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LM5021: Confirm whether he can set 1.68uF (350ms) for SoftStart.

Motoyasu Hattori
Expert 8695 points
Part Number: LM5021

Hi,

Is it no concern to set 1.68uF (350ms) for SoftStart?

On the following Figure 6 on latest LM5021 datasheet, Softstart capacitance is 1-1000nF.   He considers 1.68uF is larger than the value on this graph. That is why he is asking.

Thanks and best regards,
M.HATTORI.

  • 0
    TI__Mastermind 26840 points

    Hello Motoyasu-san,

    Thank you for your interest in the LM5021 flyback controller.

    Yes, the Soft-start cap value can be higher than what is shown in Figure 6 of the datasheet.
    In addition to extending the soft-start time, it also controls the "Hiccup Mode" timing during an overload condition. Please refer to Section 7.3.6 of the datasheet.

    I estimate that it will extend the Hiccup Mode off time between overloads to about 40 seconds, but it will also extend the allowed time to be in an overload.
    Be sure that the capacitors used to achieve 1.68uF have insignificant leakage current to avoid extending the times even more.

    In addition to those considerations, the design may need higher VIN capacitance to keep the IC powered while waiting for the reflected output voltage on AUX to get high enough to keep VIN above the UVLO threshold. 

    Regards,
    Ulrich