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LM3488 shutdown doesn't work

Diederik Bakker
Prodigy 50 points
Other Parts Discussed in Thread: LM3488

Hi,

I'm using the LM3488 and connected the FA/SYNC/SD pin with a Rfa=50K and Rpull_up10K to 3v3. in my case, a pushbutton pulls the signal, in between these resistors, to 0V. The voltage on the actual pin is either 0.7V (low) or 3.1V (high) but the 24V output stays at 24V non the less.

I've connected Vin to 12V.

How can I make this work?

  • 0
    TI__Genius 15810 points

    Hello,

    Would you please send us your schematics, layout pictures and some scope captures to get us started?

    Regards,
    Akshay

  • 0 in reply to Diederik Bakker
    Prodigy 50 points

    and a photo of the scope image

    edit: The scope image of the 24Volt output is not included because it stays at 24V no matter what. so it's a flat line.

  • 0 in reply to Diederik Bakker
    TI__Genius 15810 points

    Hello,

    It helps if you send your entire LM3488 schematic with BOM components and a zoomed out scope shot showing the switching action. It tell us if there is any instability or not. Just from what I see, your C29 should be at the ISEN pin and not after the resistor. This is just a comment.

    In a boost topology, even when the device shuts down, your output is not going to go to zero. It would be a diode drop lower than the input voltage. While the device is switching, the SW node would swing from about ground to Vout. Therefore when the device is turned off, the SW node is going to stay high. Have you probed the DR pin to see if it shuts down or not? Also, according to the datasheet, the FA/SD pin needs to be pulled high for longer than 30us. If the signal is lost in that duration, it would reset the timer. Is the 30us requirement being met as well? I couldn't make out the time base on the scope shot.

    Is the device being turned off while it is loaded or not loaded? If it was not loaded, the unspent charge in the output caps will keep Vout high for a short while too.

    Regards,
    Akshay