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LMR33630: Minimum Input voltage for 12V output

Biswadeep Indu33
Prodigy 120 points
Part Number: LMR33630

I am planning to use this IC for 12V output generation. What is the minimum input voltage for safe operation for output of 12V?

While calculation of inductance, L and Lmin, till 13.2V, the inductance values are rational. While below 13.2V input voltage, Lmin value goes more than L value calculation. So, shall I consider that 13.2V is the minimum input voltage for 12V output generation? or if there is any more calculation?

Regards,

Biswadeep

  • 0
    TI__Genius 11040 points
    Hello Biswadeep,

    The minimum input voltage will be a function of the minimum off time, RDS_on of the converter and the DCR of the inductor.

    What is your measurement for the inductor, and what frequency are you operating at?

    Thank you,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 120 points
    Dear Katelyn,
    Thank you for your response. Following is my calculation with reference to datasheet.

    VIN: 12.6V
    VOUT: 12V
    K: 0.3
    IOUTMax: 1A
    fSW: 400000 Hz
    Constant for LMIN: 0.28

    L = 4.761904762uH
    LMIN: 8.4uH

    Irrespective of the switching frequency, while the input voltage is lower than 13.2V, Lmin value becomes lower than L. Is this the minimum operating input voltage if I want to generate 12V regulated output?

    Best Regards,
    Biswadeep
  • 0 in reply to Biswadeep Indu33
    TI__Genius 11040 points
    Hello Biswadeep,

    Thank you for clarifying your question. I see how you are calculating both LMIN and L. You want to size your inductor for the nominal input voltage.

    You are correct that as your input voltage drops the inductor ripple current will decrease. However, also as the input voltage drops the frequency will begin to foldback by extending the on the time of the converter. This will allow you to operate at lower input voltages.

    Please let me know if you have any remaining questions or if anyting is still unclear.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 120 points
    Dear Katelyn,

    Thank you for the answer. Can you please provide the calculation regarding lower voltage. It will be easier for us for addressing the design aspects. As you mention that the frequency will begin to foldback, what is the calculation behind the same. It will enable us to calculate for emissions. As well, while the ripple current decreases, what is the formula for calculation?

    This IC datasheet typically provides parameters for 5V and 3.3V output but does not provide 12V output. If you can provide with the calculation, it will be further useful.

    Best regards,
    Biswadeep Indu
  • 0 in reply to Biswadeep Indu33
    Prodigy 120 points
    Hello Katelyn,

    Any update on the same please.
  • 0 in reply to Biswadeep Indu33
    TI__Genius 11040 points
    Hello Biswadeep,

    This applications note discusses the equation: www.ti.com/.../slyt293.pdf

    Use equation #8 to calculate Vout_max for your minimum input voltage.

    Please let me know if anything is unclear or if you have additional questions.

    Best Regards,
    Katelyn Wiggenhorn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 120 points
    Hello Katelyn, Thank you for your response. I have got my answer. Thanks again. :)