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Original question:

LM5157-Q1: LM5157-Q1

LM5157-Q1: Output flicker

StefanS
Prodigy 70 points
Part Number: LM5157-Q1

Hello,

I have problem with flickering output bellow  for input voltages bellow 14V.
Design should be operated 12-14V, and deliver 32V 1.8A for LED array.

As more as you go bellow 14V (13.5-13-12.5-12) it is just get more frequent flicker like strobe light (it is visible ~1/10sec).
(just to mention, there is no flicker when output current is dropped to 1.5A)

This is very wide time base to see flicker effect.

Besides flicker there is clean and steady PWM  at SW pin 1.1MHz(like on simulation)

And design schematic:

____

best regards,

Stefan

  • 0
    Prodigy 70 points

    This looks like thermal junction limit behavior, after cooling hysteresis of 15C, IC again kicks in, and so on...

  • 0
    TI__Guru* 76965 points

    Hi Stefan,

    Could you show a few zoomed in cycles at the border of beginning of the switching and stop switching?  

    Also, in the overview, could you monitor COMP, FB, SS and SW on the same cope?

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Guru* 76965 points

    Sorry I saw your first post but did not notice there was a second post.  

    If it triggers thermal shutdown, you may need to improve your PCB to dissipate the heat, and you may consider to add a heatsink.

    Thanks,

    Youhao

  • +1 in reply to Youhao Xi
    Prodigy 70 points

    I've gonne through all signals capturing as you requested, but it turns out that this paste and little heatsing ~6mm , done the job. It is working now, on 12V with no flicker... It was sure caused by overtemp. Now is ~90C and iths great. Thanks for support.
    Bottom line is , integrated fet inside is not so ideal, DS says 100mOhm max, in other hand there are discrete part with RDS in few mOhm... and there goes all heat in it
    BR