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LM25116: question about circuit design

Shinichi Inoue
Guru 21701 points
Part Number: LM25116

Dear Specialists,

My customer is considering power circuit and has a question.

I would be grateful if you could advise.

---

I am considering a circuit that outputs 12V at 12V/24V input.

Could you please see attached file.

At 24V input, step down to 12 V with LM25116.

When 12V input, it is supplied to the output through the bypass line.

At this time, power from the bypass line is input to the output (12V) of the LM25116
The reverse current prevention diode is not connected.

 In the condition when  LM25116 is turned on the input 12V and the output is 12V from bypass line.

(1) How does the circuit behave？

(2) Is there a possibility of damage?

(3) Is there any problem?

---

I appreciate your great help in advance.

Best regards,

Shinichi

question about LM25116.pptx

  • 0
    TI__Genius 11040 points
    Hello Shinichi,

    Thank you for the detailed information and providing the power point. My understanding is that there will be a 12V line which is applied to both the output and input of the LM25116? Is this correct?

    Thank you,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Guru 21701 points
    Hi Katelyn,

    Thank you for your reply.

    Yes, When 12V , both input and output apply the same voltage.

    I think LM25116 is not damage, but I'm not sure the operation at that time.

    I'm looking forward to waiting your suggestion.

    I appreciate your great help.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    TI__Genius 11040 points
    Hello Shinichi,

    You want to protect against the input voltage being lower than the output voltage. If this is possible, then the body diode of the high side FET can conduct and see high currents. This puts the high side FET at risk for damage. If this condition is possible, then I would recommend connecting a schottky diode from SW to VIN for protection. If the output voltage will be lower than the input voltage, then this condition will be okay.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Guru 21701 points

    Hi Katelyn,

    Thank you for your reply.

    In the customer's application, 12 V is supplied to the output via the bypass FET.

    Therefore, the voltage dropped by Rdson × load current of FET is supplied from 12V.

    During steady state operation, I think that it will not be a situation of your concern.

    If there is no situation of Vout> Vin, can the customer think that there is no problem?

    If so, schottky diode is not required you mentioned, is it correct?

    I appreciate your great help.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 21701 points
    Hi Katelyn,

    This is a reminder that I'm still looking forward to waiting your suggestion.

    I appreciate your great help.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    TI__Genius 11040 points
    Hello Shinichi,

    In regards to your additional questions:

    If there is no situation of Vout> Vin, can the customer think that there is no problem?

    A: This is correct.

    If so, schottky diode is not required you mentioned, is it correct?

    A: This is correct.

    Please let me know if you have any additional questions.

    Best Regards,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Guru 21701 points
    Hi Katelyn,

    Thank you for your reply, and I'm sorry for late response because I am in trouble other device.

    I understand it's no problem if Vin > Vout is kept.

    I'll share your answer with the customer.

    I appreciate your great help.

    Best regards,
    Shinichi