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TLVM13640: How to select Fsw and Rt for a given Vin range?

Tony Zhang
Prodigy 195 points
Part Number: TLVM13640
Other Parts Discussed in Thread: TPSM63604

Dear Friend

I am designing a 3.3V Vout application with TLVM13640 and an input range of Vin =5V - 12V  From the datasheet table 8-1(without Vin factor),  suggested Fsw = 0.7-0.9MHz (700MHz-900MHz should be a mistake obviously), that means suggested Rt = 18.78K - 14.5K ohm (based on equation 5).

However, based on equation 6 with Vin factor in,  I will get Fsw = 0.28MHz and 0.598MHz according to Vin = 5V - 12V respectively.  then, Rt= 47.6K - 22K range which is quite different from the range 18K - 14K.

So the question becomes: Should we follow the table 8-1 or the equation 6 when selecting Fsw and Rt for a given Vin range?

Thanks

Tony

  • 0
    TI__Guru 62887 points

    Hi Tony,

    Table 8-1 is based on Vin = 24V and targets 30-50% ripple current in the inductor (and yes that should be kHz not MHz in the table header).

    Lower Fsw is possible when Vin is lower. Equation 6 is your best guidance for this. Also, use the quickstart calculator to help with the design.

    Regards,

    Tim

  • 0
    Prodigy 195 points

    Hi Timothy

    Thank you for the useful information.

    For a given range of Vin, we should use maximum Vin as Vnor and equation 6 to select Fsw and Rt, Am I right?

    Another question is about VLDOIN, I know that when Vout = 5V, Connecting VLDOIN to Vout is good, while, my Vout = 3.3, and my Vin might exceed 12V a bit.

    So should I connect VLDOIN to GND or through a capacitor to GND?

    Thanks

    Tony

  • 0 in reply to Tony Zhang
    TI__Guru 62887 points

    Hi Tony,

    You can use Vin-nom for the calculation.

    Also, mu understanding is that 3.3V is too low for external bias (please check this in the datasheet). If so, then tie that pin directly to GND. Take a look at the example circuits in the datasheet for this module and also its counterpart, the TPSM63604, for more detail.

    Regards,

    Tim