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About TPS54350 Soft Start time

Bob Lee62837
Mastermind 7230 points
Other Parts Discussed in Thread: TPS54350

Dear support member,


My customer use TPS54350.

Soft start calculation is Css=5.55e^-3Tss listed of data sheet 7 page.

My customer want to know specifically soft start calculation, that include in the pullup current source range of ENA pin.
Pullup current source 1.8μA(min)、 5μA(typ)、 10μA(max).

How change is Start-up time in the following conditions?

1.Css=0.2μF
2.Css=0.1μF
3.ENA pin is Open.


Best regard.
Bob Lee.

  • 0
    TI__Guru**** 191445 points
    I seem to remember some confusion about how that particular equation was derived. Unfortunately it has been over 10 years ago. I will look into it but it may not be possible to answer in a short time. I think it is fairly obvious that the SS time is not particularly accurate for this device.
  • 0
    TI__Genius 11095 points

    Hello, for TPS54350, if the ENA pin is open, the soft-start time is controlled by a digital counter, which ramps up the internal reference over 1150 clock cycles. To approximate the min/max ramp times, take equation 3 of the datasheet, but recognize that the switching frequency has some accuracy that needs to be accounted for. For the internal "defaults' of 250kHz and 500kHz, the tolerance is specified in the datasheet [200k, 300k]. and [400k, 600k]

  • 0 in reply to Matt Schurmann
    TI__Guru**** 191445 points
    For the external slow start, I think the equation is Css (F) = Iss (A) * Tss (S) / Vref (V). You can modify the terms as you like to account for tolerances.
  • 0 in reply to JohnTucker
    Mastermind 7230 points

    Dear JohnTucker,

    Thank you for reply.

    May I ask a add question?

    You replied soft start equation is　Css (F) = Iss (A) * Tss (S) / Vref (V).
    Data sheet list soft start calculation is Css=5.55e^-3Tss.

    It is considered "5.55e^-3" = "Iss(A)/Vref(V)" ,
    How value is 5.55e^-3?


    Iss current source is 1.8μA(min)、 5μA(typ)、 10μA(max).
    Vref is 0.888V(min)、0.891(typ)、0.894V(max).

    example
    Iss= 1.8μA(min)、Vref=0.894V(max) case,
    Iss/Vref=2.0134×10^-6

    Iss=5μA(typ)、Vref=0.891V(typ) case,
    Iss/Vref=5.61×10^-6

    Iss= 10μA(max)、Vref=0.888V(min) case,
    Iss/Vref=11.261×10^-6


    Best regard.

    Bob Lee.

  • 0 in reply to Bob Lee62837
    TI__Guru**** 191445 points
    For whatever reason, the author assumed Vref = approximately equal to .9 V and Iss = 5 uA. Then when you put Css in uF and Tss in msec, that ration comes out to approximately 5.55E-3. I worked it backwards using standard units (S, F, V, A) to avoid ambiguity.
  • 0 in reply to JohnTucker
    Mastermind 7230 points

    Dear John Tucker,

    I'm sorry late for reply.
    I report your reply to my customer.


    Best regard.
    Bob Lee.

  • 0 in reply to Bob Lee62837
    Mastermind 7230 points

    Dear John Tucker,

    May I ask a add question?

    When carrying out the capacitor slow start by setting the ENA pin,
    Would you need to add an internal time soft-start time (internal slow start time of 1150 clock cycle)?


    soft start time is this following calculation?
    Tss ={ (Css×Vref) / Iss } + 1150× (1/ fsw)


    Best regard.

    Bob Lee.

  • 0 in reply to Bob Lee62837
    TI__Guru**** 191445 points
    It has been a few years since I have worked with that part, so I do not remember all the details from memory. Let me see what I can find out.
  • 0 in reply to JohnTucker
    Mastermind 7230 points

    Dear John Tucker,

    Thank you very much for action.
    May I ask a add question?

    Would you teach Pull-up current (Iss) and switching frequency (fsw),
    Min value and Typ value and Max value of Tj = 25℃?


    Tj = 25℃.
                   Min      Typ       Max
    Iss
    fsw


    Best regard.
    Bob Lee.

  • 0 in reply to Bob Lee62837
    Mastermind 7230 points

    Dear John Tucker,

    Could you update me the progress?
    I will need to update the customer on the progress.

    Best regard.

    Bob Lee.

  • 0 in reply to Bob Lee62837
    TI__Guru**** 191445 points
    The datasheet values are for -40 to +125 C, 1.8uA min, 5uA typ and 10uA max. I do not have data for just 25 C. TPS54350 is 12 years old now, so I do not know where the data may be. Same for Fsw. Even the IC designer is retired now.
  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points
    I would expect that a lot of the variation for SS charge current is due to process variation rather than temperature.
  • 0 in reply to JohnTucker
    Mastermind 7230 points

    Dear John Tucker,

    Thank you very much for reply.
    I will report your reply to my customer.


    Best regard.

    Bob Lee.

  • 0 in reply to Bob Lee62837
    Mastermind 7230 points

    Dear John Tucker,

    May I ask a add queestion?

    The output start time of the TPS54350, in addition to soft-start hours to, that I think that there is a start-up delay time.

    Output start time = soft-start time + start-up delay time

    Would you please teach the equation of start-up delay time?


    For The parameters used in the formula
    Please be taught to meet if there is a value in the value of -40 ~ + 125 ℃ and + 25 ℃.

    What else output start time affect value ?
    If affect value,Would you please teach?


    Best regard.
    Bob Lee.

  • 0 in reply to Bob Lee62837
    TI__Guru**** 191445 points
    So far as I know, there is no internal delay. If there is, it is not documented.

    I do not have any characterization data over temperature. Just typical values as stated above.

    Current limit can affect start up time if the required current is above the current limit, SS will be delayed. If hiccup mode is entered, you may never start up.
  • 0 in reply to JohnTucker
    TI__Genius 11095 points

    Also, for most voltage mode controllers, the PWM ramp does not start at 0V, so there's some delay while the COMP pin is charged high enough to enable switching. I've attached a graphic from another TI datasheet which illustrates this. This can be on the order of a few hundred microseconds, and it's controlled by the compensation design

  • 0 in reply to Matt Schurmann
    Mastermind 7230 points

    Dear John Tucker,  Matt Schurmann,

    Thank you very much for reply.
    I will report your reply to my customer.

    Best regard

    Bob Lee