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TPS53667: Why Phase Loop test would not across 0?

Wenhao Wu
Genius 9046 points
Part Number: TPS53667

Hi Experts,

From datasheet I found Phase line does not across 0, therefore Gain margin cannot be obtained. Could you please tell the reason why DCAP3 control topology has this kind of characteristic? Thank you very much!

Wenhao

  • 0
    TI__Expert 4060 points

    Hey Wenhao,

    Can you please give a bit more detail about what you're looking to see in the bode plot? Where, and why, do you expect to see phase crossing 0°?

    Thanks,

    Carmen

  • 0 in reply to Carmen Parisi
    TI__Genius 9046 points

    Hi Carmen,

    I saw it in TPS53667 datasheet Page 107. The customer wants to know the gain margin, but without phase crossing 0 it it hard to know. Thanks!

    -Wenhao

  • 0 in reply to Carmen Parisi
    TI__Genius 9046 points

    Hi Carmen,

    I saw it in TPS53667 datasheet Page 107. The customer wants to know the gain margin, but without phase crossing 0 it it hard to know. Thanks!

    -Wenhao

  • 0 in reply to Wenhao Wu
    TI__Expert 4060 points

    Hey Wenhao,

    For gain margin you're actually looking for when phase crosses the 180° point rather than the 0° point (see below from SLUP386).

     

    For the bode plot you're referring to in the TPS53667 datasheet, all it's saying is phase doesn't hit that point before 1MHz, the upper limit of the plot. At that point, with the TPS53667 loop bandwidth only at 100kHz, the gain margin would entirely depend on the output capacitance and board layout. 

    Multiphase designs typically deal with high current transients which require a lot of output capacitance to keep VOUT within the design targets and the 667 is no exception. That high COUT value keeps the gain margin point at high frequencies which typically aren't a concern in power supplies.

    Please let me know if you have any other questions.

    Cheers,

    Carmen

  • 0 in reply to Carmen Parisi
    TI__Genius 9046 points

    Hi Carmen,

    It is said that DCAP as well as DCAP2's loop measurement makes no sense because it does not have loop. On the other hand, why DCAP3, one part of Direct CAP control, can have loop? It seems similar to Average current Mode Control since DCAP3 has also utilized information extracted from inductor. Thanks for your reply.

    -Wenhao

  • 0 in reply to Wenhao Wu
    TI__Expert 4060 points

    Hey Wenhao,

    D-CAP, D-CAP2, and D-CAP3 at a base level are all the same modulator just with improvements as the technology was refined over the years. Taking bode plots for all 3 can be done and the results are meaningful. However, unlike voltage or current mode converters, a bit more care has to be taken when setting up the test to get good results. The app note I liked to above has information about taking bode plots with D-CAP regulators as does this Power House Blog post.

    D-CAP+ at its heart is the same as D-CAP but it's optimized for multiphase applications. Because all the phase currents need balancing there are dedicated per phase current loops providing current information to the modulator rather than using an injected or emulated signal like in straight D-CAP. Using this approach, a bode plot can be taken the same way as you would any other regulator, though the app notes and blog posts I mention still give plenty of good information that can help make the results even better.

    Cheers,

    Carmen 

  • 0 in reply to Carmen Parisi
    TI__Genius 9046 points
    Hi Carmen,

    Thanks very much for your reply. I will share this with my customer and discuss more.

    -Wenhao