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LM20: help to analyze the abnormal output

Lacey Lin
Intellectual 2900 points
Part Number: LM20

Hi team,

My customer use the LM20 in the power board in server.

The output of the LM20 is 1.4V. When they do the hot plug test of the power board, the output of the LM20 became the 2.7V and will keep 2.7V unless they power off the LM20.

And they found in the hot plug test, the 3.3V power for the LM20 has a vibration, see below.

The 3.3V power for the LM20 is the yellow one.

So can this vibration cause the problem?

How can we solve this problem?

The schematic is showed as below:

 

Please help to analyze this issue.

Lacey

Thanks a lot!

  • 0
    TI__Expert 6475 points
    Lacey,

    The waveform shows an underdamped response. To reduce ringing (vibration), I would recommend filtering the supply. I would play around with the input bypass cap value and/or adding a small series resistor (e.g. 10 to 50 ohm) to the Vcc supply. The LM20 doesn't draw a lot of current so the input RC filter shouldn't be a problem.

    -Kelvin
  • 0 in reply to Kelvin Le THS
    TI__Intellectual 2900 points

    Hi Kelvin,

    Thanks for your reply!

    So you mean the vibration will cause the abnormal output.

    Can you help to explain how the vibration cause the abnormal output and keep the abnormal output unless we power off the part and then power on.

    For the value of the bypass cap, do you have the recommendation?

    Lacey

    Thanks a lot!

  • 0 in reply to Lacey Lin
    TI__Expert 6475 points
    Lacey,

    I do not know the failure mechanism of the output latch based on the unstable supply. I will need to follow up with our designer but please keep in mind that this is an old part and it may take some time for our design team to figure out the behavior. I was assuming based on the information you gave me that the supply oscillation is the most likely cause of the abnormal output. I would try cap value in the range of 0.1uF to 0.47uF with the resistor.

    -Kelvin
  • 0 in reply to Kelvin Le THS
    TI__Intellectual 2900 points
    Hi Kelvin,
    Can you help to check how the viration of the supply affect the output of the part without Today. Customer need the conclusion to do plan the next step.


    Lacey
    Thanks a lot!
  • 0 in reply to Lacey Lin
    TI__Expert 6475 points

    Lacey,

    I just asked our design team again. But like I said, this is an old National part and it may take some time for our design team to figure out the behavior.

    In the meantime, can the customer test the RC filter at the input to see if this resolve the issue?

    Also,

    From the scope screenshot above, after the hot swap and the supply becomes stable, the output (blue) seems to oscillate from high to low every now and then.  Am I reading this right?

    This abnormal output behavior only happens when they perform the hot swap right?  Have they used LM20 before in other projects? If yes, did they experienced issues before?


    -Kelvin

  • 0 in reply to Kelvin Le THS
    TI__Intellectual 2900 points

    Hi Kelvin,

    The blue one is not the output wave. Customer say the output will remain the abnormal 2.7V even the 3.3V supply is stable.

    And then if they power off the part and power on, the output is correct. But after they do the hot swap, the issue is still.

    The output abnormal problem only happen in the hot swap test.  So if you can, please help me to reproduce the problem and analyze the root cause. I have asked customer to add the RC filter, but they said it's a little difficulty for them to do the change in the board.  They will try to do this.

    Also, if you can identify the root cause, it will be helpful for us.

    Lacey

    Thanks a lot!