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Original question:

LM5122: calculation spreadsheet

LM5122: error amplifier zero relative to load pole frequency for compensation

Kelvin Le THS
Expert 7855 points
Part Number: LM5122

In the datasheet, it recommends placing the error amplifier zero at the twice of load pole frequency (eq. 49 of  pg42)

1. Is it okay to place the zero at the pole instead of twice the load pole frequency?  Please explain why 2x the load pole frequency.

2. The load pole frequency depends on Rload which changes with Iload.  Should I calculate the compensation at the max Iload? What happens to the compensation when Iload decreases? 

  • 0
    TI__Mastermind 21720 points

    Hi Kelvin,

    Thanks for reaching out with your question and for considering the LM5122 in your design.

    1. The error amplifier zero placement is suggested at 2x the load pole to provide enough phase margin to ensure the control loop is stable. For most designs this will result in a stable control loop. The error amplifier zero can be moved around as needed to improve the phase margin as needed.

    2. It is recommend to compensation a boost regulator at the worst case operating conditions. This is when the input is at the lowest level and the output load current is at the highest level. The compensation will stay the same as the load changes, the plant function is what is changing. The load pole frequency will increase and the RHP zero frequency will increase. As the loop is compensated at the worst case operation condition the loop will remain stable.

    Please let me know if you have any questions.

    Thanks,

    Garrett