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LM5176: Compensation selection

Harsh Rana
Prodigy 110 points
Part Number: LM5176


Hi,

I'm designing varible output SMPS with LM5176. I got a very good reference www.ti.com/.../PMP21697 schematic. But I don't understand the right method to select best optimised values of compansation resistor and capacitor for variable output and variable load. I want to understand, how these values are selected in that reference board.

C65 = 390pF, C59 = 0.022uF, R67 = 12.1K
I understand the poles and zeros created by the system parameter and component selection. And I tried LM5176 design calculator "SNVC208". But trial and error method doesn't seem good for selecting good stability of the system on different output voltages and loads. 

 

  • 0
    TI__Guru 61171 points

    Hi Harish,

    this application report gives a good introduction to compensation:

    https://www.ti.com/seclit/ml/slup340/slup340.pdf

    For the optimization on the design with the LM5176 i recommend the design calculator:

    SNVC208 Calculation tool | TI.com

    Best regards,

     Stefan

  • 0
    Prodigy 110 points

    Hi Stefan,

    I know about compensation design and control system related things like poles, zeros, RHPZ, bode plot things for fixed output converter with max. load. And SNVC208 is also a very good tool for optimization that I'm already using as mentioned in my 1st question. 

    But my question is, how to design/ optimize compensation network for variable output volatge, variable load SMPS. Is there any better method except trial and error on SNVC208 calculation tool? Consider I only want to design analog feedback loop rather than making a digital SMPS.
    I'm currently placing webench suggested RC values in SNVC208 for maximum output voltage and current design for a starting point and then check bode plot response on lower voltages and currents with trial and error. But I always end up making unstable output (Sometimes negative phase margin or way below 45 degree) at lower voltages and light loading conditions.

    What should be the starting point in selection of RC compensation values for variable output SMPS?

  • 0 in reply to Harsh Rana
    TI__Guru 61171 points

    Hi Harish,

    for the compensation you need to check for all input and output voltage combinations (at least with a reasonable amount of combinations and worst case settings (min/ min ,  min /max, ....  vin=vout, ...) .

    For worst case you only need to check with highest output current.

    For all settings ensure that you have a phase margin of > 60 Degree and a gain margin of 12 dB  (lower -12 dB)

    Start with the suggested cross over frequency and use the proposed settings for the initial compensation.
    Then iterate through all settings mentioned above to find a compensation which is stable in all conditions.

    Best regards,

     Stefan