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LMZ14203: What drives the RFBT and RFBB resistor limits of 50K?

TI-CSC
Genius 9210 points
Part Number: LMZ14203

Hello E2E Experts,

Good day

Why is there a specific statement regarding this in section 8.2.2.1.2 of the datasheet that the resistors should be 1K - 50K? Here is the exact line from the datasheet "These resistors should be chosen from values in the range of 1 kΩ to 50 kΩ".

 If RFBT was say 182K what is the concern with this?

Thank you in advance for your support.

Regards,

CSC

  • 0
    TI__Mastermind 24690 points

    Hello,

    It appears it stems from the statement following it:

    "These resistors should be chosen from values in the range of 1 kΩ to 10 kΩ.

    For VO = 0.8 V the FB pin can be connected to the output directly so long as an output preload resistor remains that draws more than 20 uA. Converter operation requires this minimum load to create a small inductor ripple current and maintain proper regulation when no load is present."

    Source: www.ti.com/.../lmz14203.pdf

    In other words, the range of feedback resistance enables the minimum current drawn from the output of 20uA.

  • 0 in reply to Marshall_Beck
    TI__Genius 9210 points

    Hello Marshall,

    Good day.

    But why wouldn't you just say the feedback resistor network needs to draw at least 20uA rather than saying you should use 1K - 50K resistors? I assume this resistor impedance also affects the loop response, but that isn't discussed.

    Regards,

    CSC

  • +1 in reply to TI-CSC
    TI__Mastermind 24690 points

    Hello,

    That could be another way of explaining it

    For COT architecture, ripple injection is what dictates stability. In PCM, DC setpoint and top resistor can impact loop response as well feed forward cap calculation.

    See below appnote.

    www.ti.com/.../snva874