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LM5045: COMP Pin current-to-Duty CycleTransfer Function?

Kurt Schmidt
Genius 11125 points
Part Number: LM5045

My customer is modeling this pin as part of their MATLAB control loop simulation. Do you know what the transfer function is?

We know it is not a linear one between 0mA to 1mA because we can put as much as 500uA into the COMP pin without moving the duty cycle away from maximum (COMP pin open).

Please advise. Thank you!

  • 0
    TI__Mastermind 34465 points

    You can calculate the relationship between the duty cycle and COMP value by regarding the PWM section of the block schematic 
    in the LM5045 data sheet.

    V_ = (Vcomp-1)/2

    Vcomp = 5(1-Icomp)

    This gives V_ = 2- 2.5*Icomp

    The minimum value of RAMP signal is zero since the LM5045 has a blanking function at the start of each clock cycle.
    The value of Icomp corresponding to maximum duty cycle occurs when V_ = V+  =0.
    This state corresponds to Icomp = 2/2.5 = 0.8mA

    Now you need to know the peak value of the RAMP used in your application.
    This is normally set at  1.5V
    When V_ = 1.5V the duty cycle is minimum

    This state corresponds to Icomp =( 2-1.5)/2.5 = 0.2mA

    Between 0.2mA and 0.8mA the PWM varies linearly.

    You can  use the block schematic above to generate the transfer function.

    Regards

    John

    Regards

    John

  • 0
    TI__Mastermind 34465 points

    You can calculate the relationship between the duty cycle and COMP value by regarding the PWM section of the block schematic 
    in the LM5045 data sheet.

    V_ = (Vcomp-1)/2

    Vcomp = 5(1-Icomp)

    This gives V_ = 2- 2.5*Icomp

    The minimum value of RAMP signal is zero since the LM5045 has a blanking function at the start of each clock cycle.
    The value of Icomp corresponding to maximum duty cycle occurs when V_ = V+  =0.
    This state corresponds to Icomp = 2/2.5 = 0.8mA

    Now you need to know the peak value of the RAMP used in your application.
    This is normally set at  1.5V
    When V_ = 1.5V the duty cycle is minimum

    This state corresponds to Icomp =( 2-1.5)/2.5 = 0.2mA

    Between 0.2mA and 0.8mA the PWM varies linearly.

    You can  use the block schematic above to generate the transfer function.

    Regards

    John

    Regards

    John

  • 0 in reply to John Griffin1
    TI__Genius 11125 points

    John, I really appreciate the detailed and rapid response!

    There is one thing I'm not tracking on though. You are saying maximum duty cycle occurs at the maximum current and minimum duty cycle at a lower current. But this is opposite of what the data sheet says and what we see in practice. Specifically, the data sheet says that leaving COMP open (zero current) runs the PWM open loop at maximum duty cycle (50% nom) while 1mA of COMP input current forces minimum duty cycle (0%). This is consistent with what we see on the bench and also consistent with what an opto-coupler output driven by a shunt regulator input would provide.

    What am I misunderstanding?

  • 0 in reply to Kurt Schmidt
    TI__Mastermind 34465 points

    Yes Kurt

    You are correct. I was mistaken