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LM76003: Current Limit

Shota Mago
Expert 8130 points
Part Number: LM76003
Other Parts Discussed in Thread: TPS43060, LM76002

Hi team, 

My customer considers to use LM76003 and provided some questions.

  1. What is difference of ISC and HS current limit?

  2. Section 7.3.12 describes “The slope compensation increases with duty cycle and tends to lower the HS current limit above 60% duty cycle as it lowers below ISC. See Typical Characteristics.”.
    Which figure does "See Typical Characteristics" stand for in the datasheet?

  3. According to above description, LM76003 seems to lower HS current limit in case of above 60% duty cycle.
    Would it be possible to provide typical graph of HS current limit threshold with respect to duty cycle like Figure 20 of TPS43060 datasheet?

  4. Let me confirm the meaning of " Under non-severe overcurrent conditions when the feedback voltage has not fallen 60% below regulation, the LM76002/LM76003 reduces the switching frequency and keeps the inductor current valley clamped at the LS current limit level. ".
    In other words, does it mean if the voltage of FB pin is above 40% of feedback voltage (TYP 1.006V*40%=0.402V), LM76003 doesn't activate Hiccup mode and max output current will be LS current limit + 0.5*inductor's ripple current because of LS current limit.
    Is this understanding correct?

Best regards,

Shota Mago

  • 0
    TI__Expert 4555 points

    Hello Shota,

    Answering your questions here:

    1. What is the difference between ISC and HS current limit? - There is no difference, both are same.

    2. Which figure does "See Typical Characteristics" stand for in the datasheet? - I am sorry to say, but there is no such graph demonstrating this explanation. This will get corrected in the next revision of the datasheet.

    3. Would it be possible to provide a typical graph of HS current limit threshold with respect to the duty cycle? - Sorry to say but we don't have this particular graph, I checked internally with systems as well. 

    4. Is this understanding correct? - Yes. your understanding is correct.

    Please let me know if you have more questions.

    And please close the thread if you think my response answered your questions.

    Best Regards,

    Ankit Gupta

    Application Engineer

  • 0 in reply to Ankit Gupta95
    TI__Expert 8130 points

    Hi Ankit-san

    Thanks for your reply.

    Let me ask you an extra question.

    Section 8.2.2.5 describes "During an instantaneous short, the peak inductor current can be high due to a momentary increase in duty cycle."

    Also, section 7.3.12 describes "This operation mode allows slight over current operation during load transients without tripping hiccup."

    [Question]

    The peak inductor current does NOT over HS current limit even in the case of above conditions if the inductor which has current rating higher than HS current limit is used.

    Is this understanding correct?

    Best regards,

    Shota Mago

  • 0 in reply to Shota Mago
    TI__Expert 4555 points

    Hello Shota,

    Answering your questions here:

    Regarding "During an instantaneous short, the peak inductor current can be high due to a momentary increase in duty cycle." - I would always recommend having a saturation current of Inductor to be greater the current limit of the part. For this case, I would recommend having an inductor's saturation current to be greater than 6.8A.

    Regarding section 7.3.12 describes "This operation mode allows slight over current operation during load transients without tripping hiccup." - Your understanding is correct. Inductor peak current will never go above HS current limit. That is why Isat of an inductor should be greater than HS current limit.

    Hope this helps!

    Please close the thread if you think your questions are answered completely. Thanks!

    Best Regards,

    Ankit Gupta

    Application Engineer

  • 0 in reply to Ankit Gupta95
    TI__Expert 8130 points

    Hi Ankit-san

    Thanks for your kind explanation!

    Best regards,

    Shota Mago