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LM73606-Q1: Loop stability

Atsushi Hirai
Expert 3610 points
Part Number: LM73606-Q1

Hi team,

My customer is evaluating LM73606-Q1 loop stability over temperature.

Left hand side is room temp, Right hand side is Hot temp. The room temp result is no problem ,but for hot temp,  the phase value between 500Hz to 1kHz is lower than 0 degree and the gain is higher than 0dB.

Can we say this system is stable? If yes, how we can explain this measurement result?

Regards,

  • 0
    TI__Mastermind 47060 points

    Hi,

    When talking about a stable loop response, we primarily look at the PM and gain crossover frequency for bandwidth.

    In both cases, the PM is ~80deg and Fcross = ~50kHz.

    General rule of thumb is to have to PM > 40deg and Fcross = ~ 1/10 of switching frequency.

    I do not know your exact test setup, but I think that may be contributing to the "hashes" on your phase measurement reading. 

    Usually it is recommended to have a small AC injected signal of roughly 10mV from the frequency response analyzer/signal generator to have a clearer measurement.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    TI__Expert 3610 points

    Jimmy-san,

    Thanks. Does not need to care the phase profile lower than Fcross frequency?

    Regards,

  • +1 in reply to Atsushi Hirai
    TI__Mastermind 47060 points

    From what I've seen most customers only care about a stable design and the indicators of that are the PM and Fcross. 

    The hashes I see at lower frequency on your phase plot can be a combination of noise, test setup equipment, etc and it does not have significant impact on the system design stability.

    However at the point of interest (Fcross) the PM is >45 and Fcross is at a decade away from the switching frequency. This confirms that the design is stable.