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TPS7A4901 output capacitor question

Genius 3870 points
XY

Replies: 4

Views: 465

Hi there!

we are using TPS7A4901 in this condition : Vin=14.4V~19V, Vout=12V, Iout=80mA.  Css=CFF=10nF.

On page 17 of datasheet, it says Cout has to be larger than 2.2uF and Cout(max) should also be limited by equation(7) and (8). I am confused:

Why tss(CL) is 140us and Vout =2V in equation(8)?  I thought Vout should be 1.2V as page 16 wrote...

And in my application, Cout(max) = 140us*500mA/12V = 5.83uF. This seems to confilcts with the requirement that "Cout could use 10uF for better PSRR"?

So how should I choose Cout?

Thanks!

Genius 3870 points
XY
  • Expert 4160 points
    Mark Sellers
    Hi Yi,

    The 140us comes from equation 6 where the 14ms was calculated. If you divide that by 100 you get the 140us that is being referenced in equations 7 and 8. I believe that you are correct and the Vout should be 1.2V in that equation as well. However, this section of the data sheet is only talking about a max value of capacitor that will keep the part from going into current limit during startup. You should be able to use a 10uF Cout without a problem, there have been tests done on this part with a 100uF output cap. As can be seen in Figure 14 the 10uF cap does have better PSRR in general, so it would be recommended for increased PSRR performance.

    Hope this helps.

    Best Regards,
    Mark
  • Genius 3870 points
    XY

    In reply to Mark Sellers:

    Thanks, Mark.
    I understand the 140us comes from 14ms/100. but I do know why should tss be divided by 100. I thought the equation(7)(8) may use tss directly?
    Could you please help explain?
  • Expert 4160 points
    Mark Sellers

    In reply to XY:

    Hi Yi,

    You are not able to use tss directly in equations 7 and 8 because these equations are used to find the output cap to keep it from going into current limit. This is why the time is selected to be 2 decades before the tss to ensure that the part does not enter current limit during start up.

    Best Regards,
    Mark
  • Genius 3870 points
    XY

    In reply to Mark Sellers:

    Hi Mark,

    datasheet section 8.3.1 says "For reliable operation, the device does not operate in current limit for extended periods of time", is this "time" specified as less than  two decades below tss(1/100 of tss) ?

    What will happen if current limit is reached but other condition is good for Table1? will the IC shut down?

    Thanks!

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