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Original question:

LM317: Technical query regarding FB pin

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

Jonel Pericon
Prodigy 30 points
Part Number: LM317

Tool/software:

I understand the normal equation will be below.

However we saw on the specification this formula adding R3 and -10V in the mix.

May I know when we will use both formula and what is the model behind the second equation?

  • 0
    TI__Mastermind 34715 points

    Hi Jonel,

    In most cases you can use the first equation (in section 9.2.2) without R3 or the diode of Figure 12 installed.  The LM317 is limited on the output to the internal reference voltage.  So if you need to provide an output voltage less than Vref, you can use the schematic in Figure 12 and the updated equation to calculate Vout.  Alternatively, you can let us know your requirements (Vin, Vout, Iout, and any system requirements such as PSRR, noise, transient response, turn on time, etc) and we can recommend an LDO option for you.

    Thanks,

    Stephen