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LM317: About the output range

Yatao Ling1
Expert 1840 points
Part Number: LM317

Hi TI expert,

I recently want to use LM317.

    The input is +24V and output is +22V. The datasheet says the headroom shoud at least be 3V and below 3V, the device may drop out. I woder what is the drop out voltage? And will the device certainly drop out with my 2V difference?

Best

Yatao

  • 0
    TI__Mastermind 23361 points

    Hi Yatao, 

    LM317 is tested with a higher than 3V headroom between Vin and Vout with up to 1.5A of load current. Usually, the dropout number is proportional to the load current. For newer devices, we have test data for dropout, but for the LM317, due to the age of the device, there is no test to find the actual dropout values. 

    If your load current much lower than 1500mA and it's possible 2V headroom should work. But unfortunately, you will need to run your own validation to make sure the parts meet your specifications. The worst dropout number is usually obtained at a higher temperatures. If this is critical for your applications, are you open to another device? We may find another device that has the dropout spec for you. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Expert 1840 points

    Hi Jason,

    Thx for your detailed explanations.

    I've changed the +24V power to +27.6V. Then I think with 22V output, this +27.6V can satisfy the dropout requirements set for LM317.

    Best

    Yatao