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TPS54560-Q1: calculation of Equation 46 and 55

Takao Yamamoto
Genius 4135 points


Dear all,

Please teach me about following.

■Question 1

I can not understand how to calculate 'fco' in Equation 46.

For example, how did you calculate 29.2kHz in Equation 46?

■Question 2

I would like to know about 'RTH' in Equation 55.

This 'RTH' is equal to R_theta ja?

Which value can I use for RTH?

Best regards,

  • 0
    TI__Mastermind 35215 points

    Hi Yamamoto-san,


    The fco used in Equation 46 was based on real lab measurements of the loop bode plot. The higher fco was used to improve the transient response. The fco calculated in Equation 44 or 45 is conservative to ensure a stable design. Conservative calculation is needed here because the equations used in the datasheet are simplified for easy calculation. Alternative to lab measurements, WEBENCH or the average model can be used for more accurate compensation.

    RTH is the R_theta ja for a given PCB design. Typically the RTH is better than the R_theta ja given in the Thermal Information table because this value is on a standard test board.

    Best Regards,
    Anthony