• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 456 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LP5922: NR/SS pin internal cap and Soft-start time definition

Toshihiro Watanabe
Genius 10235 points
Part Number: LP5922

Hello,

1) It seems Cf capacitor is connected to NR/SS pin from block diagram of datasheet. When no cap connected at NR/SS pin, how much the capacitor it is? I think the cap makes soft start time.

2) How does start up time be defined? e.g. 10%-90% period.

Best regards,

Toshihiro Watanbae

  • 0
    TI__Guru 55940 points

    Hi Toshihiro,

    You are correct that adding an external capacitor to the NR/SS pin will adjust the soft start time as depicted in the following equation from section 7.3.4 Programmable Soft Start and Noise Reduction 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 (Vfb).  For LP5922 the constant current is from Vss/nr = 0 to Vss/nr = 98% of Vfb.  As such, the soft start time (start up time) is for Vout = 0 to Vout = 98% of its regulated voltage.

    We do not have an internal Cnr/ss; therefore, any capacitance on the pin without an external capacitor will be small parasitic capacitance.

    Very Respectfully,

    Ryan