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LM5170: SOFTSTART

ksma
Intellectual 290 points
Part Number: LM5170
Hello
How do I stop the soft start of the LM5170?
It is intended for dynamic switching of the number of operating phases.
In 4-phase operation, the # 2 SS capacitor was removed,
It seems that SS is not completely stopped from the attached waveform.
Pulling up to 10V at 100K had no effect.
Is the above method correct?

SW starts immediately when EN starts, but stops immediately.
After that, it resumed, but SS seems to be working.
What is the cause of the stop?

According to DS, EN2 does not work with soft start, but what should I do if I use more than one?
EN1 / 2 is integrated for circuit reasons, but will be separated in the future.

CH1:Vds(270deg)　CH2:Vo　CH3:Vds(0deg) CH4:#2_EN

  • 0
    TI__Guru* 78340 points

    Hi Ksma,

    Thank you for posting. If the SS pin has no capacitor attached, then SS will not control the switching.  What you observed is caused by two things:

    (1) The COMP voltage of your 270deg phase.  The COMP is grounded when the channel is disabled (EN=0).  The comp needs rising up to produce PWM. 

    (2) When the 270deg phase starts, its inductor current take time to rise from 0 to the steady state. 

    I also see your test condition under light load, because even the 0deg phase is in DCM.  If you run with higher load, the 270deg phase will start up faster because the higher ISETA will cause COMP rise faster.

    Hope this explains. 

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Intellectual 290 points

    Hello Youhao,

    Thank you for a quick and clear answer.
    By the way, the following email arrived, what does it mean?
    Best regard.

    ksma

    >I think one of my colleague already has the proposal in drawing. 
    >Let me pass this to him and he can share it with you.
    >Thanks,
    >Youhao

  • 0 in reply to ksma
    TI__Guru* 78340 points

    Sorry for the confusion.  I was replying to a different post but mistakenly wrote in this thread.  I had put a cancellation after I realized the mistake, but it went out to anyway before the cancellation actually executed. 

    In short it is irrelevant to your post.  Sorry for that.