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LM5176-Q1: BOOT resistor

Bill Adams1
Prodigy 190 points
Part Number: LM5176-Q1
Other Parts Discussed in Thread: LM5176

We are seeing quite a bit of noise on our boost mode design with the LM5176.  Application Note slyt740 recommends putting a resistor in series with the boot capacitor to reduce the noise from the high side FETs.  Should he resistor be placed between the HDRV1, HDRV2 pins and the gaste of the high side FETs?  Or should the resistor be placed in series with the boot capacitors?

  • 0
    TI__Guru 65340 points

    Hello Bill,

    A boot resistor for noise reduction has to be placed in series to the boot capacitor.

    You could as well use gate drive resistors which are in series to xDRVx pins but these will slow down turn on and turn off.

  • 0 in reply to Brigitte
    Prodigy 190 points

    The LM5176 datasheet states that "

    In boost operation, LDRV2 and HDRV2 are switched
    while HDRV1 remains continuously on.

    I am seeing switching on all of the gate drivers.  HDRV1 is high most of the time but switches low periodically.  The inductor is also getting rather warm and it is rated for 27.4A for saturation current.

  • 0 in reply to Bill Adams1
    Prodigy 190 points

    The compensation was not working properly.  For boost mode, the crossover frequency should be less than 0.3x fRHP.  I calculated fRHP to be 114 kHz.  All of the larger values fro crossover frequency are giving me poor results.  When I moved the crossover frequency down to 600 Hz, the regulator started to behave much better.  Is there any harm in having such a low crossover frequency?  There is 8800 uF of capacitance on the output.  This should prevent any fast transients which would require a quicker response.

  • 0 in reply to Bill Adams1
    TI__Guru 65340 points

    Hello Bill,

    In general it is recommended to set the crossover frequency as high as needed, but as low as possible. Lower crossover frequency means slower transient response but easier to stabilize, higher crossover frequency means better transient response, but harder to stabilize.

    So in general if the transient response is good enough for your design, there is no problem with having a low crossover frequency.

    When you mention that the compensation was not working properly, how did you set the compensation components?

  • 0 in reply to Brigitte
    Prodigy 190 points

    I was setting the values using the LM5176 Buck-Boost Quickstart Tool spreadsheet.  With an extremely large load capacitance, it seems to use very large resistance values for Rc1.  Thgis and the relatively small capacitance for Cc1 caused instability at the COMP pin.

  • 0 in reply to Bill Adams1
    TI__Guru 65340 points

    Hello Bill,

    If you are willing to share your result of the spreadsheet, we can have a look at it.

    Nevertheless, it seems that the solution you found solved the issue, is this understanding correct?

  • 0 in reply to Brigitte
    Prodigy 190 points

    Yes.  The low crossover frequency resolved the issue.