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TPS7H5001-SP: Duty cycle clamp questions

Brian Bubnash61
Prodigy 200 points
Part Number: TPS7H5001-SP

Tool/software:

Two questions on the duty cycle clamp:

  • The datasheet mentions that for the 75% duty cycle clamp (DCL floating) the clamp value may vary based on duty cycle of the external clock. Can you please clarify that relationship? For instance if I pass in a 45% duty cycle clock does the duty cycle clamp value vary by 5%? And does this variation always occur or must certain operational or environmental conditions be met for this to happen?
  • Do the dead times between primary and SR have any effect on the duty cycle clamp value? (Presumably if they do not then the dead time would count against SR ON time)
  • 0
    TI__Mastermind 28645 points

    Hey Brian,

    As the duty cycle of the external clock varies from 50%, the duty cycle limit will increase in variation as well.
    We have done testing on the 40-60% range for an external clock and saw an additional 5% variation from the limits of the 75% DCL.

    The dead times do subtract from the maximum on time of the part.

    Something that will vary the duty cycle clamp and is relevant is the slope compensation can end up causing a smaller duty cycle limit than what you set as well.
    The slope compensation subtracts from the comparison of the COMP pin and the CS_ILIM pin and thus can cause an artificial duty cycle limit.

    This voltage listed is subtracted from a COMP/2 value (with a small offset) and then compared to the CS_ILIM pin

    Thanks,
    Daniel

  • 0
    Prodigy 200 points

    Thanks! Some go-backs:

    • When you say an additional 5% variation from limits of 75% DCL was seen when external clock duty cycle was varied 40-60%, does that imply there is a (roughly) linear relationship between % duty cycle of the external clock and variation in the DCL? I.e. if my external clock duty cycle varies x% from desired 50% duty cycle, is it fair to say that we would see the DCL vary from its intended limit by x/2% ?
    • Thanks for pointing out the dependency on slope compensation, that's a good subtlety to keep in mind
    • The datasheet section 8.6 suggests that max duty cycle when DCL is floating and ext clock duty cycle is 50%, the duty cycle clamp range can be expected to vary from a min of 70% to a max of 80% (75% nom), over the full recommended Tj and VIN range of the device. Is that accurate?
    • Can you confirm that both T_ps and T_sp subtract from the max ON time of the part? It seems like that was your intent but I'd like to be sure

    Summarizing all of this it sounds like the worst-case range in Dmax would be:

    Dmax_ds + (D_sync - 0.5)/2 - (T_ps + T_sp) / Tsw

    Where Dmax_ds is the max duty cycle range specified in the datasheet (normalized to 0-1 range rather than a %), D_sync is the duty cycle of the sync part (again normalized to 0-1 range), and Tsw is the switching frequency of the device (T_sync * 2). Is this correct? Am I missing anything?

  • +1 in reply to Brian Bubnash61
    TI__Mastermind 28645 points

    Hey Brian,

    There is not necessarily a linear relationship between the SYNC duty cycle and the DCL limit.

    The limit variation is a bit more than that.
    It covers subgroups 9,10, and 11 in Group A testing which are defined in the SMD.
    I would review the SMD for what tests cover things exactly.

    I can confirm I meant that the dead times subtract from the max on time.

    I would not subscribe to the linear relationship of SYNC in your equation.
    I more or less agree with it when not including the slope compensation as a part of it.

    Thanks,
    Daniel