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TPS53015: Stability Testing / Bode Plot

Ben King
Expert 5296 points
Part Number: TPS53015
Other Parts Discussed in Thread: TPS53014

I am attempting to measure a bode plot on a DCAP-2 device, TPS53015, for stability testing. 

I came across the following e2e post, which describes a method for bode plot measurement requiring a pass capacitor across the resistor used to inject signals into the feedback network:

https://e2e.ti.com/blogs_/b/powerhouse/archive/2015/05/05/power-tips-how-to-measure-bode-plots-with-dcap-and-dcapx-regulators

However, for the DCAP2 measurement set-up, a Vosns pin is shown, which leads to the 'DCR injection network' as shown below:

The TPS53015 does not have this pin - only the feedback pin connected between the upper and lower feedback resistors (feedback shown below).  

I am somewhat confused about to how to include the DCR injection circuit path through the capacitors described in the previous e2e post when attempting to measure the frequency response in a bode plot.  Any guidance here would be much appreciated.

Thank you!

  • 0
    TI__Guru**** 191445 points
    For TPS53014, you can just measure directly across the 10 ohm resistor that you show. I have personally measured without that cpass capacitor. There may be valid reasons for some ICs, but I have never used it.
  • 0 in reply to JohnTucker
    TI__Expert 5296 points

    Thank you for the quick reply John.

    Perhaps you could help me make sense of the results I'm observing?

    The transient response for load steps that I observe in lab appears to indicate a stable system.
    However, in the bode plot I am only seeing ~16 degrees of phase margin. 
    This values seem quite low for stability.

    Is it correct to assume that phase margins well below 45 degrees are still acceptable in D-CAP2 systems due to the lack of a high DC gain error amplifier in the control system?
    Or am I perhaps making a mistake in my bode measurements and/or the system is not as stable as it seems?

    Thanks for any help or additional resources!

  • 0 in reply to Ben King
    TI__Guru**** 191445 points

    The general shape of the plot looks normal.  It is definitely normal to have PM lower than 45 deg.  My concern is that PH is dropping below 0 deg, just after the LC corner.  Usually that is not allowed.  Maybe your LC corner frequency is too low.  Can I see your schematic and design inputs?

  • 0 in reply to JohnTucker
    TI__Expert 5296 points

    Just FYI, this is for an internal project that I am doing simply to better my understanding of power design, so it is by no means a priority. 
    If you have time, any feedback you on the design is highly appreciated.  Feel free to email me directly if it is more convenient, b-king@ti.com

    The specifications are as follows:

    Input:  6V - 17V  (12V nominal)

    Output: 3.3V @ 3A 

    Aiming for high efficiency with <=3% deviation on the output (including output ripple and transient responses) 

    Thanks again for the help!
    -Ben