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LM3478: about startup

ACGUY
Guru 11170 points
Part Number: LM3478

Hello E2E,

Please let me know about start up behavior of LM3478.
-1- Does LM3478 have soft start function?
-2- When start up, does LM3478 add to limit for on-duty or maximum current?
-3- How long the soft start time?

Best regards,
ACGUY

  • 0
    TI__Guru 68875 points

    Hi Acguy,

    Thank you for reaching out.  To answer your question:

    1. The datasheet indicates that the LM3478 has internal soft-start function. 

    2. It limits the ON duty, but the cycle by cycle peak current limit also in function, so the peak current limit may truncated the ON duty, too.

    3. The typical Soft Start time is 4ms, which is indicated in the EC table as Tss.

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Guru 11170 points

    Hi Youhao,

    Thank you for reply.

    Please see the waveforms on following URL;
    lab.fujiele.co.jp/.../f6f77a1616e27fc34eb1a81aa7dc6262.png
    Cyan : Vout 24V settings
    Green : Vds
    Magenta : Iout Typ 600mA
    Blue : Vin 5.5V

    Under the above conditions, start up time finished until from Vin start to about 40msec later.
    I think that it's limitting behavior and some parameters at start up. Peak current, Duty, fsw, ect..
    What paramater limitting at during start up time?

    Regards,
    ACGUY

  • 0 in reply to ACGUY
    TI__Guru 68875 points

    Hi Acguy,

    if you see the Magenta trace, the internal soft start seems completed at about 4 ms after VIN is applied, so you see the loop starts to taken over and the current rises up.  However, you Vout is not yet established until after 30ms later.  During this period, your circuit may high the peck current limit.  Until after 30ms, the circuit starts to operate in steady stages. I need to see your schematic to understand the last step in the blue trace.  Can you monitor the input current?  You don't need to monitor Vds and change that trace to the input current. 

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Guru 11170 points

    Hi Youhao,

    Please see the following URL, Magenta is input current.

    https://lab.fujiele.co.jp/fujiele_wp/wp-content/uploads/2020/02/4d2898cc519c4468d541cbcfe0072ef6.png

    We are using an electric load in CC-mode. After the inrush current period, it didn't finish the start up time. Does this device will add limit to max_duty or max_current at startup?

    Regards,

    ACGUY

  • 0 in reply to ACGUY
    TI__Guru 68875 points

    Hi Acguy,

    The new waveform has different timing than the one of Feb 19th. Seems your test condition is different.  Anyway, the wave shape is similar, just the inrush period is shorter (~10ms) than the previous ~25ms.

    For this new waveform, the start up time is already finished at about 10ms, as you can see the output voltage is already established.  The following 42ms (the input current between the input to your cursors a and b position, is actually related to your input voltage. Somehow your input voltage source get stuck at that level before it finally settles to the last level.  Please check your Vin source property. 

    Thanks,

    Youhao

     

  • 0 in reply to Youhao Xi
    Guru 11170 points

    Hi Youhao,

    Please answer about questions from me. Please.

    What parameter limiting at during startup time? Does this device will add limit to max_duty or max_current at startup?

    Best regards,

    ACGUY

  • 0 in reply to ACGUY
    TI__Guru 68875 points

    Hi Acguy,

    The IC will limit both max duty and the max inductor peak current.  The SS basically limits the max duty, and if the inductor current hit the peak limit, it will also truncate the duty. 

    Thanks,

    Youhao