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LM43601 Cross Over Frequency

Ross Parlour
Intellectual 1240 points
Other Parts Discussed in Thread: LM43602, LM43601

Hi there,

I have a customer working on a design with our LM43601 and LM43602 parts, whom I am trying to support. Can someone confirm that I am reading the datasheet correctly please?

Looking at page 30 in the datasheet (SNVSAA0A –JULY 2015–REVISED AUGUST 2015), equation 21 is used to estimate cross over frequency when using low ESR output capacitors. Equation 22 shows a relation between the crossover frequency, feedback network and feed-forward capacitor.

Can equation 22 be used to estimated cross over frequency based on selected feedback network and feed forward capacitor?

Kind regards,

Ross

  • 0
    TI__Mastermind 23815 points

    Yes you can. So equation 22 gives you the estimated value of CFF to get your desired crossover frequency

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    TI__Intellectual 1240 points

    Hi Arief,

     

    The plan is to use 2 SMPS with the frequency between 0.28MHz and 0.53MHz. In this range the EMC spec do not have any noise margin, aiming for reduced EMC future problems.

     

    Because the current is high (1.2A nominal for 5V and 0.4A for 3.3V), and low frequency and low inductor ripple current (also as a EMC potential noise source) are desired. The inductor value is calculated in the range of 33uH to 47uH and output capacitor in the range of minimum 220uF.

     

    This implies that electrolytic capacitors need to be used.

     

    With electrolytic capacitors on the output how can the crossover frequency be controlled? Reading the datasheet equation 22 only applies if low ESR caps are used meaning ceramic capacitors.

     

    Thanks,

    Ross

  • 0 in reply to Ross Parlour
    TI__Mastermind 23815 points

    Hi Ross,

    If you are running at low frequency such as 0.28MHz, you might not need the CFF. The electrolytic capacitor ESR will add the zero into the system instead.

    I would say that you can use webench to give you a starting point by entering your parameters for VIN, VOUT, Fsw, and Iout. Let me know if you have any further questions

    Thanks

    -Arief