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LM3481 soft start function

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: LM3481
Hello,

Please see the attached file as start up waveform of LM3481.
6431.2046.20140930_LM3481 start up waveform.pdf

In the attached file soft start time is varied when we change the input voltage.
Could you please let me know this reason?
Also , please let me know the culculation of  tss(soft start time).

Best Regards,
Ryuji Asaka
  • 0
    Guru 20090 points
    Hello,
     
    I have additional question.
    In attached file page 4, Vout was increased as step by step at start-up.
    I thought that this cause is current limit.
    However Isen peak voltage is about 70mV.

    Is this waveform correct ?
     
    Best Regards,
    Ryuji Asaka
  • 0
    TI__Intellectual 890 points

    Hi, I'll have a look at this and get back to you.

    regards

    john

  • 0
    TI__Intellectual 890 points

    Hi, In your 5VIN plot it looks like there is a 4 or 5ms delay from VIN to  VOUT starting  to come  up, this delay is less than 2ms when VIN is at 8V, If you refer to  UVLO  section on page 13 of the datasheet, R7 and R8 sets up the UVLO rising and enable values, for example if UVLO is ~4V and VIN is 5V, this may account for this inital delay before VIN starts to rise.

    Also looks like the start up time is internally fixed at 15ms.

    regards

    John

  • 0 in reply to John Bowie
    Guru 20090 points
    Hello John san,

    Thank you for the reply.I understood.
    I will check the UVLO pin voltage.

    Also , I have additional question about start up.
    Please see the attached image.
    In attached image, VSW pulse was skipped at start up.
    As a result , Vout is increased as step by step.
    Is switching pulse skipped at start up due to softstart function?
     
    Best Regards,
    Ryuji Asaka
  • 0 in reply to Ryuji
    TI__Intellectual 890 points

    Hi Asaka san

    Yes this is expected behaviour, the soft start feature allows the output to gradually reach staedy-state operation, thus reducing inrush current.The internal soft start brings the output up in about 15ms.

    regards

    John