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LM5117: Duty cycle problem in PSPICE simulation, tHO(OFF) limited to 1us

Jonathan Geronga
Expert 7930 points
Part Number: LM5117
Other Parts Discussed in Thread: LM5145

Hi Team,

Our customer is trying to simulate the LM5117 with Vin=47V and Vout=44V, Iout=6A. 

In pspice simulation, they can't reach the output voltage of 44V because min. tHO(OFF) is around 1us. Vout is around 40V.
RT is set to 42k2 for 120 kHz.

According to the datasheet, it should be possible to reach 44V. Duty cycle = 44/47 = 0,936 should be possible at 120 kHz.

They have already checked the current signal on Rs and RAMP. Everything seems ok.
They using an inductor of 47µH.

If the Vin increases to 55V in the simulation, it works.

Is there a problem with the pspice model? Is it possible to reach a duty cycle of 0,936 at 120 kHz.

Please let me know your comment.

Thanks,

Jonathan

  • 0
    TI__Guru 73072 points

    Hi Jonathan,

    The LM5117 is not suitable for such a high duty application as some margin is required in terms of the PWM off time (320ns typ, 440ns max). Please consider the LM5145 controller that has a toff-min of 140ns typ, 200ns max. You can use the Excel quickstart calculator available by download from the product folder to assist with component selection.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 60 points

    Hello Tim,

    If we consider fSW=120kHz (periode time = 8,33µs):

    max. Duty Cycle = (8,33µs - 440ns)/ 8,33µs = 0,94717

    Required Duty Cyle for Vout=44V and Vin=47V: 44V/47V = 0,9361

    As I see it, there is still margin and it should work. It also complies with Figure 28 in Datasheet.

    Ether the simulation model for pspice is wrong or the information in the datasheet.

    I don't want to switch to LM5145 because I need the current monitor output of LM5145 for adjustable CV/CC regulation according to

    Is there a reference circuit for CC/CV regulation with LM5145. I current and voltage limit have to be adjustable by microcontroller (38V-44V / 1A - 6A)

    Could you please give me advice in this case.

    Best regards

    Florian

      

  • 0 in reply to user5069983
    TI__Guru 73072 points

    Hi Jonathan,

    The duty cycle will be slightly higher with FET Rdson and inductor DCR voltage drops at 6A load current. Also, some duty cycle margin to react to load changes or transients is useful. It appears the simulation is too conservative here. My recommendation is to reduce Fsw to 100 kHz.

    Regards,

    Tim