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LM5022: LM5022 several questions

Hollis Liu
Intellectual 2545 points
Part Number: LM5022

Hi Team,

Could you please help to answer below questions in datasheet? Thanks. 

1. current limit equation 36

How do we get equation 36? Especially the part, （Vo-Vin）*3*D/（ L*Fsw).

Why does（Vo-Vin）*3*D/（ L*Fsw） impact R_SNS calculation? 

2. R_S2 resistor

Is there any other impact for R_S2 resistor besides the current limit function? Can we set R_S2 to be 0? 

in actual test, we tried to set R_S2 to be 0. And that will bring unstable issue(duty changing).

  • +1
    TI__Genius 15425 points

    Hi Hollis,

    Thanks for reaching out to us via e2e.

    The LM5022 ia controller that uses a Peak Current Mode Control feedback loop.
    Datasheet Description: Output voltage regulation is based on current-mode control

    The part uses an inner Peak Current Mode Control loop and an outer Voltage feedback loop.
    The Peak Current Mode Control is always active, it cannot be disabled.

    Therefore, as per the datasheet, the Current sense input pin is used for the following:
    Input for the switch current used for current mode control and for current limiting.

    In other words:
    The CS pin is not just meant for over current protection purposes.
    The part cannot work properly by only using FB and COMP for the feedback loop and connecting CS to GND 
    because the most important inner current feedback loop would be missing and it cannot really regulate that way. 

    So, please connect the CS pin as described and calculate the resistors and capacitor as described in section 8.2.2.8 and 8.2.2.9.

    Best regards,
    Harry

  • 0 in reply to Harry Hofner
    TI__Genius 15425 points

    Hello Hollis,

    I have not heard from you for two weeks now.

    Can we close the thread?

    In case there are new questions in the future you can always re-open it or just create a new one.

    Thanks and regards,

    Harry