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LM3421: About LM3421 LED Dimming Question

Wen Yung
Expert 1220 points
Part Number: LM3421

Hi Sir,

Regarding to datasheet simply buck-boost schematic.

May I know CF & Q5 &CB has what function ?

 

  • 0
    TI__Mastermind 25500 points

    Hello Wen,

    There are actually two circuits circled. The Rf and Cf simply add some delay to the turn on/off of the dimming FET Q2. Q5 works in conjunction with Rov1/2 to level shift the OVP and cancel out the input voltage. That way you are only measuring the output voltage rather than Vin + Vout for your over-voltage protection.

    Best regards,

    Clint

  • 0 in reply to Clinton Jensen
    Expert 1220 points

    Hi Sir,

    Thanks for you help.
    I draw schematic now.

    May I know OVP function can explain as below FIG?

    ex. Data sheet reference design page 47
    VTURN-OFF(OVLO) = 40V, trigger OVLO
    VHYSO = 10, Remove OVLO

    LM3421 Circuit.pdf

  • 0 in reply to Wen Yung
    TI__Mastermind 25500 points

    Hello Wen,

    If the output voltage rises to the 40V threshold the LM3421 will stop switching. When it stops switching the output capacitor will discharge. It will begin switching again when the output voltage falls to 30V (10V hysteresis) where it will begin switching again. It will cycle on/off like this until whatever fault is causing OVLO (usually an open LED string) is corrected.

    Regards,

    Clint