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Question of LM317L current limiter

Jacob Kim
Prodigy 250 points
Other Parts Discussed in Thread: LM317L, LM317, TL431, LP2951

Hello There

My customer reqeusted to check LM317L as 100mA current limiter.

They wants 8.5V Vout with regulated 100mA.

In this case, What about output voltage accuracy?

Main current will flow through R1? I mean R1 need bigger wattage spec?

Do you have any reference schematics about current limiter?

Thank you.

 

 

 

  • 0
    TI__Expert 3920 points

    Jacob,

    I'm not aware of any reference schematics we can share, but If you want to regulate both current and voltage, you should be able to place 2 LM317 regulators in series, one configured as a current limiter as you have shown above, and the other configured as a voltage regulator.

    I'm not aware of a way to regulate both V and I with one LM317.

    Tim

  • 0
    TI__Guru*** 140106 points

    Jacob,

    How about something like this. The TL431 regulates voltage and the LM317 limits current.

    2161.BattCharger.tif

    R1 wattage is I^2*R or I*V (100mA*1.25V=125mW)

  • 0 in reply to Ron Michallick
    TI__Prodigy 250 points

    Thank you Tim and Ron

    Customer can control Vin to get 8.5V for Vout.

     

    I think it doesn't need FB part.

    If I recommend bleow, Vout can generate 8.5V within 100mA?

    (Vin supplies 8.5V too)

     

  • 0 in reply to Jacob Kim
    TI__Guru*** 140106 points

    Jacob,

    The circuit has some problems. The LM317L will have about 2V drop out at 100mA and the voltage drop on the 12.5 ohm resistor will be 1.25V @ 100mA. So input voltage should be 11.75V. Even with this, the current limit will be accurate, but the voltage output will vary considerably.

    If a current limit near 150mA is acceptable then a LP2951 can be used if VIN was 9V or greater. 
    Two external resistors can set Vout to 8.5V, the near 150mA current limit is built in.