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TPS61085: How to calculate Soft Start time

Nariaki Ogasawara
Intellectual 1215 points
Part Number: TPS61085

Hi Team,


 I think Soft Start time is calculated by below equation.
 Tss = Vss x Css / Iss

 In case of Css is 100nF, Tss is 12ms.
 Vss = 1.238V, Iss = 10uA,  Css = 100nF
 Tss = 1.238 x 100e-9 / 10e-6 = 12.38ms

 The soft start time at Figure 13 Soft Start in datasheet seems 2ms.
 Is the calculation wrong?
 How to calculate the Soft Start time?

Regards,
Ogasawara 


 

  • 0
    TI__Mastermind 40080 points

    Dear Sir，

    From section 7.3.1, we can that "the SS voltage directly controls the peak inductor current" ,  and the peak inductor current doesn't has a linear relationship with the VFB.

    There's no formula to calculate Tss for this part since it controls the inductor current. So different Vin , Vout, and load current will have different soft startup time.

  • 0 in reply to Helen chen
    TI__Intellectual 1215 points

    Hi Helen,

     Thank you for your response.
     I have some questions.
    1. Does the SS voltage control the inductor current? Is the SS voltage unrelated VFB?
    2. When the customer set the target SS time,  he can confirm by Webench? How does accurate the Webench result?   
    3. How much does the rise time variation at same condition?

    Regard,
    Ogasawara

  • 0 in reply to Nariaki Ogasawara
    TI__Mastermind 40080 points

    Hi Ogasawara-san,

    1. Yes the SS voltage controls the inductor current, it is unrelated to the VFB.

    2. Webench gives out an typ value. But we need to take the tolerance into account.

    3. for the rise time, we need to take the Cout tolerance into account. it is around +-30%, also we need to take the inductance  and charge current tolerance into account. Considering the all the tolerance, suggest take at least +- 60-70% tolerance into account.