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LM536253QEVM: LM53625 EMI performance

Sushrut Kumar Pattar
Prodigy 40 points
Part Number: LM536253QEVM


Hello,

I am using LM53625xQEVM board to check my CISPR-25 EMI measurement setup to see if my setup gives the similar results as that of the users' guide.

Conditions of the setup:

Vin = 12V, Vout = 5V, Iout = 3.5A (external resistors are connected at the output)

LISN: TEKBOX 5uH

Signal analyzer : N9000A, 9kHz-3GHz

The setup is as shown in the figure. Two LISNs are connected to Power and return lines. The length of the LISN cable to the input of the board is at least 20cm. Noise from the power line is measured using signal analyzer and return line LISN is terminated by 50Ohm to avoid the noise interference from outside world.

I did the measurements with similar signal analyzer settings as showed in the page 18 of the user's manual. I have following questions regarding the measurement.

1) I am getting different results than the of the user's guide. At lower frequency from 150kHz to 30MHz, there is huge spike at the beginning and it is passing the limits afterwards. But from 30MHz I could see there is a noise and it is failing the limits, please check fig.1 in the measurement document. Not sure why there is such a noise. Could you please help me understand this? 

2) I noticed that the limits used in the user guide are different than that of the CISPR-25 limits. Also the sweep time, RBW are different compared to CISPR-25. For example, according to the table2 (https://www.alldaq.com/fileadmin/user_upload/files/manuals/tekbox/AN_Conducted_Noise_Measurement_Tekbox_LISN_TBOH01_and_EMCview_ALLDAQ.pdf) between 41-88MHz, 88-108MHz the limit is around 28dBuV. Where as in the user's guide in the mentioned frequency range, the limits are 25dBuv and less than 20dBuV respectively. Could you please let me know if there is any updates in the limits from CISPR-25, if not please let me know why there is change in limits?

3) The sweep time between 150k-30MHz is around 300s, considering the RBW = 9kHZ, but in the user's guide it is only 10s. Why is there is a reduction in sweep time?

4) Measurement done as per CISP25 standard, there is lesser noise (fig2) compared to the measurement done as per the user guide (fig1).

Measurement.docx

  • 0
    TI__Mastermind 28020 points

    Hi Sushrut,

    I will need to check with the EVM designer and get information from them. I will let you know what I find out.

    Regards, Jason

  • 0
    TI__Mastermind 28020 points

    Hi Sushrut,

    Can you please clarify a couple things about your EVM set up? I see the power supply current meter says 3.86A, I assume that is the current limit setting and not the current being delivered? Also, the marking on EVM shows it’s adjustable EVM, what VOUT is used?

    The Load resistors are bent over to the IC, this will couple Switch node noise into the LISN setup and bypassing EMI filter. The Load resistors should be better bent upwards or to the LISN facing and not over to the Switch node.

    If Spread Spectrum is ON there is naturally a bit larger noise in the lower frequency band from Spread Spectrum it self. Why it’s so large in his case I can’t explain, maybe it’s coming from the input power supply used.

    The user's guide used 200mm wire length power wire hanging at level as EVM, see the user's guide. Shortest allowed 20cm wire have better results than using longer wires. If the wires are separated like in above pictures they can form a parasitic inductance or can pick up noise and measurements are not very repeatable. Use similar wire setup as in user guide.

    Regards, Jason

  • 0 in reply to J Arrigo
    TI__Mastermind 28020 points

    Hi Sushrut,

    It has been several days, so I hope you were able to have better EMI results with the recommendations above. 

    I will close this post, but you can respond if you still have questions.

    Thanks, Jason