• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 64 subscribers
  • Views 150 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.

TPS54678: Slow start calculation

Onur Kusakoglu
Intellectual 960 points
Part Number: TPS54678

Good morning, 

I am trying to understand the soft start timing formula in the datasheet on page-14.

If I understand it correctly, It says 2.2uA charges the SS from 0.15V to about 0.8V for the handoff of the SS voltage to reference voltage. 

given Formulla-> Css( nF) = 3 x Tss(mS). 

For example If I have a 3N3 %10 capacitor connected to SS pin, then I expect to see 1.1ms with a %10 tolerance as a soft start time. How shall I correlate this to output voltage rise time?

I assume this formula is a rough formula and I need to take into account the tolerances of reference voltage and tolerances of Iss which charges the external slow start capacitor to calculate the soft start time?

Thanks in advance for your help,

Best regards

Onur

  • 0
    TI__Expert 5425 points

    Helllo Onur,

    the voltage at the softstart capacitor is used as a voltage reference for the output voltage. So the output voltage will follow the capacitor voltage increasing. So the output voltage will increase from 0V to nominal output voltage within the time you use for calculating the capacitance for the softstart capacitor. As you figured out already the parameters related to this are not controlled and specified very accurate for this device. This is usually not needed. What are your softstart timing requirements?

    Best regards,

    Juergen

  • 0 in reply to Juergen
    Intellectual 960 points

    Hi Juergen,

    Actually I met with all of my rise time requirements but I am a bit off my calculated rise time and measured rise time.  And yes I can't calculate an expected accurate range with the information in the datasheet. I was measuring the rise time of the output voltage from 10 to 90% but I suppose this needs to be measured from 0 to 100%(nominal output).

    Thanks for your help

    Best regards

    Onur