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UCC2818: UCC2818 query

Star Xu
Genius 9645 points
Part Number: UCC2818

Hi

We are using UCC2818 in our existing design...In nearly 10% of PCBs it is making hissing sound for 2-3 sec while initial turn ON..Can you tell us the reason for this hissing sound...After replacing the ICs problem is getting resolved.

Please give some suggestion for this problem.

Thanks

Star

  • 0
    TI__Mastermind 34465 points
    Hello Star,
    With a very long soft start the output of the error amplifier may be less than 0.33V for a significant amount of time.
    This is recognized by the UCC2818 as a zero power condition that bypasses the normal PWM switching.
    It may be that the gate drive is bursting on and off at full pulse width for very low pulse repetition rates.
    This will cause audio noise issues like the hissing sound you report.
    This issue is worst for long startup times and if you startup into a no load condition.
    You should increase the soft start time to the minimum time required or ensure that the output of the error amplifier (VAOUT) is always above 0.33V.
    Increasing the resistance on the IAC input helps to increase the value of VAOUT
    Regards
    John