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LMR60430-Q1: phase margin

TETSUYA TAKAHASHI
Prodigy 10 points

Part Number: LMR60430-Q1

Hello team,

I would like to design a circuit under the below conditions.
Vin: 8–16V
Vout: 1.2V
Iout: max1.5A
fsw: 2MHz
I ensuring a phase margin of at least 45 degrees.
When the output capacitance is set to 22µF, WEBENCH® POWER DESIGNER shows a phase margin of over 45 degrees. 
However, under the same conditions, the LMR604X0-CALC tool yields a phase margin in the 30-degree range, indicating that the output capacitance must be increased to achieve 45 degrees.
Which of these tools provides the correct result?

Thank you for your support.

  • 0
    TI__Mastermind 29470 points

    Hi Tetsuya,

    An expert will let you know the minimum C value you need for the VOUT.

    Thanks,

    Andrew

  • 0
    TI__Prodigy 460 points

    Hi Tetsuya,

    Both these calculators use simplified, linearized models of the buck converter transfer function to calculate your bode plot. As your duty cycle is very low (D = 1.2/16 = 7.5%), these models will become less accurate. Simulating on our power electronics software, I found your current design has a 13-degree phase margin (assuming a 1.5uH inductor). Increasing the output cap to 50uF, with all else left the same, yields a much more desirable 44.8 degrees of phase margin. Increasing the output capacitance more will further increase the phase margin but also draw down the bandwidth of the device. 

    Let me know if you have any further questions about this,

    Thanks,

    Aidan

  • 0 in reply to Aidan Moran
    Prodigy 10 points

    Hi Aidan,

    Thank you very much for your helpful response.
    I will use L = 1.5uH and set Cout to 50uF or higher, taking tolerance and DC bias into consideration.
    Regarding the feedback resistors, while 100kΩ is standard for RFBT, I am planning to set RFBT = 47kΩ and RFBB = 220kΩ. Do you see any issues with this?
    Additionally, could you please advise on the optimal capacitance for the feedforward capacitor (CFF)?
    Best regards,

  • 0 in reply to TETSUYA TAKAHASHI
    TI__Prodigy 460 points

    Hi Tetsuya, 

    your selection of RFBT and RFBB is fine, as long as the math works out so your output voltage and reference voltage are correct. I can attach an app note on selecting a Cff. However, in your case, because the reference voltage and your output voltage are so close together, the feed forward capacitor has a very small voltage across it (0.2V), and thus it will have a minimal effect on the loop transfer function. The largest knob to tune an internally compensated buck converter is with the output capacitance. 

    Cff App Note

    Thanks,

    Aidan