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TPS40422: CH2 output failure

JH Shin
Mastermind 9815 points
Part Number: TPS40422

Hello,

My customer has a problem that the output of the TPS40422 CH2 below does not operate normally during production.

TPS40422.pdf

Failure symptom is as follows.

  1. A drop in the 5V output voltage of Phase 2 occurred in some parts.

  2. When they reduced the 5V load this symptom did not occur. However, the actual load was about 10A, which was sufficient for the design.

  3. When resistor R20 was removed, Hiccup symptom occurred.

  4. When the resistance of R20 was changed to 4.7kΩ, the same symptom occurred.

  5. When the resistance of R20 was changed to 1kΩ, this symptom did not occur.

Q1. Is it okay to use 1KΩ as the resistor value for R20? Are there any other recommended values for R20?

Q2. Can they use a value lower than 1KΩ for R20? What are the acceptable MIN values?

Thank you.

JH

  • 0
    TI__Mastermind 27330 points

    Hello, 

    The RC time constant should be higher than the inductor time constant. Reducing R20 to 1K, the RC time constant R20//R28*C22 is 391uS. Assuming the inductor DCR = 1mOhm, the inductance time constant is 0.82uH/0.001Ohm = 820uS. 

    I suggest to increase R28 to 10K and keep R20 at 2.49K. Please try it and let me know if it fix the issue.

    Regards

  • 0 in reply to Mahmoud Harmouch
    Mastermind 9815 points

    Hello Mahmoud,

    Thanks for your help.

    The customer has some questions about the L3 inductor in the above TPS40422.pdf file.

    1. Does the inductance value of the inductor affect the OCP operating point of the PWM controller?

    2. Unlike the circuit diagram, the actual assembled inductor is 0.68uH (refer to the datasheet below). Then, should the values of the peripheral elements (R, C) also be changed?

    6518.PG0077.801NL.PDF

    3. Can you please recommend the values of peripheral elements (R, C) when the inductor is applied as a 1uH product?

    Thank you.

    JH

  • 0 in reply to JH Shin
    TI__Mastermind 27330 points

    Hello,

    1- The RC filter is to remove the large AC component of voltage across the inductor. The voltage across the inductor series resistance appears at the current sense amplifier. The voltage across the inductor series resistance = DRC * (Iload + Iripple). The inductor value affects the OCP because the ripple current is affected by the inductor value.

    2- Yes the RC should be changed according to inductance value and its series resistance DRC. DRC * (Iload + Iripple) should be calculated and the voltage at the sense amplifier input should be less than 60mV at maximum load.

    3- Using 1uH, the RC can be calculated according to 1, 2 and the explanation in datasheet page 17. Inductance series resistance DRC should be known for the calculation

    Regards