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TPS54540: Compensator check

Part Number: TPS54540

Hi Sir,

Would you help check the compensator? is it appropriate for switching frequency 950KHz and 650KHz?

  

Vin=9V to16V, Vout=5V, Iout=3.5A, Maximum ambient temperature=85C, switching frequency= 950KHz and 650KHz

Inductor= 10uH, Output capacitor: 470uF electrolytic capacitor//10uF MLCC

Thanks!

Vito

  • Vito,

    That looks good. I ran the sim in WEBENCH and got a phase margin around 97% at 13kHz so it will be good to react, a bit slow to correct, but it should be stable.

    Here's a link to my WEBENCH design based on your inputs. You can use the, "Re-Comp" tab if you'd like to change your phase margin and crossover freq.

    -Sam

  • In reply to Samuel Jaffe:

    Hi Samuel,

    Thank you for your support.

    We set the frequency to 950KHz and observed a voltage spike during start-up. The output voltage go up to around 8V. Could you teach me how to solve this issue?

  • In reply to Vito Liao:

    Vito,

    This should only happen if the converter is switching for too long - VOUT is regulated but it keeps switching for some reason. Possible reasons:

    1. FB is low when it shouldn't be (could be due to layout).
    2. COMP values are incorrect (but they look okay).
    3. The IC is unstable (comp values or layout).

    Please share the layout and I'll take a look.

    -Sam
  • In reply to Samuel Jaffe:

    Hi Samuel,

    Our customer told me that they did not see the voltage spike when the TPS54540 operated at 650KHz. I will try to get layout file.

    BTW, can you re-check the compensator? We need them operate at both 950KHz and 650KHz.

    Thanks!

    Vito

  • In reply to Vito Liao:

    Hi Samuel,

    Enclosed please kindly find the layout file.

    I have a question. The TPS54540 has built-in an output overvoltage protection. Why was the OVP not triggered?

    MAIN-P4C1249ZA-PP0-PCB-180717_DCDC.pdf

    Thanks!

    Vito

  • In reply to Vito Liao:

    Vito,

    Switching frequency is not tightly linked with stability so you should not have to recompensate for a different switching frequency. The only time that would be the case is if the lower frequency bandwidth was wide enough to interfere with the crossover freq but you're seeing issues at a higher frequency which leads me to believe compensation is not the issue.

    OVP would trip if FB (with respect to the GND pin) was >106% ref. Clearly FB is above 106% and it looks like it's still switching (for a millisecond or two). This leads me to believe there's an issue with the GND part of the FB to GND measurement.

    You will have to check the IC's operation as this issue occurs. Use a scope to see what's happening on the pins. Is FB noisy? Is VIN noisy? How does this all compare to the 650kHz board? Here are some thoughts of things to try from what I saw in the layout:

    Your attached layout shows the GND pin is not connected to the thermal pad or to a large GND plane. Instead it goes straight to a via to a plane in the mid-layer. That means all return (GND) current must go through mid/bottom layers and through that via to get to the GND pin of the IC. All this parasitic inductance may be resonating at a harmonic of 950kHz (but not 650kHz) which could cause a GND bounce issue. This would explain what you're seeing. You can try some careful soldering to connect the GND pin pad to the thermal pad under the IC to confirm if this would help fix the issue.

    The VIN caps should also have a shorter loop. C77 is close to VIN (good) but it goes through C78 which will require a long path and multiple vias to get back to the GND pin of the IC. This should be a very small loop. You can try connecting a wire directly from C78 GND pin to the GND pin of the IC to see if this helps.

    Other layout guidelines are in the datasheet.

    -Sam