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WEBENCH® Tools/LM3488Q-Q1: Startup limitation current with my DC supply

Raul Rosa
Prodigy 60 points
Part Number: LM3488Q-Q1
Other Parts Discussed in Thread: LM3488

Tool/software: WEBENCH® Design Tools

Good morning,

I designed a SEPIC using LM3488Q,

the problem is that when i turn on my power supply the output voltage stays at 3V (instead of 10V),

the Power mosfet does not have enough Vgs and the amount of current in the inductor keeps growing.

The scheme is the following:

the only difference is the power mosfet that I am using: FDD120AN15A

  • 0
    TI__Guru* 78070 points

    Hi Raul,

    Thank you for designing with the LM3488.  Please first check if your transformer polarity is used correctly.  If no problem with the transformer polarity, could you monitor the following signals:

    (1) VIN Pin

    (2) DR pin

    (3) U2 Drain

    (4) COMP pin

    Capture all these four signals on the same scope picture, and show zoomed out (at least 10 cycles) and zoomed in (two cycles)?

    In another scope picture, can you monitor 

    (1) DR pin

    (2) ISEN

    Thanks,

    Youhao XI, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 60 points

    Thanks a lot,

    I found the issue, the sense resistor sets the current limit higher than the power supply limit,

    in this way the peak current never reach the threshold to start the transfer of power and begin the normal operation.

    The solution was increeasing the value of Rsense to 100mOhm, then to avoid ripple current limitation I also increased the value of the inductor to 68uH.

    thanks for your time