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LMR36015AEVM: PGOOD Behavior after Load-Dump Test

I Chou
Prodigy 10 points
Part Number: LMR36015AEVM

Tool/software:

Hi experts,

I have used LMR36015AEVM for load-dump test, the voltage was clamped at 48V which is within the input voltage spec (60V) of the EVM.

However, after the load-dump test was finished, the output waveform of the EVM would have some fluctuations rather than keeping at some constant values.

When the PGOOD (pin2 of J4) was not connected to V_CC or V_OUT, I can still get about 0.512V at PGOOD.

I would like to know is the behavior of PGOOD normal as described?

Thanks!

  • 0
    TI__Mastermind 31165 points

    Hi,

    Technically, PGOOD is allowed to be floating if not used, and the voltage it is within part operation spec. However, if you are using it as a MCU control signal, recommend connecting to either VCC or VOUT. 

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 10 points

    Hi Richard,

    Thank you for your reply!

    I had also connected the electronic load to LMR36015AEVM after the load-dump test with four different cases of PGOOD being connected to V_OUT, V_CC, shorted to GND, and kept open.

    In each test case, the load current were 0 A, 0.5 A, 0.8 A, and 1 A.

    It was observed that when the first test case was performed (connected to V_OUT), the output voltage would fluctuate with all four different values of load currents were applied, and high-frequency noise could be heard during output voltage fluctuation when the load current of 0.5 A to 1 A were used.

    However, the output voltage fluctuation occurred only at performing the first test case for the first time, the phenomena didn't occur in the second time or when other cases were performed.

    Is there something wrong with the EVM?

    Thanks!

  • 0 in reply to I Chou
    TI__Mastermind 31165 points

    Hi,

    I think it may be an issue with the test setup. When device is operating at no-load, device is operating in PFM which means frequency is modulated. It is possible that the burst of pulses is interacting with the electronic load setup and causing you to hear the noise. 

    If you replace the e-load and/or power supply, do you notice the issue?

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 10 points

    Hi Richard,

    In the first attempt to test the EVM with no load,  the noise could be heard when the load current was 0 A, 0.5 A, 0.8 A, and 1 A.

    However, as soon as the first attempt was finished, the second attempt was carried out, and no noise was heard when all the four values of load current were applied.

    I am not sure what is the mechanism causing the noise to disappear and the output voltage to be stable in the second attempt of testing the EVM with no load.

    Thanks!

    I Chou

  • 0 in reply to I Chou
    Prodigy 10 points

    Hi Richard,

    The table shows  the data of the first and second tests of testing the EVM with no load.

    Hoping the table will describe the issue more clearly!

    Ian Chou

  • 0 in reply to I Chou
    TI__Mastermind 31165 points

    Hi Ian,

    Thanks for sharing the data, and I understand the issue now. I see that at 1A, the output fell to 7.05V. 

    Do you have any picture of the output voltage waveform or noise issue during the first test? If not, the next step is to see if it is replicable. Can you confirm if the power supply was current limited during the first test? 

    if you swap ICs, does the issue track with a known good unit? 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 10 points

    Hi Richard,

    Thanks for providing further information!

    1.These are the photos of the output voltage waveforms:

    2.The current limit of the power supply is 1A in all first and second tests.

    3.I only had one LMR36015AEVM with me right now, son unfotunately it is not possible to swap the ICs.

    Thanks!

    Ian Chou

  • 0 in reply to I Chou
    TI__Mastermind 31165 points

    Hi Ian,

    I agree with you that the output voltage waveform is not good, though I'm curious about the large amount of noise at the VOUT pin. Are you probing vout with a tip and barrel setup? Are you able to replicate the waveform? 

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Prodigy 10 points

    Hi Richaed,

    I didn't use the tip and barrel setup to probe vout.

    Instead, the vout was connected with the tip of the passive probe and the probe was coonected to ground with an allegator clip.

    This is the passive probe I used:

    I will update you on how the vout waveform looks like once I get the resul!

    Thanks!

    Ian Chou

  • 0 in reply to I Chou
    TI__Mastermind 31165 points

    Hi Ian,

    When probing VOUT with the ground lead, it may actually pick up a lot of the switching noise so it is difficult to tell what part of the waveform is due to the switch node and what is due to the output voltage falling. Let me know once you get the results.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 10 points

    Hi Richard,

    I have received a new LMR36015AEVM today.

    The V_OUT of this new EVM looks stable, so I will take this new EVM for load-dump test.

    Really appreciate your time for answering my questions in the past week.

    Wishing you a pleasant and fantasic weak!

    Ian Chou