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LM3478: About SEPIC

ACGUY
Guru 11170 points
Part Number: LM3478

Hello E2E,

Please let me know about SEPIC with LM3478. I refered schematic figure 1 on "snva742" and I made the testing circuit board for SEPIC topology. Please see the following URLs wave-forms.

lab.fujiele.co.jp/.../a3ac422b5c6c3615c6cbec59ebb8f4de.png
lab.fujiele.co.jp/.../31ef343ec905f75f5c4ccd0f52824d93.png

Blue: Vgs
Pink: Iin
Green: Vout
Light blue: Vin

Q1 : LM3478 start the FET switch after 20msec since Vin reach to 2.72V. Please let me know the reason that why switching start deraied. Is it normaly behavior?

Q2 : Immediately after switching starts, duty and frequency are sporadic. What the reason? Does this design(snva742) have same behavior too?

Best regards,
ACGUY

  • 0
    TI__Mastermind 46905 points

    Hi ACGUY,

    Could you share your schematic? 

    the input voltage in the application seem quite low, the startup voltage of the device is 2.9V. do you see problem at high VIN, for example 5V

  • 0 in reply to Jasper Li
    Guru 11170 points

    Hello Jasper,

    Thank you for reply.

    >> Could you share your schematic?
    Schematic is there on document "snva742".

    >> do you see problem at high VIN, for example 5V
    When input voltage is 5V that the switching frequency and duty are stable.

    >> the startup voltage of the device is 2.9V.
    I know that this device has 2.9V startup voltage, the startup voltage has characteristic variation too.

    Questions;

    Q1 : LM3478 start the FET switch after 20msec since Vin reach to 2.72V. Please let me know the reason that why switching start delayed. Is it normally behavior?

    Q2 : Immediately after switching starts, duty and frequency are sporadic. What the reason? Does this design(snva742) have same behavior too?

    Regards,
    ACGUY

  • 0 in reply to ACGUY
    TI__Mastermind 31255 points

    Hi ACGUY,

    Sorry to hear that you are having trouble. 

    For Q1: It is possible that your input power supply is faulty, since I would expect it to rise to a steady level, instead of shifting from 2.72V (below threshold of part), to what looks like just above the part limitation. 

    Q2: At lower input voltages, you may be running into current limit, or you are simply pulling COMP down such that it cannot allow for a consistent duty cycle or frequency during startup. Additionally, your input voltage may be too low such that the internals of the part are unable to function as expected. I recommend using a higher input voltage or changing to a different supply. 

    Thanks

    Richard