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LMG1210: DCM operation with small inductor

Chris Smith
Intellectual 825 points
Part Number: LMG1210

I'm trying to run the LMG1210 demo board in a bit of a non-standard way and it seems to be cutting off prematurely.  I'm not sure why.

I am using hysteretic feedforward regulator to react to my changing input waveform.  I have a 200nH inductor with 100nF capacitor.  I have the lower switch set to off with the independent control of the PWM inputs.  The upper switch gets the pulse signal.  The device starts switching but then cuts out.  See attached picture.  Yellow is gate signal and green is output voltage.  

The way the output voltage is moving initially is exactly what we wanted but then it cuts off.  It will periodically turn back on.

What could be causing this?

  • 0
    TI__Intellectual 2100 points

    Hi Chris,

    Thank you for reaching out! It looks like your boot capacitor is not maintaining its charge. Typically, the low-side side circuitry helps create the charge for the boot cap; however, you mentioned that you are keeping the low-side switch off.  You will need to pull HS (switch node) to ground every once in a while to help charge the boot cap.

    Another option is to create the charge using a charge pump. Our team actually has an article available on how to implement a charge pump for continuous gate drive operation: Link.

    Let me know if you have any questions!

    Best,

    Alex Weaver

  • 0 in reply to Alex Weaver
    Intellectual 825 points

    So the bottom diode is definitely conducting when my inductor discharges.  This period may not be very long but I've seen it on the scope.  What is the UV on the bootstrap cap voltage?  I think I saw a consistent 4V on that node.  

    C

  • 0 in reply to Chris Smith
    Intellectual 825 points

    I’m also seeing that the datasheet says BST is only enabled when the bottom switch is on? Is there a way to disable that?

  • 0 in reply to Chris Smith
    TI__Intellectual 2100 points

    Hi Chris,

    Here is a look at the relevant UVLO ratings:

    The HB-HS differential voltage needs to stay above 3.8V. 

    There is no way to disable the low-side charging the boot cap; however, you can implement a device such as SN6501 or SN6505 to generate the high-side bias and charge the boot-cap without turning on the low-side output. I'm linking an article that discusses component selection for the bootstrap circuitry: Link. Section 3.1 on page 3 discusses the importance of sizing the boot capacitor properly to stay above the UVLO threshold and avoid the high-side turning off prematurely.

    Best,

    Alex Weaver