• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 448 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.

LM20146: Setting the Start up Time (CSS)

Dice-K
Mastermind 8108 points

Part Number: LM20146

Hello, support team.

 

Please let me know the maximum capacity that can be used for CSS.

 

The maximum is 120nF in the page-18 of SNVS563C.

 

Regards,

Dice-K

  • 0
    TI__Guru**** 191445 points

    Dice,

    During slow start, the output capacitors are charged at a linear rate, so the current is constant and equal to Cout * Vout / tSS.  That current plus the load current should be less than the minimum current limit of 7.35 A for reliable start up.  That defines the minimum SS time.  There is nothing specific to limit the maximum time.  Typically the SS time will be specified as 1 to 10 msec.  The datasheet table includes 15 and 20 msec as well, which are not unreasonable.  You could extend the SS time a little more if needed.  There is no specific limit, but as stated in the datasheet, the SS capacitor is discharged under fault conditions or when EN is toggled.  If the Css capacitor is large, there may not be enough time to discharge it if EN is pulsed rapidly.  What SS time do you require?

  • 0 in reply to JohnTucker
    Mastermind 8108 points
    Hi John,
    Thank you for your support.

    What I want to know is to slow down the rise of less than 200mV.
    However, looking at the graph below, it seems that the rise of less than 200mV does not become slow even if Css is increased.

    SNVS563C Page-8 Figure 21. Start-Up (Soft-Start)

    Please let me know if there is a way to slow down the rise of less than 200mV.

    Best regards,
    Dice-K
  • 0 in reply to Dice-K
    TI__Guru**** 191445 points

    Dice-K,

    The internal default is when no capacitor is used. Let me check if that is true for small SS capacitor as well. It may take a few days.

  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points
    Dice-K,

    I checked this in the lab. You cannot decrease the SS time faster than the default.