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TPS7A05: Fluctuation of Start-up time

user6142633
Intellectual 470 points
Part Number: TPS7A05

Hi,

I use TPS7A05 series following conditions.

TPS7A0518 : Vin : 3.2 V

                      Vout : 1.8V

                      Cin&Cout : 1 uF ± 20%

                      EN is separate with Vin

                      Tj : -10 ~ 70 ℃

TPS7A0508 : Vin : 1.8V

                      Vout : 0.8V

                      Cin&Cout : 1 uF ± 20%

                      EN = Vin

                      Tj : -10 ~ 70 ℃

Let me know about "Start-up time" in these conditions.

①Minimum value of "Start-up time"

②As figure 35, When difining Tstr = Tdelay + Trise, which region fluctuate when "Start-up time" fluctuate ? "Tdelay" or "Trise" or Both ?

if "Tdelay" fluctuate, I have no problems.

if "Trise" fluctuate, I must satisfy "Slew rate of Vout"(red dot line in Figure 35) < 0.018 V /us.

Regards,

  • 0
    TI__Guru 50060 points

    Hi ,

    We don't test for the minimum startup time in our production test which is why we do not state a minimum in the datasheet. That being said, I look at our test data and The minimum time seen is EN assertion to VOUT=95%*1.8V is 1.17ms. For 0.8V, the fastest start-up time we saw was 1.33ms.

    What is the application and what is the concern over the minimum start-up time? The reason we show the maximum is because, after this time, we guarantee that the device is enabled. The minimum time shows the fastest it could be enabled.

    I hope this answers your question.

    Regards,

  • 0 in reply to JCHK
    Intellectual 470 points

    Hi, John.

    Thank you for quick response.

    I concern about "Slew rate of Vout".

    Because of requests of load, Vout must rise slowly( slew rate < 0.018 V/us ).

    As figure 35, When defining Tstr = Tdelay + Trise, which region fluctuate? "Tdelay" or "Trise" or Both ?

    If fluctuation of Tstr means that of Tdelay, I have no problems in terms of "Slew rate of Vout".

    However, if Trise fluctuate, I must consider about it.

    Regards,

  • 0 in reply to user6142633
    TI__Guru 50060 points

    HI Yoshida-san,

    To control the slew rate you need an LDO with a Soft-Start Function. Does the second LDO need to be a really low IQ LDO? If Not, something like the TPS7A90 would help to control the slew rate.

    Let me know what you think.


    Regards,