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TPS62745: soft start

Andy_Lin
Intellectual 2610 points
Part Number: TPS62745

Hi Team,

my customer is using TPS62745 and have 2 questions of soft start,

1. Is there's data of min and max of Tsoftstart?

2. customer is measured that Vout have overshoot while enable the device , since this device's soft start is fixed ,how to adjust Vout overshoot when under this condition? (customer spec Vin=5V ,Vout=3.3V , Iout=0.3A(max), Cout=10uF*2)

below is d/s reference

  • 0
    TI__Guru**** 191445 points

    Andy,

    The (not guaranteed) minimum and maximum SS time is 523 usec / 932 usec.  I don't really see any overshoot in that waveform.  It is just from the datasheet.

  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points

    Andy,

    Looking at the control topology for TPS62745, it is likely to have a small amount of overshoot if the load current is less than the reduced current limit during SS time.  During SS the part ramps up with the output current equal to the (reduced) current limit.  At the end of the SS time, it takes a few switching cycles for the   switch current to reduce down to the required load current and the circuit to reach the steady state operating point.  That excess current during this time will result in a very slight overshoot on the output voltage. I ran some simulations today foe a similar device and that is what I observed.

  • 0 in reply to JohnTucker
    TI__Intellectual 2610 points

    Hi John , 

    Nice to reach out again ,since the overshoot it's not shown every time when customer measured and seems the overshoot value is within their spec, so they didn't provide the waveform for me , but I'll check with customer how much load current is set when measurement, maybe load current is less than the reduced current limit during SS time.

    and I think this is little typo , but I assume , what you mean's is that at the end of SS , it take a few switching cycle for current to reduce (not reduce current lime value) , and the circuit (Vout) to reach the steady state operating point.  is that right?

    I ran a simple WEBENCH of start up simulation  , the condition is as customer spec (5Vin, 3.3Vout , I=0.3A)

    and found the same result as you says . is this can fixed by adjust design?

    Andy 

  • 0 in reply to Andy_Lin
    TI__Guru**** 191445 points

    Andy,

    I went back and corrected my typos.  Looking at your Webench sim, that is something different altogether.  Can you share your Webench design so i can look at it?  There may be something wrong with it.  In the mean time, I will run a simulation in pspice today.  I am reasonably certain that pspice will reveal the exact phenomenon I am expecting.

  • 0 in reply to JohnTucker
    TI__Intellectual 2610 points

    Hi John ,

    I simply type the customer spec in WEBENCH and run the start up simulation then get above result, I didn't change any parameter so it would be easy to duplicate from your side.

    do let me know the pspice simulation result and kindly explain to me what should be the normal start up operation result per your means , thanks!

    Andy

  • 0 in reply to Andy_Lin
    TI__Guru**** 191445 points

    Andy,

    I do not know why Webench behaves that way (it is Webench after all...).  The Psipce simulation I ran does not have that transient effect.  I Think you should post a separate topic to the Webench forum and ask them to explain it.

  • 0 in reply to JohnTucker
    TI__Intellectual 2610 points

    Hi John,

    got it , thanks for response.

    since customer say's that overshoot is within their system spec and it's occasionally , so we close this issue at this moment.

    Andy