• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 62 subscribers
  • Views 104 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

TPS54262-EP: Soft Start details / additional information

Christopher Liang
Prodigy 10 points
Part Number: TPS54262-EP

While I have generally had a good experience with this device and its datasheet, I am seeing some large inrush currents on turn-on (which I realize will also be influenced by capacitance on my design) as well as somewhat excessive output ringing. I was hoping to mitigate this by adjusting the soft-start capacitance and adjusting how the turn-on enable signal is handled.

1) The published documentation is not exactly the most verbose regarding details on the soft start function. All that is mentioned is that TI recommends a 100 nF soft-start cap for a certain start-up load current and there is no further discussion. The design examples at the end of the datasheet only say to use the 100 nF value and do not go into any detail. Can anyone provide more knowledge regarding the soft-start behavior for this device? Is there a limit to the maximum capacitance that can be applied to this pin?

2) I am interested in playing with some turn-on delay using a low pass network with the EN pin function -- are there any issues with a slow- rising signal on the EN pin? 

Thanks for any insights.

  • 0
    TI__Genius 13825 points

    Hi Christopher,

    The 100nF capacitor is a starting point listed in the datasheet for a typical load of 1A. You should start with 100nF recommended in the datasheet, and adjust the capacitance to meet your soft start requirements. Using soft start will help with the inrush current by slowing the Vout ramp-up to limit the amount of current needed to charge the output capacitance.

    Using an RC delay on the EN pin will delay the device from turning on after the signal has been applied, but once the EN voltage reaches the required threshold, the device will startup normally. An enable delay does not change the soft-start time and would not prevent inrush current for the device turning on.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 10 points

    Hi Ridge,

    Thank you for the response.

    Do you know if there is a maximum limit to the capacitance I apply to the soft-start pin? Is the relationship between capacitor value and soft-start time directly proportional? ie. if I double the soft start capacitance, does the soft-start time also double?

    Agree regarding the RC delay on the EN pin. I didn't do a good job explaining, but that is more for supply sequencing (multiple supplies used on the design) than for soft-start / inrush mitigation.

    Chris

  • 0 in reply to Christopher Liang
    TI__Genius 13825 points

    Hi Christopher,

    Generally, soft start is achieved by regulating the output voltage to the voltage over the SS capacitor as it ramps up (see Section 7.2 block diagram, pin 11). The datasheet states the soft start current (the current that charges the SS capacitor) to be 50uA typical. You can use the SS current, Vsense (the feedback voltage), and the capacitance to estimate the soft start time, but you will need to validate your soft start time experimentally and adjust from there. 

    For example: Css = (Iss * tss)/Vsense

    Again, this would just be for estimation. You should start with the datasheet recommended value of 100nF value and adjust the capacitance from there to experimentally validate your design. 

    Best regards,

    Ridge