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LM5176: Vin resonance issue

Tommy Tzeng
Genius 5690 points
Part Number: LM5176

Hi Sir,

We have problems in the application of LM5176. Due to our EMI requirements, we need to add an inductor PL4 (200uH/4A) at the input, as shown in the red circle in Figure 1 attached. In our application, LM5176 Vin=5V~36V, VOUT=8.25V (Tool is in the attachment). this problem is that when load on the output, the Vin voltage will have unstable resonance, as shown in Figures 3 and 4 attached. Different loads will also appear at different voltage.

The voltage is unstable (as shown in Figure 5). This problem was only seen after adding an inductor(PL4). Only when the inductor is paired with the LM5176 circuit can it be seen (the same for EVM test.). 

we would like to ask your comment. Does your experience suggest any solutions? The schematic and test waveform is in the attachment. The current test as below:

1. we try add E-Cap to Vin, and it improved. However, the space height is limited and it is impossible to add this type of E-capacitor.
2. To use MLCC Cap, we need to add at least 32pcs 10u/50V, but the space is limited and it is impossible to add so many capacitors.
3. Reduce the filter inductance(PL4) value to 150 uH no much improvement (<150uH does not help much with EMI).

LM5176 test report.xlsxAC FLEET.PDFLM5176 Buck-Boost Quickstart Tool_Joe_1208.xlsm

  • 0
    TI__Mastermind 29750 points

    Hi Tommy,

    Thanks for using the e2e forum and sending all the design details.

    It seems that the inductor at the output is influencing the LM5176 regulation loop until it becomes unstable.
    The quickstart calculator and the schematic look okay to me, so I agree on the assumption that the PL4 inductor is the issue here.

    You mentioned this inductor is necessary for EMI compliance. Would it be possible to adjust the output filter without violating their EMI specifications?
    We have additional resources on output filter design, as for example given in this app note:
    https://www.ti.com/lit/an/snva871/snva871.pdf

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 150 points

    Hi Sir,

    Thanks for your suggestion. We adjusted the inductor from 200UH to 15UH, which can solve this problem and meet EMI test.

    But we have another issue, the LM5176 input voltage has drops when the output is loaded as shown the figure 2 in the attached file. We did the some tests as follows and the issue still happen, we would like to ask your comment. Does your experience suggest any solutions?

    1. The issue still happen when I cut out the LM5176 output and connect the electronic load(as figure 3).
    2. The issue still happen when we try to add E-cap 220uF input and output in LM5176.
    3. I removed or bypassed some components to make the circuit on our board the same as the EVK(as figure 4).
    4. Similarly, I removed the input(100uF) and output(330uF x 2) E-caps on the EVM to make the circuit on the EVM the same as our circuit design.
    5. Compare our board to the EVM to make the circuit consistent.


    0447.LM5176 Buck-Boost Quickstart Tool_Joe_1208.xlsm0842.AC FLEET.PDFTest instruction.xlsx

  • +1 in reply to Hung HanTse
    TI__Mastermind 22510 points

    Hi Hung,

    I believe the resonance is due to the stability of the input filter. You may refer to Input Filter Design for Switching Power Supplies.

    I can see 0.2V voltage drop at the input. Does this come from the resistance of the cable between the power supply and the LM5176? You may try to use remote sense function of the power supply to see if it helps.

    Best Regards,

    Feng Ji