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TPS24710: Formula for on-time calculation

Guy Phillips62
Prodigy 20 points
Part Number: TPS24710

I am having a hard time correlating the formula for on-time with the example in the datasheet and the Excel calculator.   The values do not seem to add up.  When I use the values in the example, I get 34usec+2310usec-470usec=1.877msec.  The datasheet shows 0.614msec.  If I use the Excel spreadsheet tool, the calculation shows 1.1msec.  Can you clarify for me whether the formula for on-time is correct.  Is there an error in the datasheet and a different error in the calculator?  The formula in the spreadsheet calculator shows the formula as "=Start_up!L2", but I cannot see any spreadsheet called "Start_up".

I would also like to know what the first and third term in the ton formula represent.  I've figured out that the 2nd term represents the time it takes the output to ramp up to 12V given a constant input power.

  • 0
    TI__Guru 57737 points

    Hi Phillip,

    I will look into this and get back to you today.

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Philip,

    If the power limit setting value is less than product of Ilimit and Vin(max), the device will start up only in power limit and will not enter power limit. Then the turn on time can be calculated from the equation below.

    In case the power limit setting value is greater than product of Ilimit and Vin(max) then the device will startup in power limit and then transition to Current limit. The startup waveform will look like below.

    In this case, the turn on time depends the current limit and power limit values (as the device can start in power limit and then transit into current limit) and the below equation has been derived for startup time in power limit.

    Please ignore the last term in the equation in the datasheet. it is an error. The above equation is correct. For more understanding refer to section 2.2.1 of the below document.

    http://www.ti.com/lit/an/slva673a/slva673a.pdf

    Also, I have unlocked the Design calculator so that you can access all the pages in the design calculator sheet. You can find the attachment here:

    Copy of TPS24710_Design_Calculator_REV_E.xlsx

    The turn on time calculated as per design calculator and the equation above should be close. 

  • 0 in reply to Praveen GD
    Prodigy 20 points

    Hi Praveen,

    Thanks.  I created a model and compared the results with the spreadsheet calculator and the equation.  They all seem to be matching.  Thanks for your help. I have a couple of other questions on this part, but will ask under a new subject.

    ...Guy