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LM117: What is the dropout voltage?

Part Number: LM117
Other Parts Discussed in Thread: LM1086

Hello,

I am looking for the maximum dropout voltage of the LM117 in my circuit.

R1 = 1.87k / R2 = 16.58k

This gives an output voltage around 13.16V.

By applying 15V at the input, we have a differrential voltage of 1.84V.

My question is, is it enough? (Output current expected is less than 100mA.)

Datasheet was not clear to me on that point.

Thank you

Best regards

Benjamin

  • Hi Benjamin, 

    Below is the Dropout Voltage plot from the datasheet, as you can see it will depend on the junction temperature as well. As you can see the lower temperatures could become a concern if your application needs to operate at or below Tj=0C. 

    If that is a concern for you, then you could consider LM1086 whish is also a floating regulator. If you don't need the floating capabilities of LM117 & LM1086 then we have many more modern LDOs which may be able to support your needs such as TLV709.

  • Hi Kyle,

    Thank you for the answer.

    My main concern is that the Line regulation and the Vref parameters are guaranteed for (VIN‑VOUT) ≥ 3 V. So, with 15V input and 13V output (Vdelta = 2V < 3V), we may get the desired voltage out, but not properly regulated or not regulated at all.

    This graph seems valid only for the regime where the LM117 is not regulating anymore and the output voltage depends fully on the input voltage and load current. Which is not what we want while regulating.

    What are your thoughts about it?

    Thank you

    Best regards

    Benjamin

  • Hi Benjamin, 

    You are correct, Linear Regulator dropout is defined as the point where the the output voltage is no longer regulated. So being at dropout will result in a output voltage hit. Old Linear Regulators like this have much larger dropout voltages than more modern devices. 

    Note that for Vdo<Vin-Vout<3, the LDO will be regulating but the performance may be worse than the EC Table shows since you are outside the tested EC Conditions.

    Below is an example using the graph of Vout vs Vin for a 5V regulator, and as you can see even with Vin-Vout=2V, the output regulates at 5V. Now this graph is not zoomed in enough to visually see the Line regulation spec of 0.05%/V (2.5mV/V for Vout=5V), but it does show that it is regulating. 

    If your application requirements mean you to have better accuracy (or known accuracy performance), not just regulation, than you can be sure of with LM117 then I'd recommend considering switching to a newer device with a lower dropout. 

    As I mentioned above, without knowing all your requirements, I'd consider LM1086 which is also a floating regulator. If you don't need the floating capabilities of LM117 & LM1086 then we have many more modern LDOs which may be able to support your needs such as TLV709.

      

  • Hi Kyle,

    thank you very much for the detailed feedback.

    Best regards

    Benjamin