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LM317: convert 110~140V input to 100V/0.1A output with LM317

Wayne Wang20
Expert 4360 points
Part Number: LM317

Hi team,

My customer wants to convert 110~140V input to 100V/0.1A output with a LDO. The high voltage LDO is too expensive so customer wants to use LM317 to do a low cost solution. But LM317's max input voltage is less than 110V. So I want to check with you can we make a floating GND for LM317 GND pin so it can endure 110V input to the system GND? Do you have such design experience? Thanks.

Best regards,

Wayne

  • 0
    TI__Expert 6825 points

    Hi Wayne,

    In theory this can be done as long as you maintain the device within its absolute maximums at all points of operation. The most common failure point would be if you connected a fully charged input while the output is still at GND. You would have a 140V differential across the part in this scenario and that would cause it to fail. 

    There are several examples online showing how to get around this by putting a zener between the output and input to clamp the maximum voltage that can be seen between the input and output and the output and ADJ pin. The problem I see with implementing something like that in your case here is that your input to output voltage range is right on the edge of the operating max of the device, so to find a zener that would not interfere with your regulation and still offer enough margin to adequately protect the device would be difficult.

    I would not recommend doing this unless you can limit your Vin range to a smaller value where an adequate zener can be selected for protection.

    Regards,
    Mark