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LM5023: Trestart Question

jacob scaria
Prodigy 110 points
Part Number: LM5023

Hi,

What is th function of Trestart block in functional diagram of datasheet?

Is it  the time interval between a turn off event and turn on event even if duty cycle_off can be shorter than that? 

Jacob

  • 0
    TI__Mastermind 30380 points

    Hello Jacob, 

    Thank you for your interest in the LM5023 QR flyback controller. 

    The Trestart function is not adequately explained in the LM5023 datasheet. 

    Ordinarily, the next switching cycle of a QR controller is started at the first valley of the previous cycle's resonant ring after core demagnetization. The resonant ringing frequency is typically at 100's of kHz.  There may be some rare situations where the resonant ringing is dampened out very quickly, or the ringing stops for some reason, before the 130kHz frequency clamp limit (~7.7us) expires. 
    If the QR zero-crossings (at 0.35V threshold) stop before the frequency clamp timer expires, the next switching cycle cannot be triggered and switching stops.   

    The Trestart function is to provide a high signal after 12us to enable the AND gate in the block diagram to allow the OUT flip-flop to set the Q output high and start a new switching cycle, even if the QR signal has been lost.  So essentially, it restarts the switching independently of the QR input, but only after the ~12us delay expires without external QR crossings.   

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 110 points

    Ulrich,

    Thank you

    My undersding of your explanation, Please correct me if i am wrong.

    So Trestart only get engaged when Vqr(0.35v threhold) reached before 130khz window, this way unit can still operate at legit low frequency ( T>12us )?

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  • +1 in reply to jacob scaria
    TI__Mastermind 30380 points

    Hello Jacob, 

    Yes, your understanding is correct, AND that if/when Vqr is reached before the 130kHz window and the ringing does not re-cross Vqr after the 130kHz window is completed. 

    Regards,
    Ulricn