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LM317: Maximum input voltage

Part Number: LM317

Hi Team,

The maximum input voltage in the product page of LM317 in SOT-223 package shows that it is Vin (Max) = 40V. Upon checking the datasheet, there is no exact value of Vin. The Vin (Max) of 40V contradicts with the input-to-output differential voltage of VI - VO = 40V.

It also contradicts with the minimum output voltage of 1.25V if the input-to-output differential voltage is 40V because the total would be 41.25V. 

What is the maximum input voltage of LM317DCY (SOT-223)?

Regards,

Danilo


  • Hi Danilo,

    The LM317 is one of the handful of linear regulators that does not directly connect to ground.
    As such, the requirement is that Vin-Vout remains below 40V at all times.
    The reference voltage is across the top feedback resistor.
    That is, it connects from Vout to the adjust pin.
    So the bottom feedback resistor will see the most significant voltage, if you are using this at high voltage.

    I hope this clarifies the requirements.

    Thanks,

    - Stephen

  • Hi Stephen,

    Thank you for your quick response.

    I would like to clarify if the input voltage is 60V and the output voltage is 24V at less than 1amp load, is the device going to work fine since the Vin-Vout = 60V-24V= 36V? The VIN-Vout is below 40V and matched the requirement stated in the datasheet.

    According to the product page below, The Vin (Max) is 40V for LM317 and LM317M, it is in conflict with the Vin of 60V in the example above.

     

    Regards,

    Danilo

  • Hi Danilo,

    If the Vin-Vout differential is less than 40V, then this device will work.
    There are numerous examples of this device being used at hundreds of volts on Vin, with the output being 40V lower (or less).
    Please see the following links:

    https://electronics.stackexchange.com/questions/188734/floating-lm317-used-at-high-voltage-protected-by-two-zener-diodes

    http://www.ti.com/lit/an/snva583/snva583.pdf

    If you are using the device at 1A with 24V differential, then you will dissipate 24W across the device.
    You will need to consider thermal heat sinks or fan cooling to keep the device below its maximum junction temperature.
    Using the KCS package junction to ambient thermal resistance, and 24V differential, you will see a 100 degree rise in junction temperature with 175mA of load current.
    See the Thermal Information table in the datasheet. 25C ambient + (24V*175mA)*23.5C/W = 123.7C.

    At 100C allowable temp rise above 25C ambient, and 24W dissipation, you will need a thermal heat sink that can provide 4C/W or better thermal resistance.

    Thanks,

    - Stephen