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Original question:

LM317: I want a detail explanation on Output Voltage Calculation of LM317

LM317: 24V to 3.3V conversion

Kapil K
Prodigy 190 points
Part Number: LM317

Tool/software:

Hi,

I am interested in using this part (LM317LMX/NOPB) to generate 3.3V @20-30 mA. I observe the reference circuit in which R2 is the variable resistor. Is it necessary to have a variable resistor or can I maintain the fixed values of R1 and R2 and generate 3.3V?

Vin: 24V

Vout- 3.3V @ 10-40 mA.

Verf= 1.25V

Which combination value should I maintain in generating 3.3V?

Regards

Kapil

  • 0
    TI__Mastermind 37065 points

    Hi Kapil,

    You can use fixed resistors to generate the 3.3V output.  You do not need to use variable resistors for this.

    You need a minimum load of 5mA to keep this LDO stable, which you can get from the feedback resistors if your DC loading will not always be available.  In that case your resistors should be no larger than 249 ohms and 407 ohms. 

    If your DC load will always be available then I would scale those values up but you'll have to consider the large adjust pin current which acts as an error term in the output voltage equation. Use equation 1 to guide you in your resistor selection.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 190 points

    Hi Stephen,

    • You need a minimum load of 5mA to keep this LDO stable, which you can get from the feedback resistors if your DC loading will not always be available.  In that case your resistors should be no larger than 249 ohms and 407 ohms. 
      • So for this condition, we can ignore voltage generated by Iadj current (Iadj*R2) right? Is my understanding correct?
      • So the value of R2 will be 408.36 ohm. while keeping R1 249 ohm.
    • If your DC load will always be available then I would scale those values up but you'll have to consider the large adjust pin current which acts as an error term in the output voltage equation. Use equation 1 to guide you in your resistor selection.
      • In this case, I should go with worst condition and keep Iadj- 100uA. And calculate the value of R2. While keeping R1 is in the order of 1-3.3K ohm. Correct me if I need to change anything in this.
  • 0 in reply to Kapil K
    TI__Mastermind 37065 points

    Hi Kapil,

    This is correct.

    Thanks,

    Stephen