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TPS7A8300: NR/SS pin internal cap and Soft-start time definition

Part Number: TPS7A8300
Other Parts Discussed in Thread: TPS7A83A

Hello,

1) I think TPS7A8300 doesn't have internal capacitor at NR/SS pin. If no cap at NR/SS pin, TPS7A8300 works without soft-start control. Is my understanding correct?

2) If no internal cap is there, I guess stray cap is existing at NR/SS pin. How much is it approximately. I guess a couple of pF.

3) When Cap is connected at NR/SS pin, how does start up time be defined? e.g. 10%-90% period.

Best regards,

Toshihiro Watanabe

  • Hi Toshihiro,

    We do not have an internal Cnr/ss; therefore, any capacitance on the pin without an external capacitor will be small parasitic capacitance.  If no external capacitor is added to the NR/SS pin, there will be no soft-start.

    An external capacitor to the NR/SS pin will adjust the soft start time as depicted in the following equation from section 8.1.1.2 Noise Reduction and Soft-Start Capacitor (Cnr/ss) of the datasheet:

    This equation is simply an equation for the time it takes a constant current to charge a capacitor from 0 V to a known voltage (Vref).  The soft start time (start up time) is the time from Vout = 0 V to when Vout reaches its regulated voltage.

    I would also like to make you aware of TPS7A83A which is the next generation of TPS7A8300.  TPS7A83A is pin to pin compatible and a bit more user friendly as it does not have any sequencing requirements.

    Very Respectfully,

    Ryan

  • Hello,
    There is a start-up waveform when CNR/SS=0nF in Figure 47. It seems to take ~20us turn on time it should be faster from calculating value of CNR/SS.
    I guess this time is due to charge the output capacitor. Is my understanding correct?
    If yes, how do we calculate this time from Vout and Cout? I guess CxV/I is time. Is this I =2A good fit for this?
    Best regards,
    Toshihiro Watanabe
  • Hi Toshihiro,

    The soft-start time equation mentioned previously is valid for noise Cnr/ss values within our recommended range of 10 nF to 1 uF. A 10 nF capacitor yields a startup time of 1.29 ms nominally. The 20 us is considered negligible at this point. Keep in mind that the tolerance of the external capacitor can vary the softstart time by a larger margin.

    Vout will track Vref during startup unless enough output capacitance is placed on the LDO output and/or a high enough output current to cause current limit to engage. In the case of Figure 47 we are showing that the internal reference takes 20 us to reach its final value. This is independent of load current.

    Very Respectfully,
    Ryan
  • Hello Ryan-san,
    Thank you for your answer. Do you think figure 44 rise time is faster if output capacitance is lower than 22uF?
    Best regards,
    Toshihiro Watanabe
  • Hi Toshihiro,

    TPS7A8300 is designed, characterized, and tested for a minimum output capacitor of 22 uF. Reducing the output capacitor to less than 22 uF will take you outside of datasheet limits.

    Assuming that you could achieve stability with a lower capacitor on the output, we would not expect the internal reference to startup any faster than depicted in Figure 47.

    Very Respectfully,
    Ryan