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LM5045: Minimum on time

Ella KIM
Expert 4212 points
Part Number: LM5045

Hi team,

I wonder the minimum on time of LM5045.

I believe during the period that I checked with the blue circle, the PWM pulse for primary FETs operates in the minimum on time. Is it correct?

Thanks,

  • +1
    TI__Mastermind 32775 points

    Hi Ella,

    The blue circled area you highlighted is not necessarily operating at minimum on-time. The key point about this portion of soft-start (SS) is that the voltage across the SS capacitor is controlling the duty cycle of the primary MOSFETs. COMP is not yet taking over the control until the SS reaches the 2V threshold. The blue circled area may begin at minimum on-time but as the SS voltage is increasing, the duty cycle is proportionally increasing according to SS voltage. More detail is provided below as:

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    TI__Expert 4212 points

    Mappus, 

    Thank you for the information. 

    Could you also let me know the min. on-time spec of LM5045? It's not specified in DS.

    + One additional question is :  what would the primary PWM signal be like if there's no prebias? I guess there would be no PWM OFF period. Could you please check it as well?

  • +1 in reply to Ella KIM
    TI__Mastermind 32775 points

    Minimum on-time is not a measured parameter and therefore not published in the data sheet, but in general, the controller output should be able to achieve minimum on-time on the order of 10-20ns. However, minimum on-time seen at the power stage will depend upon the amount of gate charge being driven. Higher gate charge will result in longer rise/fall times and less achievable minimum on-time. PCB layout also plays a role in the minimum achievable on-time. COMP, RAMP and CS need to be very clean with minimal induced noise when trying to achieve narrow duty cycle, minimum on-time. 

    If the output is not prebiased, the input should proceed continuously through a normal soft start cycle but there is still a handoff that happens where SS passes control to COMP. 

    Regards,

    Steve

  • 0 in reply to Steven Mappus1
    TI__Expert 4212 points

    Steve, 

    To clarify the 'handoff' that you mentioned, even though there's no prebias on the Vout, the primary pulse plot would show the short OFF period just like the figure 15?