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TPS3808G33 calculation of delay time min/max

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: TPS3808
Hello,
 
In the TPS3808 datasheet page9, Delay(s)=Ct(nF)/175 + 0.5 x 10^-3(s)
If I use 10nF as Ct, Delay time = 57.64ms.
I think that 57.64ms is typical value.
I would like to caluculate min/max value of the delay time.
 
(1)
Please let me know the accuracy of "175" and "0.5 x 10^-3(s)"

(2)
Please let me knot the background of "175" and "0.5 x 10^-3(s)"  

<Use Conditions>
 Vdd=4.75~5.25V
 Pullup resister=10kΩ
 CT=10nF
 Temp range=-40~85(degree)
 Setting = Figure 14(c)
 
Best Regards,
Ryuji Asaka
  • 0
    TI__Guru 55940 points

    Hi Ryuji,

    You can get a good sense for the accuracy by looking at the td spec in the EC Table on page 3.  Here we give typical, min and max for multiple Ct values.  Please note that the accuracy will be dominated by the accuracy of the capacitor selected.

    The 175 comes from the voltage/current of the capacitor when it is charged.  The 0.5us comes from the internal delay required to detect that an external capacitor is present.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Guru 20090 points
    Hello,
     
    Thank you for the reply.
    In the datasheet,  EC table on page 3.
    td spec accuracy is about +/-29%.
    Is this accuracy include capacitor tolerance ?
    Should we add capacitor tolerance to +/-29% ?
     
    If yes, is calculation method as below ?
     
    ---
    Delay(s)(typA ) =Ct(nF)(max)/175 + 0.5 x 10^-3(s)
    Delay(s)(typB ) =Ct(nF)(min)/175 + 0.5 x 10^-3(s)
     
    Delay(s)(typA) x (1+29%) = Delay(s)(max)
    Delay(s)(typB) x (1-29%) = Delay(s)(min)
    ---
     
    Best Regards,
    Ryuji Asaka
     
     
     
     
  • 0 in reply to Ryuji
    TI__Guru 55940 points

    Hi Ryuji,

    Typically we do not include external components such as capacitors and resistors in our accuracy specs.  This is the case for TPS3808.  Your calculation method looks correct.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Guru 20090 points
    Hello Ryan san,
     
    Thank you for the checking.
    I checked again td calculation by EC table CT.
    In this result , td=1.071ms at the Ct=100pF
    and td=1.029s at the Ct=180nF.
    this is not typical value of EC table.
     
    I think that , my calculation was incorrect....
     
    ----
    (1)
    Could you please kindly let me know the correct calculation?
     
    (2)
    Our customer use Ct=10nF.
    So, please let me know min/max value of td at Ct=10nF at least....
    ----
     
    Best Regards,
    Ryuji Asaka
     
     
     
     
     
  • 0 in reply to Ryuji
    TI__Guru 55940 points

    Hi Ryuji san,

    Your calculation is correct.  The equation that you are using is accurate and came from the designer of the IC.  Please use the equation to calculate the delay time min/max.  Measured typical values are determined by characterization of a sample.  Depending on the sample, a typical value may shift slightly; however, this will not affect the min/max spec.  The equation is the best way to calculate the delay time min/max you can expect.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Guru 20090 points
    Hello Ryan san,
     
    Thank you for the checking again.
    I understood.
     
    Best Regards,
    Ryuji Asaka