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TPS549D22: Visualizing loop stability

Obinna Ezeilo
Intellectual 2385 points
Part Number: TPS549D22
Other Parts Discussed in Thread: TPS54678

Hi team, 

For a design my customer is planning on implementing the TPS549D22 switcher. They have simulated the load transient response with the Webench tool and changed the inductor & capacitance values. The voltage drop seems OK, - please see the attached Webench report.

They would like to see if the loop stability is ok by looking at the bode plots, phase/gain margin and crossover frequency. However, in the Webench tool, these are not available for the TPS549D22 (for the TPS54678RTER these are available when they generated the Webench report). Is there an alternative way of getting these details?

WBDesign35_TPS549D22_0V9.pdf

Thanks for the support, 

Obinna. 

  • 0
    TI__Guru 56815 points

     

    Unfortunately we do not have a math calculator that accurately predicts the frequency domain Gain and Phase plot of the TPS549D22's D-CAP3 constant on-time control like we have for the TPS54678's current mode control.

    I will see if I can put something together for you tomorrow though.

  • 0 in reply to Peter James Miller
    TI__Intellectual 2385 points

    Hi Peter, 

    Thanks for the prompt response.

    I will be looking forward to hear if you were able to put something together. 

    Thanks, 

    Obinna. 

  • 0 in reply to Obinna Ezeilo
    TI__Guru 56815 points

      

    Based on the default switching frequency and compensation settings of 425kHz Switching Frequency and 1x RC internal ramp time-constant along with 6x

    This looks good and stable, but I am concerned about the DC bias derating and loss of capacitance along with parasitic phase-delays.

    If I reduce the capacitance by 30% (70uF per capacitor) the bandwidth increases to 180kHz and the phase margin drops to 40 degrees, so you will need to check the DC bias derating of the output capacitors and you may want to provide for an additional 1 or 2 output capacitors (7 or 8 total).