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LM73605-Q1: Reference Design Schematic Review

Michael Lu (Santa Clara)
Mastermind 29655 points
Part Number: LM73605-Q1
Other Parts Discussed in Thread: LM73605, LMS3655

Hi Team,

I am currently working on a reference design where I would like to design in an LM73605-Q1 to support an application where the input can range from 9V-30V and the output must be 3.3V and draw up to 1.5A.

Is it possible to perform a schematic review before I move forward into layout? For example, I could use some guidance about whether there is a more optimal choice for switching frequency and output series inductor.

Please find the schematic attached.

HSDC056A(001)_Sch_MainPWR_LMH73605-Q1.PDF

Michael

  • 0
    TI__Genius 11040 points

    Hi Michael,

    Overall your schematic looks good.

    The switching frequency will dictate the inductor necessary.  Currently the design looks good for 500kHz operation, with L=3.3uH and the RT pin open.  By increasing the switching frequency, you can decrease your inductance and output capacitance values at the expense of lower efficiency.  If you are flexible on size for this reference design and application, I would recommend designing the LM73605 to operate at 350kHz - 500kHz for the highest efficiency.

    Second, for improved efficiency you can tie the Bias pin to the output.  The Bias pin acts as the input to an internal LDO, and by powering the LDO with the 3.3V output instead of the 12V input, the efficiency will be improved.

    Please feel free to let me know if anything is unclear or if you have any more questions.

    Best Regards,

    Katelyn Wiggenhorn

  • 0 in reply to Katelyn Wiggenhorn
    TI__Mastermind 29655 points
    Hi Katelyn,

    Thanks for the quick response (and good to see a familiar name on the other side)!

    A couple questions and I should be good to go:

    1. If I choose to go bigger for inductor at the output, say, 4.7 uH shielded inductor, does this buy me anything compared to 3.3 uH? We are flexible switching-frequency wise. I left the header pin options for changes in the switching frequency in case changes are needed for EMI optimization.

    2. From an EMI perspective, I'm curious about whether it would be helpful to add anything else. I noticed that there's an option in Webench for an EMI filter that involves potentially adding some external components. Any tips there?

    I'll connect the bias pin to 3.3V instead of the 12V input. Thanks again for your help!

    Michael
  • 0 in reply to Michael Lu (Santa Clara)
    TI__Genius 11040 points
    Hi Michael,

    1. For automotive applications, you want to operate below or above the AM band for EMI purposes. I would recommend operating at 400kHz, but for EMI purposes you can also operate at 2.1MHz. Second, it will be important to use a shielded inductor for improved EMI performance. We have seen good results with the XAL6060 series.

    2. If you are trying to pass CISPR 25 Class 5 standards with this design, I would recommend adding an input EMI filter. I can share an EMI test report and the corresponding EMI filter via e-mail.

    3. Depending on your design schedule, a second option would be to use the LMS3655 (36V, 5.5A, 400kHz) device. This device offers pseudo-random spread spectrum which dithers the switching frequency to lower the peak emissions. This device also offers hotrod packaging which eliminates the parasitics associated with wirebonds and consequently greatly lowers switch node ringing. I can also share an EMI test report and input filter design via e-mail.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    TI__Mastermind 29655 points
    Hi Katelyn,

    Thanks for the recommendations. I think for my current design, I should be in good shape. However, I will look into implementing your recommendations for a second reference design board I am making which will be more centered around EMC performance.

    Let's continue our discussion further over e-mail.

    Thanks,

    Michael