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TMS320F28069: Q26 limit

user4921965
Prodigy 40 points
Part Number: TMS320F28069

I have built a buck converter based on the TMS320F28069 MCU as its controller, I've implemented the 3P3Z module, calculated the control coefficients and inserted them into my code, the issue is that if I use a crossover frequency higher than 1KHz, the control coefficients become larger than 31.999999 which is the limit for the Q26 numbers.

the coefficients become this big because my output is 12 volt so I have a big divider ratio. and the external reference for the ADC is also 3.3V max

is there a work around this issue?

  • 0
    TI__Genius 10995 points

    Check your Bode Plot output and verify the control loop design. Then you will know whether your coefficients have optimum values and you have adequate phase margin and gain margin. 

    Shamim

  • 0 in reply to Shamim Choudhury
    Prodigy 40 points

    I did exactly that, and i have verified my control loop coefficients, for the system to work as desired the values of the control coefficients must be larger than 31.999999 (Q26 Limit), is there away to get more range using DPLib?  

  • 0 in reply to user4921965
    TI__Genius 10995 points

    Then you just have to change the 3p3z code for Q25 coefficients. This will allow you to use max coeff of 63.999.

    When you change the code for Q25 coeff, change the remaining part of the code accordingly.

    If the max control output is also Q25 then you will have to map the max control output to 100% duty of the DC/DC converter. So you have to verify when control output is max then the compare value you generate for the C2000 PWM Compare Register should be such that it results in 100% duty.

    Shamim