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LM25145: nearly 100% duty cycle operation

Shinichi Inoue
Guru 21701 points
Part Number: LM25145
Other Parts Discussed in Thread: LM5145, LM5176

Dear Specialists,

My customer is considering LM25145 and has questions.

I would be grateful if you could advise.

---

(1) According to the datasheet P1. , says the LM25145 continues to operate during input voltage dips as low as 6 V, at nearly 100% duty cycle if needed

I am thinking to use LM25145 at 24V/12V input and 12Voutput.

In this case, 12V input and 12Voutput should be almost 100%, is it possible to use?

(2) Could you please let me know the disadvantage compared with normal PWM duty cycle.

e.g. increase ripple voltage, etc.

(3) Is it possible to simulate with 12Vinput and 12Voutput.

I'd like to confirm in this condition.

Any simulator is ok.

---

I appreciate your great help.

Best regards,

Shinichi 

  • 0
    TI__Mastermind 40040 points
    Hello Sinichi,

    If operating in dropout the switching will not be periodic, the output will drop and the voltage ripple will increase. the nearly 100% statement is alluding to the fact that this device can operate at 99% (typ) in a certain condition. and is advertising suitability for uses in applications where the input momentarily dips to near output voltage setting as in automotive applications, for example.

    Please refer to the Datasheet electrical specifications for the Dmax specification as well as the toff min specification. Also pay close attention to the max specification for worse case.

    Also you may find this link below informative.

    www.ti.com/.../slyt293.pdf


    If you require a 12V in and 12V out, the LM5145 may not be able to regulate the output in this condition. The decision to move ahead with the LM25145 will depend on the following:

    1. Output voltage accuracy requirement
    2. Load current
    3. Parasitic impedance in the power path during the D and 1-D period.

    For example, if you need 12V +/- 5% with high current greater than say 3 to 5A, then I believe you will need to use a buck boost converter. Perhaps the LM5176 maybe more suitable?

    Hope this helps?
  • 0 in reply to David Baba
    Guru 21701 points
    Hi David,

    Thank you for your reply.

    I shared your suggestion with the customer.

    I appreciate your great help.

    Best regards,
    Shinichi