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LM3478: Stop switching after few cycles

Ivan Gehri
Prodigy 10 points
Part Number: LM3478

Dear TI-Team

The attached circuit (see above) shows a very special behaviour. It is already on a PCB and we try to run it by applying 24V at 24V_IN. Except for the On/Off button nothing else is connected to the board.
Once the MAIN_SWITCHED is being turned on by pressing the button, the Mosfet (Q5) switches a few times (3-4) before it stays closed. The gate is driven by the Pin DR of U2 and seems to aim this functionality. If we give 24V and allow more than 1.5A current Q5 simply burns thru (happened once already).
According to our simulation in Webench (see below) it should work just fine.

  • Removing R25 didn't help (removing the lowpass filter)
  • increasing R27 to 75mOhm didn't help (reducing the maximum allowed output current).
  • The load itself at 24V_MAIN seems to be fine. Applying 24V_MAIN directly worked perfectly with very little current consumption.

We run out of ideas what to look for. Thank you very much in advance for your help!

Best regards,
Ivan

  • 0
    TI__Expert 7450 points

    Hi Ivan,

    Thank you for using LM3478. I need more information to fully understand the problem. What is the voltage at MAIN_SWITCHED?

    Thanks,

    Yinsong

  • 0
    Prodigy 10 points

    Hi Yinsong

    Thank you for responding! In the meantime we found a solution to the problem thanks to other related posts in this forum. We needed a RC filter infront of Vin to delay turning on the LM3478 as long as Q3 is limiting the inrush current.

    To answer your question anyway:
    MAIN_SWITCHED is just a net name, as this point of the circuit equals (more or less) the input voltage (24V) and can be switched on or off by TP14.

    Thanks again for your help and best regards
    Ivan