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LMZ21701 noise

Ray-Chang
Prodigy 40 points
Other Parts Discussed in Thread: LMZ21701, TPS54328

Hi,

I am looking for a buck converter. And I notice LMZ21701.

I have some question as below.

1. As datasheet Page7 Figure 10. Radiated EMI on EVM==> For Radiated Emissions. Does it will be worse as this condition? Vin=9V and Vout=5V Iout=1A 

2. I have 3 device need LMZ21701 to provide power. And the 3 device are separate in 3 area. Do I need to add bead in case of noise couple to other device?

3. For the GND. Doesn't it need to separate as PGND and AGND? Does it need to add more GND via for thermal and noise?

Please help to reply.

Thanks

  • 0
    TI__Mastermind 19020 points
    Hello Ray-Chang,

    It sounds like you are worried about noise. Going with a module is probably a good idea, because the layout will be much easier and the switch node will be quite small.

    Let me try to answer your questions.

    1. The noise in a buck converter depends quite a bit on how the input is bypassed (input capacitor placement) and the switching current at the input. Since converting 9V to 5V at 1A will require more input current than 12V to 3.3V at 1A, it is possible that you can see more noise from the input. As always, it will depend on the board layout and how closely you can place the input capacitor to the device. You can also consider adding a small LC filter on the input so noise does not get conducted back to the supply.

    2. If you use an input LC filter for conducted EMI mitigation (as in 1.), that would prevent switching noise coupling to other devices through the input lines. See this application note for some tips on EMI filter design www.ti.com/.../snva489c.pdf

    3. The power ground connection to the chip power MOSFETs happens at pin 5 of the device. This is where you want to place your input capacitor (across pin 5 and pin 8). For thermal, you can place vias under the center GND pads to get heat away from the chip and into the PCB copper. See the pin description on page 3 of the datasheet.

    I hope this helps.

    Cheers,
    Denislav
  • 0 in reply to Denislav Petkov
    Prodigy 40 points

    Hello Denislav

    Thanks for your reply.

    For Question 2. I think I may need LC filter for input/output of LMZ21701. And placement as close as possible to input/output capacitor. Right?

    For Question 3. I have confused about output capacitor. For LMZ21701 page 28. Output capacitor GND connect to center GND (No GND via to GND plane) .

    But for other TI buck(TPS54328), output capacitor will use via to GND plane. Does it better to have GND via for output capacitor?

    One more question. Because of LMMZ21701 is 100% duty. It should be able for Input=6V and output=5V Right? 

    Thanks for your reply

    Regards,

    Ray