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TPS54620 Inductor DCR Effect on Stability

Mateo Begue
Intellectual 2805 points

Other Parts Discussed in Thread: TPS54620

Hello everyone, 

The customer that I"m working with, PMC, wanted to simulate their TPS54620 schematic on WEBENCH but received the following error when inputting their inductor value: 

"The following limits have been exceeded which may cause instability in your design:

- The property DCR with value '0.047 Ohm' is higher than the maximum recommended value of '0.02 Ohm" 

The design seems stable and has a phase margin of 48 degrees. I'm just curious how strict the 0.02 Ohm recommendation is. 


Thank you, 

Mateo

  • 0
    TI__Guru**** 191445 points

    Generally speaking the DCR has minimal effect on stability.  It does affect the Q of the output filter, and may cause the phase to change more rapidly at resonance . Who knows what the webench limitation is? I'll move this to the webench forum.

  • 0
    TI__Mastermind 28950 points

    Hi Mateo,

    I am not sure of the exact design input conditions used to reproduce this limitation. However, the maximum DCR is based on conduction loss in the inductor and is typically a certain percentage of the output power. If you increase the inductor DCR, that should lower the efficiency. As John mentions, the Q of the filter will get affected but since this is peak current mode control, the stability should not be an issue.

    Regards,

    Amod

  • 0 in reply to Amod Vaze
    TI__Intellectual 2805 points
    Thank you for the response Amod.

    Another question, the excel app note calculator for the TPS54620 is telling me that the the minimum inductor value that I can use is 1.8uH for a fsw = 480kHz and 1.3uH for a fsw = 665kHz. My problem is that WEBENCH is telling me that 1.8uH is lower than the minimum recommended value of 2.774uH. How are these limits calculated?


    Thank you for your help.

    Mateo
  • 0 in reply to Mateo Begue
    TI__Mastermind 28950 points

    Hi Mateo,

    Inductor selection is based off of ripple limit of 30% of the Iout value. If you can provide the input-output design conditions, I can take a look at this. 

    Regards,

    Amod