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LM317-N: Need of 8.4.2 Protection Diodes

Part Number: LM317-N

Dear Specialists,

My customer is considering LM317-N and has a question.

I'd like to get your advice for him.

---question

According to the datasheet of LM317-N (P.14) of 8.4.2 Protection Diodes, it says, for output capacitors of 25 μF or less, there is no need to use diodes.

This means output protection diode depend on the output capacitor value (25uF).

Is there no limitation of capacitance type?

Is it also possible to use ceramic at this time?

---

 I appreciate your great help.

Best regards,

Shinichi

  • Hi Shinichi,

    Please note that the two diodes protect against different events. D1 is not required if the output capacitor (C1) is 25uF or less. D2 is not required if the output voltage is 25V or less AND C2 is 10uF. Under these conditions the device is internally able to handle the discharge of the capacitors. If larger capacitors (or voltages for D2) are used, the current will flow through the protection diode.

    As mentioned in Section 8.4.1 External Capacitors, we recommend solid tantalum capacitors for this device.

    Very Respectfully,
    Ryan
  • Dear Ryan,

    Thank you for your reply.

    I understand the purpose of D1 and D2.
    It depends on capacitance and output voltage.

    On the other hand, you mention output capacitor is tantalun,
    but other post is also able to use ceramic capacitor.

    e2e.ti.com/.../135490

    Which is recommended?
    The customer would like to use ceramic.

    I appreciate your great help.

    Best regards,
    Shinichi
  • Hi Shinichi,

    The datasheet recommendation is solid tantalum. As stated in the datasheet and in forum post you link to, it is possible to use ceramic capacitors with LM317-N; however, the recommendation with this older device is solid tantalum.

    Very Respectfully,

    Ryan

  • Dear Ryan

    Thank you for your reply.

    following line, ceramic capacitors are also good.

    I think the customer is able to use.

    In addition, nowadays ceramic capacitor is the better high frequency performance than tantalum. 

    I appreciate your great help.

    Best regards,

    Shinichi