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TPS22810: CT capacitor

K.Hamamoto
Guru 12155 points
Part Number: TPS22810

Hi

The our customer is consider about CT capacitor.

How much capacitor is it possible? I would like to know the maximum value(limit).

Regards,

Koji Hamamoto

  • 0
    TI__Genius 15780 points

    Hi Koji,

    There is no strict maximum value for the CT capacitor, but instead this is dependent on the customer load. During turn on, the power dissipation across the device is very high since VIN >> VOUT when the FET is being turned on. If the inrush current is very high (due to the output capacitance) then the device may overheat if the rise time is too long because it is in a high power dissipation mode for a long time. However, the TPS22810 also has thermal shutdown, and this means that if the device overheats then it will shut itself off. If the device heats up too much during turn on, then it will cycle itself on and off until the load is fully powered.

    For more information on your specific use case, could you please provide additional information about your application? This includes:

    • VIN voltage
    • Output capacitance
    • Desired CT capacitance
    • Output current load
    • Ambient temperature range

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    Guru 12155 points
    Hi Alek,

    I am sorry for the delay in my reply.

    I talked with the customer about the maximum of CT capacitor. They understood but they asked us the additional question.

    What is the constant value(46.62) in the calculation formula (SR=46.62/CT)?
    And does the value has any variation? The customer would like to consider the variation of SR at a certain capacitor.

    Regards,
    Koji Hamamoto
  • 0 in reply to K.Hamamoto
    TI__Genius 15780 points

    Hi Koji,

    The constant value from the formula is derived using the data collected in table 2 of the datasheet. The calculated slew rate may vary due to many factors, and these are described in the application note below:

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    Guru 12155 points

    Hi Alek,

    Thank you for your prompt reply.

    I understood about the application note. In this case, the customer would like to use the large CT capacitor (up to 1uF).

    So, I think the board parasitic and traces around device does not affects so much as mentioned on the application note.  

    On the other hands, the rise time has some variation over temperature as shown at  figure 7.

    How much should we estimate the margin for the temperature if we consider the variation of rise time at over all temerature? (46.62 is typical value at 25degC, isn't it?)

    Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Genius 15780 points

    Hi Koji,

    What VIN voltage is being used? We took timing data across temperature for device characterization, and I can let you know how much the rise time varied to give you an idea of what to expect.

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    Guru 12155 points
    Hi Alek,

    Thank you for your support.

    The VIN is min=4.85, max =5.15V.

    Regards,
    Koji Hamamoto
  • 0 in reply to K.Hamamoto
    TI__Genius 15780 points

    Hi Koji,

    Please find below a table of data averaged from 5 units at VIN=5V. This should give an idea of how the device timing varies with temperature.

      -40C 25C 85C 105C Maximum deviation from typ (25C)
    470pF 72.4 67.9 56.8 59.1 16%
    1000pF 93.8 93.4 80.7 82.5 14%
    2200pF 199 197 170 168 15%
    4700pF 418 406 346 349 15%
    10000pF 931 863 745 735 15%
    27000pF 2290 2220 2020 2010 9%

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    Guru 12155 points

    Hi Alek,

    Thank you for you support.

    I appreciate that data. 

    I talked with the customer with the data. They would like to know how much the rise time will be at 68000pF.

    It will be fine if you have a data at CT=68000pF. However if you do not have a data, please give us how can we estimate the time will be at CT=68000pF.

    At the data what you give us above, the deviation of rise time will be smaller than small CT value if the CT is bigger.

    If the trend is linear,the rise time will be smaller than 9% at CT=68000pF. Is that correct?

    Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Genius 15780 points

    Hi Koji,

    The relationship between CT and rise time is pretty linear, so for an approximation, you can use the equation:

    tR (us) = 0.0814 x CT (pF)  + 28

    Using this equation, I was able to estimate 5563us for the rise time at CT = 68000pF.

    As for rise time variation, a higher CT will not always give better variation, so 15% would be a better estimate since it was found at several other CT values.

    Thanks,

    Alek Kaknevicius

  • 0 in reply to Aleksandras_Kaknevicius
    TI__Expert 4185 points
    Hi Alek,

    Do you have any calculated excel file for SR of TPS22810. A little bit hard to understand.
    Thank you in advance.

    Regards.