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LM51770: Compensation Network Resistor Calculation

Clayton Wrobel
Prodigy 20 points
Part Number: LM51770


I am designing a power supply with this part.  I am having trouble with the Compensation Resistor calculations.  

There are 2 diffeerent formulas  in the datasheet, equation 3 and 46.

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The value for A-CS is not in the datasheet.  Equation 3 does not give 1.9K for the example design.

 

  • 0
    TI__Guru 68215 points

    Hello Clayton,

    I recommend using the quickstart calculator for selecting the compensation components: LM51770-QUICKSTART-CALC Calculation tool | TI.com

    Please share your calculation for RC1 that we can compare your and our calculation.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 20 points

    I am trying to validate the quick Calc sheet.  I Have some concerns with the crossover frequency.  It recommends 17 KHz, and it  is valid at low input voltage, but drops below 10 KHz at the high input.

    Here are my calculations.  Acs is a guess, since it doesn't seem to be in the datasheet.

    6685.Calcs.xlsx

  • 0 in reply to Clayton Wrobel
    TI__Guru 68215 points

    Hello Clayton,

    Thank you very much for sharing your calculations. I will need some time to check them out. The crossover frequency will change over input voltage.

    Be aware that the calculator is always just a starting point as it cannot include all parameters coming in through parasitics for example.

    So most important for any type of power supply are measurements on the bench as for example input impedance, load impedance, board parasitics, temperature, external component variations cannot be included in such a calculator.

    Especially startup, load transients, line transients are important measurements.

    Best regards,
    Brigitte