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LMZ21701: are the conducted EMI mitigation techniques discussed in Application Report SNVA489C worth pursuing with this device?

Britton Kerin
Intellectual 310 points
Part Number: LMZ21701
The LMZ21701 datasheet section 9.3.2 mentions SNVA489C only in the context of avoiding stability issues with long input power leads. What I wonder is if the other techniques in SNVA489C (input LC filter) are worth pursuing with the LMZ21701, or if the device already uses these techniques internally? The LMZ21701 emphasizes low conducted EMI as a feature, so I don't want to invest time trying to master the LC filter approach if it would be redundant and pointless?
  • 0
    TI__Expert 7500 points

    Hi, 

    Jean will look into this and feedback to you soon.

    Thanks,

    Lishuang

  • 0
    TI__Intellectual 1125 points

     

    Regarding EMI, there is no substitute for careful attention to layout, input filter design, etc.

    DC-DC converters do generate conducted EMI at various levels. Use of EMI filter is the most common way of controlling it, it is typically unavoidable.

    The discussion of 9.3.2 is more about the potential pitfalls of using LC filter and impact on stability. A switching supply presents a constant power load, and it presents a negative impedance. An LC filter, if without any damping, will have tendency to resonate (Zout of the filter becomes very high a resonant frequency) and it will be even worse if the load has negative impedance.

    Jean

  • 0 in reply to Jean Picard
    Intellectual 310 points
    It sounds like this means that an input LC filter should be included for designs using the LMZ21701, including a damping capacitor. I noticed that webench power design often has "Add an Input EMI Filter" as an advanced input option. Unfortunately when the LMZ21701 is first entered in the search field at the top of the page, the toggle for this option seems to be inactive. So I guess I would need to try to do this myself by following the advice in SNVA489C. I have a couple questions about how to do that: 1. My current design uses a shield can for the entire converter layout (following advice of cell module manufacturer). However, it looks like the large damping electrolytic capacitors required (>40 uF in my case according to SNVA489C ) would require a large can. Is it safe to instead place the input filter outside the can if a shielded inductor is used for Lf? 2. Alternately, is it ok to use an MLC cap together with a series resistor to give the required ESR instead of an electrolytic for Cd? If I understand correctly the ESR of Cd is only important at the cutoff frequency, so large ESR at other frequencies shouldn't be important, correct? 3. Do you have any advice about whether it is reasonable to leave sockets for the input filter components in the PCB design, then investigate exact values later? Britton
  • 0 in reply to Britton Kerin
    TI__Expert 4795 points

    Hi Britton,

    Jean will feedback to you next Monday. Thanks.

  • 0 in reply to Britton Kerin
    TI__Intellectual 1125 points

     

    1) the first thing to make sure is to have the high frequency ceramic capacitor inside the can and very close to the switching circuit. Your suggestion might be ok although it depends on how good the shielding is.

    2) discrete resistor in series with ceramic cap is OK

    3) emi components need to be soldered, PCB layout is very important. Socketed parts is not recommended, it also adds elevation over the board making them ineffective. You'll need to have footprint on the board and then experiment with soldered components (surface mount).

    Regards

  • 0 in reply to Jean Picard
    Intellectual 310 points
    sorry, by socket I means I would leave some pads without things mounted for experimentation as you suggest. Is there any chance the webbench might get updated to allow input filter design for designs baesd on LMZ21701? Assuming that that feature is doing what wer're talking about here for designs based on other parts
  • 0 in reply to Britton Kerin
    TI__Intellectual 1125 points

     

    I doubt there is any plan to do it for this device but I can check and let you know.

    Note that EMI filter design is quite complex and typically requires multiple iterations. It also depends a lot on component parasitics and PCB layout.

    Regards