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LM317M: Cadj recommend value

Yuhao Wang
Prodigy 330 points
Part Number: LM317M

Hello，

Good day our customer have a question about the recommend Cadj in this diagram and why.

  • 0
    TI__Genius 9250 points

    Hello,

    Thank you for your question.

    As stated in the datasheet, the adjustment terminal capacitor improves ripple rejection by preventing the amplification of the ripple:

    We recommend using a Cadj value of 10uF. Specifications with adjustment resistor is shown in figure 13 and the ripple rejection spec below:

    Please let me know if you have any additional questions or concerns.

    Best Regards,

    Hannah

  • 0 in reply to Hannah Grace
    TI__Prodigy 330 points

    Hello Hannah，

    Really appreciate your reply, our customer had concern about if the Cadj is 10uF the Cout could be 120uF which is too big for their PCB. They wonder if they can remove the Cadf, how much ripple could it be, any other influence could have.

    Thanks,

    Yuhao

  • +1 in reply to Yuhao Wang
    TI__Genius 9250 points

    Hello Yuhao, 

    Thank you for your quick response.

    You are correct in stating that the customer may remove the Cadj, but it may amplify ripple:

    In the datasheet we have graphs regarding the ripple rejection and output impedance without Cadj to give some additional information on this part:

    Ripple rejection would slight be worse at room temperature for this part. 

    Over temperature ripple rejection is slightly better than room temperature.

    Without Cadj the output impedance would also be slightly larger across frequency about 1mohm:

    Overall oscillations without Cadj will be more present in the system, and the peaks for transients will be higher, so the customer will need to take this into design consideration.

    Please let me know if you have any additional questions.

    Best Regards,

    Hannah