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Question of LM34922

Takahiro Nishizawa
Genius 5575 points
Other Parts Discussed in Thread: LM34922

I have questions about LM34922.

 

Question 1.

Does this IC have light load mode?

 

Question 2.

If this IC has light load mode, please tell us the operation about light load mode.

For example, pulse skip mode and so on.

 

Question 3.

Please tell us the maximum output current when Vin is 15V and Vout is 12V.

 

Question 4.

Please tell us the accuracy about current limit.

Best regards,

Takahiro Nishizawa

  • 0
    TI__Mastermind 35115 points
    Hi Nishizawa-san,

    Please find below my answers to your questions.

    The LM34922 inherently has light load mode with variable fsw at light load because it is a non-synchronous constant on-time converter. However the Iq is relatively large so there is likely a newer better part if you need to optimize the light load efficiency.

    That maximum current is mostly determined by how the current limit is programmed with the current sense resistor. The maximum recommended average current is 2 A. Was there a specific concern with supporting Vin 15 V and Vout 12 V?

    The accuracy of the current limit is given by the VILIM threshold voltage at CS specification in the electrical characteristics table.

    Best Regards,
    Anthony
  • 0 in reply to Anthony F
    Genius 5575 points
    Thank you for your answer!


    I understood this IC has light load mode.
    Please tell us the operation of light load mode.
    pulse skip or single pulse control or bang-bang control?

    About accuracy of current limit,
    I checked datasheet "Threshold voltage at CS".

    min : -146mV
    typ : -130mV accuracy : 12.3%

    typ : -130mV
    max: -115mV accuracy : 13.04%

    Accuracy is about 12~13%.
    Is this understanding correct?


    Best regards,


    Takahiro Nishizawa
  • 0 in reply to Takahiro Nishizawa
    TI__Mastermind 35115 points
    Hi Nishizawa-san,

    Your understanding for the current limit is correct.

    With a constant on-time converter, what typically happens at light load is the fsw decreases as the load decreases. There is a single pulse but at a lower fsw that depends on the load. This happens because the constant on-time pulse is triggered when the FB voltage falls to the reference voltage. At lighter loads it takes a longer time for the FB voltage to fall to the reference voltage so the fsw is lower.

    Hope this helps.

    Anthony