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LM36274: VNEG short-circuit protection

Peter Xu
Expert 3565 points
Part Number: LM36274

Hi team,

For the VNEG short-circuit protection, I wonder how the filter works. In my understanding, I think the filter works right after the ramp finishes. The pictures are attached below. Besides, is there any relationship between the ramp time and filter time? For example, should I set a long filter time if a short ramp time is set? 

Best Regards,

Peter

  • 0
    TI__Expert 3565 points

    Hi team

    I got another question. I wonder why VNEG uses voltage protection while VPOS uses current protection. What makes the difference?

    Best Regards,

    Peter

  • +1 in reply to Peter Xu
    TI__Mastermind 22980 points

    Hi Peter,

    In datasheet there is no explanation about relationship between filter and ramp. But in my understanding, The filter time is set to prevent falsely triggering a short to GND fault. So the filter is working all the time whatever there is ramp or not. 

    As for different detection way about VNEG and VPOS, I need to further analyze and reply will be sent then.

    BR

    Patrick Yao

  • 0 in reply to Patrick Yao
    TI__Expert 3565 points

    Hi Patrick,

    Is there any progress for VNEG/VPOS detection difference?

  • +1 in reply to Peter Xu
    TI__Mastermind 22980 points

    Hi Peter,

    In bias application there are two output voltage level called VPOS and VNEG. VPOS is the output of a BOOST converter and VNEG is the output of a charge pump.

    When short fault occurs, there will be a large current in boost converter. So the detection method is to measure the current.

    For charge pump, the typical circuit (not the real structure in LM36274, just a reference to help you to understand) is shown below:

    As you can see, when short fault occurs, there is still a capacitor between Vin and GND, so the current won't be as large as boost converter and won't last for a long time. But the output voltage will drop. So the detection method is to measure the voltage.

    I can't find the real inside circuit since there is only a block diagram on datasheet. But I think this is the main reason for different fault detection method.

    BR

    Patrick