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LMZ1050x Rfbt Calculation

Nikolay Gniteev
Prodigy 80 points
Other Parts Discussed in Thread: LMZ10503

In the datasheet for LMZ10503 it is said that Rfbt should be calculated is defined based on the voltage loop requirements and then Rfbb is then selected for the desired output voltage. (section 8.2.2.2.1 of Datasheet http://www.ti.com/lit/ds/symlink/lmz10503.pdf 

There is no equation for Rfbt in datasheet but there is a link for AN-2013 http://www.ti.com/lit/an/snva417d/snva417d.pdf

  • 0
    TI__Genius 15810 points
    The LMZ10503 is a voltage mode control based buck regulator. In this architecture, the value of the upper feedback resistor will influence the loop response. The application note discusses that and gives the equation.
    Regards,
    Akshay
  • 0 in reply to Akshay Mehta
    Prodigy 80 points

    Sorry, somehow the general question has been washed away from my initial message.

    The question is that there is not enough data for calculation Rfbt (and therefore Rcomp and Rcomp) in latest revision (Rev D) of app note.

    The background of the question is that some time ago we used Rev B of app note for calculation and things were simple. But today we encountered problems with LMZ10503 (ICs are burning out) so after datasheet revision we had found Rev D of app note. Besides formulas change in app note, recomended values for LMZ10503 also has been changed. And specialy changed value of Rfbt for high value Co - it is much greater than in Rev B. So I'm trying to understand if loop correction should be done in our device.

    According to Rev D of app note it's assumed that Rcint (equation 6) and Rout (equation 8) are known values. But I hadn't found any of this for LMZ10503.

    Without knowing these values it's impossible to calculate Rfbt and therefore Rcomp and Ccomp if using Rev D of app note.

  • 0 in reply to Nikolay Gniteev
    TI__Genius 15810 points
    I just updated your other post with the values of RCINT, CCINT, and ROUT.
    Would you kindly give us the background on the LMZ10503 blow up? What is the application and under what condition was the IC blowing up? I would definitely suggest performing loop correction on your design. If you'd like, I can be of assistance with that.
  • 0 in reply to Akshay Mehta
    Prodigy 80 points
    Thank you for pointing out about Rcint and Ccint. Still there is a question about ROUT, which I asked in thread e2e.ti.com/.../546030.

    My coworker already started thread about failures and you already saw this (e2e.ti.com/.../548852).
    I suggest continue conversation about failrues in his thread and use first thread for questions about compensation. In that case this thread could be clossed so we can concentrate.

    I'll post soon into two threads above.