• State Resolved
  • Locked Locked
  • Replies 9 replies
  • Answers 5 answers
  • Subscribers 66 subscribers
  • Views 886 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS22975: TPS22975 Rising time calculate request

kerr chang
Genius 4075 points
Part Number: TPS22975

Hi Team

       I want adjustable rise time. TPS22975 Datasheet P.15  Vout rising time formula  is Vbias =5V.

         SR = 0.43 * Ct +  26 

But my Vbias = 3.3V.

         How did I calculate the slew rate ?  same Calculation formula ? 

  • 0
    TI__Genius 10855 points
    Hi Kerr,

    Thanks for reaching out on E2E!

    I will need to look at our data to provide you with an equation when Vbias is 3.3V. The equation that we have in the datasheet provides a general estimation of the rise time. With Vbias at 3.3V, I expect the rise times to be slightly faster than the equation in the datasheet.

    Thanks,
    Arthur Huang
  • 0
    TI__Genius 15760 points

    Hi Kerr Chang,

    For the TPS22975, we did not take the data for VBIAS=3.3V, but as an estimate I used VBIAS=5V and VBIAS=2.5V data to come up with the following equations for VBIAS=3.3V:

    VIN=3.3V: tR(us) = 2.14 x CT(pF) + 136

    VIN=2.5V: tR(us) = 1.65 x CT(pF) + 110

    VIN=1.8V: tR(us) = 1.23 x CT(pF) + 88

    VIN=1.05V: tR(us) = 0.773 x CT(pF) + 63.9

    Again, please note that these are good for estimations, but process and temperature variation may cause some deviations from the equations.

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    Prodigy 40 points
    Hi Alek,
    Please also help to provide the time calculation method of Ton when Vin=Vbias=Von=3.3V, thank you very much.
  • 0 in reply to joey lee
    TI__Intellectual 1770 points
    Hi Alek,

    May I know what's the tolerance of those equation?

    Thanks
    Shaq.
  • 0 in reply to Shaquille Chen
    TI__Genius 10855 points

    Hi Joey and Shaq,

    Since TON doesn't affect the timing characteristics of our load switches, you can use the equation that Alek provided above to calculate the rise time: VIN=3.3V: tR(us) = 2.14 x CT(pF) + 136

    Regarding the tolerance to these equations, what type of accuracy are you expecting (ex:25%, 50%)? Does this rise time need to be accurate across temperature?

    I just looked up some lab data we took across 5 devices, and it looks like for the temperature range of -40C to 105C we see a maximum deviation of 10%. However, we cannot guarantee that the deviation won't be larger since there are many things which can affect rise time (device variation, temperature, CT capacitor variation, board layout).

    Thanks,

    Arthur Huang

    To find the latest information on Power Switches, visit www.ti.com/powerswitch

    Please click "This resolved my issue" button if this post answers your question.

  • 0 in reply to Alpha H
    TI__Intellectual 1770 points
    Hi Arthur,

    What's the propagation delay at turn on and turn off?
    Customer want to calculate it as accurate as possible.

    Thanks
    Shaq
  • 0 in reply to Shaquille Chen
    TI__Genius 10855 points

    Hi Shaq,

    We normally don't spec tdelay for each VBIAS / VIN condition, but we were able to pull some information from our characterization data. Please find a chart below that shows the tdelay time across CT:

     CT VIN = 3.3V
    0pF 75
    220pF 215
    470pF 334
    1000pF 552
    2200pF 1088
    4700pF 2048
    10000pF 4531

    Using this data, the following equation can be used to extrapolate the tD time (for CT > 0pF):

    VIN=3.3V: tD = 0.438 x CT(pF) + 104.4

    Thanks,

    Arthur Huang

  • 0 in reply to Alpha H
    TI__Intellectual 1770 points

    Hi Arthur,

    Customer test the delay time on their PCB.

    Result as below:

    According to the tD equations .

    VIN=3.3V: tD = 0.438 x CT(pF) + 104.4 = 0.438 x 3300pF + 104.4 = 1549.8us.= 1.549ms.

    Consider the variation of capacitor (+-10%) / High Temperature(-12%) / equation  (+-10%).

    The worst case calculation is 1.863ms ( max )

    But the actual measurement tD is 2.48ms.

    Customer want to know why this variation is so high.

    Thanks

    Shaq.

  • 0 in reply to Shaquille Chen
    TI__Genius 10855 points

    Hi Shaq,

    Thanks for providing additional details. The delay time of the load switch can also be affected by the load resistance and the load capacitance. If you have a larger output resistance, it will require more gate drive until the output voltage starts to rise. For reference, you can refer to section 2.2 and 2.3 of our app note, Timing of Load Switches, for additional details.

    Have you been able to try a similar configuration on the EVM? What tdelay times are you seeing there?

    Thanks,

    Arthur Huang