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LMR16006: What does SS mean for the input of the error amplifier?

Tristan Gao
Prodigy 60 points
Part Number: LMR16006

Tool/software:

Hi, friends

This is the block diagram of a BUCK switch converter LMR16006. I want to know what the SS means for the input of the error amplifier. I know the 0.765V is the reference of error amplifier, but what is the function of this SS input?

Thank you!

  • +1
    TI__Guru* 76135 points

    Hi Tristan,

    Thank you for posting.  The expert of this device is currently out of office.  I took a look of the datasheet and I think I can answer your question here. 

    In this device, there are two non-inverting inputs, one is fixed at 0.765V (typ) and the other is the SS voltage which starts to rise from 0V after enabling.  Between this two non-inverting inputs which lower takes over as the reference voltage for the error amplifier.  In other words, the FB's reference will first be the SS voltage therefore the Vout setting voltage is rising gradually as SS voltage increases, until it reaches 0.765V then the reference voltage becomes the fixed level so does the output setting voltage. In this way, the output voltage go through soft start.  

    Hope this clarifies.

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Prodigy 60 points

    Hi Youhao,

    Thank you very much for your answer.

    I now know the SS voltage is used for soft start, and I still get a further question.

    Since the soft start voltage increases from 0 to 0.765V, the turn-on time of PWM would be much short, and then the switcher would be quickly turned on and off. Does this process generate a lot of ringing or noise? In other words, what is the cost for soft start?

    Thank you again.

    Best regards,

    Tristan

  • +1 in reply to Tristan Gao
    TI__Guru* 76135 points

    The cost of soft start is a relative slower Vout rising time, but the advantages it brings in are more important because it prevents large inrush current and also the output overshoot during startup.  

    Thanks,

    Youhao