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LMR23630: About LMR23630 Block Diagram

GARAM AN
Intellectual 1990 points
Part Number: LMR23630

Hello,

Could you explain me what the "Precision Enable" and "Zero Cross" are, and how they work functionally?

Also, Could you explain the value of the Rc and Cc in the compensation filter?

How are they calculated and affected for the internal compensation system?

Thank you

  • 0
    TI__Mastermind 22301 points

    Garam,

    Precision Enable block describe the EN/UVLO circuit with hysteresis.

    Zero-cross block measures low-side MOSFET current (inductor current) and will turn off the low-side switch when the current goes negative (crosses zero). Preventing negative inductor current allows for highest light-load efficiency.

    Zero cross function is only available in PFM devices, FPWM operation allow negative inductor current.

    Rc and Cc are internal type-2 compensation to stabilize the control loop.

    From the datasheet: "The internal compensation is designed so that the loop response is stable over the entire operating frequency and output voltage range"

    Hope that helps,

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Intellectual 1990 points

    Thank you for the answer! Orlando

    So, Zero cross block does not work in the FPWM mode, is this correct?

    Could you please explain more about the "Precision Enable" block?

    you mean that it is for the EN/UVLO circuit with hysteresis, could you explain why the EN / UVLO need hysteresis?

  • 0 in reply to GARAM AN
    TI__Mastermind 22301 points

    Garam,

    Yes zero-cross detection is not used in FPWM mode.

    Yes the precision enable allows hysteresis on the EN/UVLO pin. It will allow a different turn-off voltage than the turn-on voltage.

    Allowing EN/UVLO hysteresis will allow designers to account for cases where VIN could drop temporarily but they do not want the converter to shutdown.

    This can help avoid VIN shutting down from input voltage ripple.

    Or with designs with lower input capacitance where the VIN capacitor could be drained temporarily by a sudden increase in load.

    -Orlando

  • 0 in reply to Orlando Murray
    TI__Intellectual 1990 points

    Thank you Olando,

    I have question about Zero Cross, and DCM Mode.

    I thought that LS switch is off automatically when inductor current goes to under zero at light load, because of MOSFET principle and MOSFET has internal diode.

    If there is no Zero Cross block or function, then countercurrent(backward current) can be flowed at the light load when the LS switch is on and inductor current goes to under the zero, is this correct?

  • 0 in reply to GARAM AN
    TI__Mastermind 22301 points

    Garam,

    The LS MOSFET is not off automatically, when the FET is on and conducting current can flow both directions.

    The internal diode is parallel to FET. When the FET is on the diode is shorted and has no effect. 

    You are correct for FPWM device there will be negative inductor current when the output current is less than half of the ripple current.

    Certain applications required fixed consistent switching independent of load, and FPWM is how to guarantee that.

    Hope this helps,

    -Orlando