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LMR23625-Q1: Manual Spread Spectrum via external clock < 300KHz

Martin Seeger
Expert 5411 points
Part Number: LMR23625-Q1
Other Parts Discussed in Thread: LMS3655-Q1, LMS3655, LMR23625

Dear Team,

similar request like my previous post about the LMS3655-Q1, but the customer has new specs (lower output currents):

  • V-in = car battery
  • V-out = 5V
  • I-load = 0.65A constant and occationally peaks with 2A
  • max. switching frequency < 300KHz

Is it possible to implement a frequency sweep / manual spread spectrum with this device?

The idea would be to use a MCU, connected to EN/SYNC. We would apply a 250kHz... 300kHz clock with maybe +/-5% deviation. Similar idea what the LMS3655-Q1 has integrated with its Spread Spectrum Control.

Would this work / would the device lock fast enough to the clock?

Would it make a difference if we take a device with or without FPWM?

Thanks and best regards
Martin

edit / ps 22/03 1:30pm (Europe): some copy&paste went wrong. Rewrote some parts

  • 0
    TI__Mastermind 23835 points
    Hi Martin,

    What is your modulation frequency? Also the LMS3655 part eventhough it has a +/- 3% deviation, it uses a pseudo random generator cycle by cycle frequency hopping. Therefore even if you synchronize the part with an FM frequency of 10kHz be aware that you might see this frequency as a beat frequency and might not be as effective as a built in spread spectrum as the LMS3655 part have.

    As for the FPWM or non-FPWM, the SYNC or synchronization only happens when the part is in CCM mode. Therefore if you want to synchronize from 0A to 2.5A, you might want to use the FPWM part.

    Hope that helps

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    TI__Expert 5411 points
    Hello Arief,
    thanks a lot for your answer.

    Frequency Modulation: That's a good question. I thought that the LMS3655-Q1 used a sawtooth FM scheme. But maybe I am wrong about that. Let me see which flexibility the customer has, maybe they can generate a similar scheme in the MCU anyhow.

    But in general, you do not see any problems implementing something like this? So for example that the part cannot lock to a changing frequency?

    My idea was to order an EVM and ask the customer if they wanted to test device like they did with the LMS.

    Best Regards,
    Martin
  • 0 in reply to Martin Seeger
    TI__Mastermind 23835 points
    Hi Martin,

    The part should be able to lock to the frequency that you set to. Just make sure that you are selecting a FPWM part in order to sync from 0A to 2.5A. I would still think that the LMS3655 is a better option in terms of EMI mitigation because it is a hot rod package and there is no bond wire like the LMR23625. Therefore it reduces parasitic inductance internal to the IC that might cause ringing on the SW node. You can also see the pin layout in the LMS3655 is optimized for the EMI (Input capacitor on both sides of the IC) with opposing current loop that helps on EMI.

    Page 22 on this datasheet helps explain the spread spectrum technique used on the part
    www.ti.com/.../lms3655.pdf

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    TI__Expert 5411 points
    Hi Arief,

    thanks for the answer & back ground information. That helps a lot!

    I will discuss the options with the customer.

    Best Regards
    Martin