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LM317L-N: Current Limiter

Kotaro Yamashita
Intellectual 2180 points
Part Number: LM317L-N

Hi,

My customer would like to achieve current limiter circuit as Figure 20. However, Figure mentioned limited value, 12 < R1 < 240.

What does this value come from? They would like to achieve 2mA current limit with 600ohm resistor.

Regards,

Kotaro Yamashita

  • 0
    TI__Expert 4350 points
    Hi Kotaro,

    LM317L-N has a minimum load current for proper operation as listed in the Electrical Characteristics table.
    For Vin - Vout <= 40 V, the minimum load current is 3.5 mA typical and 5 mA max. R1 = 240 ohms results in Iout = 5 mA.

    For smaller Vin - Vout, the minimum load current is less. With 3 V <= Vin - Vout <= 15 V, the minimum load current is 1.5 mA typical and 2.5 mA max. If the customer operates in this lower range of Vin - Vout, they can use larger R1. R1 = 600 ohms for Iout = 2 mA would be acceptable. To guarantee the minimum load current is met, the customer could use R1 = 480 ohms for Iout = 2.5 mA.
  • 0 in reply to Eric.Preiss
    Prodigy 10 points
    Hi,Eric. Related Question.
    In case that smaller load current , such as Vin-Vout=20V, Iout=1.2mA with LM317N , what will happen to ?
    Unstable? or Oscillated?
  • 0 in reply to Shusaku Goto
    TI__Expert 4350 points
    Hi Goto-san,

    The output voltage will rise if a minimum load current on the output is not present.

    From section 7.1 in the datasheet:
    "Because the 100-μA current from the adjustment terminal represents an error term, the LM317L-N was designed
    to minimize IADJ and make it very constant with line and load changes. To do this, all quiescent operating current
    is returned to the output establishing a minimum load current requirement. If there is insufficient load on the
    output, the output will rise."

    Figure 6 in the datasheet shows typical quiescent current, which flows out of the output pin.