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LM43603: Cff calculation

Andrejs Gasilovs
Expert 1335 points
Part Number: LM43603

Hi everyone,

I'm following the datasheet and trying to calcualte Cff value for LM43603. I'm following section 8 as a design exmaple.

Crossover frequency, fx, calculated using equation 21 and given Vout = 3.3V and Cout = 150uF is 10.7kHz

Based on that, calculated Cff using equation 22 (and taking Rfbt = 1Mohm and Rfbb = 432kOhm) is 27pF

 

Which then gives you a zero at 5.88kHz and a pole at 19.5kHz , so crossover frequency is somewhat in the middle, as suggested.

But the datasheet for some reason chooses 47pF, that gives a zero at 3.39kHz and a pole at 11kHz (almost at a crossover). That doesn't make much sense...

  • 0
    TI__Mastermind 27030 points

    HI:

    the cff is used to boost the gain and phase for higher cross frequency to improve the transient performance. the zero impact will from 0.1x to 10x, so both 47pF and 27pF(actually need 22pF or 33pF) is helpful for improve the cross frequency, and final result need verified based on the practical loop test. 

    Thanks

  • 0 in reply to Daniel Li14
    Expert 1335 points

    Hi Daniel,

    What do you mean by "improve the cross frequency"?

    It is problematic to perform gain-phase analysis of the controller as it is internally compensated, so I try to follow datasheet recommendations and it's a bit confusing when datasheet suggests one thing but does something opposite.

    I will, of course, finalise the circuit including Cff on the bench, but I need to have some kind of a model with predicted gain and phase frequency responses which I will then use to compare against the actual circuit.

  • 0 in reply to Andrejs Gasilovs
    TI__Mastermind 27030 points

    HI

    you can use the wenbench mode to performance the loop analysis to see the cff impact on the gain and phase.

    Improve the cross frequency means to get a higher cross frequency which will be helpful to the transient.

    below is wenbench sim result of 33pF and 47pF 

  • 0 in reply to Daniel Li14
    Expert 1335 points

    Thanks Daniel, I'll give it a go and then let's see how accurate simulation is.