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UCC256302: About soft start and burst mode threshold

Masazumi Ishii
Genius 3945 points
Part Number: UCC256302

 Hello.

I have a question about UCC 256302.
I have a question about the following two points.
① About soft start
② About burst mode threshold

Detailed content is described in the attached document.

Thank you.

UCC256302burst.pdf

 

  • 0
    TI__Mastermind 26675 points
    Hi Masazumi-san,

    Thanks for your interest in UCC256302. Yes, the abrupt jump in the LL/SS pin voltage is the end of the soft start period. The FB voltage waveform looks reasonable as well.

    If the feedback is oscillating above and below the burst threshold, then you will see burst-like behavior. Could you test without C1 and R2 and share the feedback waveform with me?

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Genius 3945 points

    Hello.

    Ben-san

    I have additional questions about UCC256302.

    ③Insert a resistor in series with the soft start capacitor of the LL / SS terminal. This limits the maximum oscillation frequency at startup.
    However, if a certain resistance value or more is attached, a vibration of about 3.5 MHz is generated in the voltage of the LL / SS terminal
    . (330 Ω: No vibration, 1 kΩ: with vibration)
    The occurrence of this oscillation will be at the time of normal operation after completion of soft start (no vibration in the soft start period).
    The larger the resistance value, the greater the vibration.
    Why does this oscillation occur?
    Please tell me the cause.

    Please see the attached document for details.

    Since ZCS triggering causes unstable operation at startup, we want to limit the maximum oscillation frequency at soft start.

    The resistance value you want to add now is a resistance value of several tens of kΩ.

    UCC256302-3.pdf

  • 0 in reply to Masazumi Ishii
    TI__Mastermind 26675 points

    Hi Masazumi-san,

    Inserting a large resistance between the LL/SS pin and the soft start capacitor can cause the LL/SS pin voltage to oscillate. I would suggest tuning the VCR capacitance to limit the maximum switching frequency. Increasing the capacitance will lower the maximum switching frequency during startup.Section 2 of the UCC25630x practical design guidelines has some more detail about soft start switching frequency tuning:

    Best Regards,

    Ben Lough