have to give that a try.
    The 3.3V rail is shared with other devices(not really needed but belts and suspenders recommendation), I've replaces it with a 0R link and still get the same waveform on the output.

    Dandanache
  • 0 in reply to Dandanache
    Guru 140200 points
    Hi Dandanache,

    if you want to use the ferrite bead keep in mind that they can show a rather high low frequency inductance. This can create a resonance unless properly damped. A usual ferrite bead has 1...2µH low frequency inductance. In combination with 1µ ceramic cap a ringing can be observed at arround 130kHz according to Mr. Thomson. To dampen the series resonance a damping resistance of R >= SQRT(2L/C) = 1R8 would be needed. Put this resistance in series to the ferrite bead. Another method is to use a cap with enough ESR to also dampen the resonance, like a tantal, for instance.

    Kai
  • 0 in reply to Dandanache
    TI__Mastermind 19580 points
    Hi Dandanache,

    I agree with Kai's observation about the ferrite bead with capacitor oscillation. Can you remove this beads to see if this is the cause of the problem?

    Can you replace the LM4140 with another LM4140 or test the LM4140 with a separate power supply to make sure that the device does not have problems with the input?
  • 0 in reply to kai klaas69
    Prodigy 40 points

    Hi Kai,

    I've added the 1R between the output of the LM4140 and the output capacitor. This had a big impact, thank you for the suggestion! (see below)

    Input voltage is clean, replacing the ferrite bead wit a 0R link, or supplying the LM4140 from an external PSU did not improve the output.

    I do not have Tantal caps on hand so I did not test that option.

    As such, it is safe to assume that this stability issue is down to the ESR of the output cap, do you agree?

    Thanks,

    D

  • 0 in reply to Marcoo Z
    Prodigy 40 points
    Hi Marcoo,

    As stated in my reply to Kai, I've replaced the ferrite bead with a 0R link - no improvement, replaced the LM4140 - no improvement, used an external PSU(Hameg HMP4040) to supply the input voltage to the LM4140 - no improvement.
    Only the 1R in line with the output helped, confirming my suspicion that it's down to the output capacitors ESR.

    If you don't agree with my assessment, please let me know.
    Thank you both for your time and suggestions, they are much appreciated.

    Regards,
    D
  • 0 in reply to Dandanache
    TI__Mastermind 19580 points

    Hi Dandanache,

    I am glad that it looks like your issue was resolved. You took the exact steps I would of taken to resolve the problem. I agree that the LM4140 was unstable. Section 8.1.2 has ESR examples for ESR regions of stability for future designs using this device.

    -Marcoo Z

  • 0 in reply to Dandanache
    Guru 140200 points
    Hi Dandanache,

    yes, the oscillation had to do with the missing ESR of 1µ cap. :-)

    Kai