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LM5161-Q1: Variable voltage output: 0 - 12V

Steven Plaskon
Prodigy 40 points
Part Number: LM5161-Q1
Other Parts Discussed in Thread: LM5161

I am designing a variable output voltage converter.  The input voltage range is 20 - 50VDC.  The output is variable from 0V to 12V @ 0.5A.  Is the LM5161 a viable candidate for this design or will the minimum on time prevent operation below an output voltage of 1.2V?  Are there any other suggested solutions?

  • 0
    TI__Mastermind 22390 points

    Hi Steven,

    The LM5161 is a constant on-time converter, so when the output is below the feedback regulation target, it will issue a pulse of the programmed on-time and check to see if FB is above the reference. If not, it will issue another pulse of that same duration every period. With this, combined with a network that injects current into the feedback pin, you can probably can use the LM5161 for your application, but I don't think you can truly bring the output down to 0V since you will trigger high-side peak current limit. 

    Please see an app note here discussing possible schemes: www.ti.com/.../slyt777.pdf

    Regards,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 40 points

    Richard,

    Thank you for the quick response.  A couple of additional questions and clarification.

    I am using the "Analog voltage input" as described in the reference paper to control the output.  I provide a DAC output through a resistor feeding to the FB pin of the controller.  With this approach the voltage at the feedback pin (FB) will always be satisfied even with the output voltage near zero.

    You commented that "I don't think you can truly bring the output down to 0V since you will trigger high-side peak current limit".  Can you please elaborate on this and why it initiates a high side current limit..  Will this happen even when using "analog voltage input" approach and satisfying the FB pin voltage?

    Will this controller operate in burst mode or skip cycle if the voltage on the FB pin is above the reference?  Can it run down to zero duty cycle if I pull the FB above the reference?

    Thanks,

    Steve

  • +1 in reply to Steven Plaskon
    TI__Mastermind 22390 points

    Hi Steve,

    I understand your use-case a bit more clearly now. 

    I made this comment based off of the datasheet in the section describing the regulation comparator. Basically, when FB is pulled down to gnd, the regulator will try to operate with minimum off-time per cycle, which means at certain load currents, you may run into current limit, since the switch is on for a long period of time. I think your use case will be fine, but I would make sure to test the condition of low output voltage and high current. 

    The LM5161 is not a typical peak-current mode buck converter, since it is a COT device. This means that it will skip cycles if FB pin is above the reference, and will only issue a pulse as long as the programmed on-time once FB is below reference. 

    Regards,

    Richard