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LM317-N-MIL: Multiple LDO devices, Max/Min input voltage request

Genius 13859 points
Part Number: LM317-N-MIL

Hi Experts,

Good day. I am reaching out to guide us completing the information requested by a known Cx:

OPN MIN INPUT MAX INPUT
LM317K STEEL/NOPB ? ?
LM350K STEEL/NOPB ? ?
LM317HVK STEEL/NOPB ? ?
LM79L05ACM/NOPB ? ?
LM79L05ACMX/NOPB ? ?
LM79L15ACMX/NOPB ? ?
LM79L12ACMX/NOPB ? ?
LM79L12ACZ/NOPB ? ?

I am having difficulties interpreting the minimum recommended headroom or dropout voltage, for example LM317K part to have regulated Vout. Is it 3V? I know that the device can work even hundreds of input voltage as long as input-output voltage differential will not be exceeded (40V). The device is recommended to operate up to 37V (for current greater than 1.5A), so I am thinking it would support up to 77Vin in this application, correct?

Hence, would be glad more to help complete the requested data.

Thank you for your guidance!

Regards,
Archie A.

  • Hi Archie, 

    The LM317K has an adjustable pin rather than a ground pin - also known as a floating ground pin. 

    Because the regulator is floating and detects only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input-to-output differential is not exceeded. That is, avoid short-circuiting the output. 

    The max differential being 40V means that the part could potentially support a 77V VIN to 37V VOUT with a current of 1.5A. 

    That being said, the thermals of the part must be considered.

    PD = (VIN-VOUT)(ILOAD) + (VIN*IGROUND), which influences the temperature at the junction of the package. The junction temperature cannot exceed 125C. 

    Depending on your application and PCB layout, you should review the "10.3 Thermal Considerations" of the DS and add the appropriate heatsinks if possible. 

    The heatsinks necessary to support a large power dissipation may require a large amount of space. 

    Best,

    Juliette

  • Hello Juliette,

    Thank you for confirmation.

    But my main question is, what is the minimum dropout voltage, for LM317K? I see some notes about LDO that normally its 3V. If this particular LDO has minimum Vout of 1.25Vout, then to have a safe regulation, a recommended minimum Vin should be 4.25V, correct?

    Regards,
    Archie A.

  • Sorry for the misunderstanding Archie, 

    Figure four in the DS goes over the dropout voltage based on the output current of your application. 

    3V is the headroom that would apply to any application regardless of output current. 

    Best,

    Juliette

  • Hi Juliette,

    Thank you for your responses.

    After interpreting each info sheets, I came up with the results. Please confirm if the following information I concluded is correct for the respective parts:

    OPN MIN INPUT MAX INPUT
    LM317K STEEL/NOPB 4.25V 77V
    LM350K STEEL/NOPB 4.2V 68V
    LM317HVK STEEL/NOPB 4.25V 117V
    LM79L05ACM/NOPB -3.2V -82V
    LM79L05ACMX/NOPB -3.2V -82V
    LM79L15ACMX/NOPB -3.2V -82V
    LM79L12ACMX/NOPB -3.2V -82V
    LM79L12ACZ/NOPB -3.2V -82V

    Given that all devices are operating at specified excess current rating.

    Thank you!

    Regards,
    Archie A.

  • Hi Archie, 

    I will look into this for you and get back to you tomorrow. 

    Best,

    Juliette

  • Hi Archie, 

    I have looked into this for you and these parts are not spec'd with a min and max value.

    They are only spec'd with a differential between IN and OUT, not with respect to ground.

    As long as the thermals of the part are not violated and the differential between IN and OUT are not exceeded, the part is expected to work. 

    Best,

    Juliette