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WEBENCH® Tools/LMR23630: Is Rent needed for frequency synchronization if there is an external EN signal?

Lucy Gong
Prodigy 80 points
Part Number: LMR23630

Tool/software: WEBENCH® Design Tools

Hello,

I have been using the WEBENCH tool to help me design a circuit with LMR23630 (with RT pin, WSON-12 package). The intent is to convert 5-19 V input to a 3.9V output at 0.5A, synced with a 312.5kHz clock. The design provided by WEBENCH includes a UVLO setup with Rent = 124k, Renb = 205k. Looking at the datasheet, it says that Renb is not needed if the UVLO feature is not needed. However, in addition to syncing the buck to an external clock, I want to be able to enable and disable it with an external enable signal. I'm assuming that Rent essentially acts as a pull-up resistor, so the converter will always output 3.9V whenever there is an input voltage present, which is not what I want. Would it be possible to remove the Rent and use an external Enable signal instead? Would providing no consistent positive voltage and only an external clock damage the converter, or will the converter simply be disabled?

Thank you,

LG

  • 0
    TI__Genius 11040 points

    Hello LG,

    In order to use the Sync function of the EN/SYNC pin, Renb and Rent are not necessary.  The Sync signal can be applied from an external source.  In order to enable the device continuously, the average of the sync signal must be above the Enable threshold (1.7V):

    In order to use the sync function, the SYNC amplitude threshold of 2.8V (typical) is required to trip the internal synchronization pulse detector, and the minimum SYNC clock ON-time and OFF-time must be longer than 100 ns (typical).  

    Please let me know if you have any additional questions or if anything is still unclear.

    Thank you,

    Katelyn

  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 80 points

    Hello Katelyn,

    Thank you for the clarification - it was confusing since the datasheet also showed this example for external clock sync:

    Also, since the amplitude of the AC signal must be at least 2.8V, does that mean for a 1.7V EN signal, the HI SYNC signal must be 1.7 + 2.8 = 4.5V?

    Thank you,

    LG

  • 0 in reply to Lucy Gong
    TI__Genius 11040 points
    Hi LG,

    I understand your confusion. The datasheet example would be one way to connect the SYNC signal.

    The AC signal must have a peak amplitude of at least 2.8V, and it must have an average voltage (Duty Cycle x Peak Amplitude) must be greater than 1.7V.

    Please let me know if this is still unclear or if you have any additional questions.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 80 points

    Hi Katelyn,

    Does that mean it would be possible to have both an AC external clock to sync the device AND an EN signal to enable and disable the device? Something like this:

    Assuming a duty cycle of 50%, the 3.3V clock alone would not have an average voltage of 1.7V - but with a 1.8V enable, would the converter turn on and be synced with the 312.kHz external clock? Or is this not an option?

    Thank you for all the help,

    LG

  • 0 in reply to Lucy Gong
    TI__Mastermind 23845 points
    Hi Lucy,

    Yes, the configuration that you show on the schematic above should be ok. I don't think you need the diode though.

    Thanks
    -Arief