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TPS54620: TPS54620

jinlong liu1
Prodigy 150 points
Part Number: TPS54620

I want to measure the gain and phase margin for this -9V output circuit use TPS54620.
Could you tell me how to connect this circuit to FRA?
As it is -9V. Should I connect the ground of FRA to -9V output?

Thanks

5Vto-9V.pdf

  • 0
    TI__Mastermind 35165 points

    Hi Jinlong,

    For an inverting buck-boost, you would hook up the frequency response analyzer to the same pins as if the device was being used as a regular buck. What I mean by that is you would use the GND pins of the IC as the reference point for sensing. The injection resistor should still be at the "top" of the VSENSE resistor divider, between the resistor divider and the board ground. Hope the below image helps explain.

    Let me know if you have any issues.

  • 0 in reply to Anthony F
    Prodigy 150 points

    Hi Anthony

    Thanks for your answer.

    I measured the phase margin and gain margin as your way.

    But the result is not good. The result of gain always is higher than 0V.

    Could you give me some advice to fix this problem?

    I simulated 14Vto9V 0.01A in TI webench power design.

    And the result is the same as my measurement result.

    Could this device or circuit not be used for very low output current applications?

    my TI webench power design file's link for TPS5462014Vto9V 0.01A:

    webench.ti.com/.../SDP.cgi

  • 0 in reply to jinlong liu1
    TI__Mastermind 35165 points

    Hi ,

    The image you had attached didn't come through. Can you try posting again?

    It's not clear to me what the concern is. I do not see any issue with the loop response chart for the WEBENCH design. Could you help point out what you are referring to?

    However, I do see some issues with the components selected by WEBENCH that I will request our WEBENCH team to improve. The selected inductance is very large resulting in extremely small ripple current in the inductor. With peak current mode control, it is important to ensure a minimum amount of ripple current as the ripple current is used as part of the ramp at the PWM comparator.

    Anthony

  • 0 in reply to Anthony F
    Prodigy 150 points

    Hi Anthony

    for the webench power designer:

    Could you run the new simulation: Bode Plot in the simulation tab.
    You will see the graph and performance summary.

    Sim ID        Low Freq Gain        Cross Freq         Phase Marg        Gain Marg

    1                 76.56 dB                  51.98 kHz          62.05 Deg           NaN dB

    I think there is no gain marg for this circuit. Is it right?

    Could you help me to make gain marg less than -10dB @14Vin9Vout0.01A?

    Otherwise, Is this device or circuit cannot be used in low current situation?


    for the measurement of my circuit:

    Jinlong

  • 0 in reply to jinlong liu1
    TI__Mastermind 35165 points

    Hi Jinlong,

    Something does seem off with the measurement as the gain has not yet reached 20 dB all the way up to 1MHz. Usually this part you will see a loop bandwidth (or 0 dB gain frequency) of near 100kHz or less. Is the pulse-width at the PH pin stable? If you are having stability problems, the first thing I would suggest trying is adding more capacitance in parallel with C80705. 10µF is quite small but I was assuming there is also some capacitance after L80703. More capacitance after L80703 would also help with loop stability.

    In WEBENCH the gain margin is NaN because the phase has not dropped to 0 degrees yet. However the average model used in WEBENCH for this part does start to lose accuracy at high frequencies (>100 kHz). Tradeoffs are made to ensure accuracy at the crossover frequency for more accurate phase margin simulation as this is usually the most critical.

    There is not any issues using this part in a low current situation. It may just not be the most optimized solution as its power FETs are optimized for higher current so the switching loss will be relatively high at low currents.

    Also, in case you haven't seen it already, I want to point out the following app note which can help with designing with the TPS54620 as an inverting buck boost.

    https://www.ti.com/lit/pdf/slva458

    Anthony