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Missing pulses in UCC28950 burst nmode

Francesc Casanellas
Intellectual 375 points
Other Parts Discussed in Thread: UCC28950, UCC27324

In burst mode sometimes I see some missing pulses in OUTA, while OUTB is correct. I mean these are not missing pulses caused by the burst mode, but missing pulses within a pulse train. Tmin = 15K. Rsum= 130K to GND, RDELAB = RDELCD=15K, RT = 61K9.

All imputs seem right and out of burst mode all pulses are rigth too.

What may be the cause?

  • 0
    Intellectual 375 points

    I have seen that the missing pulses are caused by the input of  the driver UCC27324 short circuiting the ouput of the UCC28950(! ).

    If I put a 510 ohm resistor between the UCC28950 output and the driver input, both seem to work well. And as I said, out of the burst mode all is OK.

    The ground path between both IC's is very short.

    Any ideas?

  • 0 in reply to Francesc Casanellas
    Intellectual 375 points

    More about this:

    If I put a Schottky diode very  between inputs and +supply of UCC27324 (very close to the IC), the problem is solved.

    So I think that the problem is that if the UCC27324 latches and its iputs go shorted, if one of its inputs receives a glitch higher than 0,3V the supply.

    The  positive glitch may come from the inductance of the tracks.

  • 0 in reply to Francesc Casanellas
    TI__Mastermind 31580 points
    Hi Francesc,

    I will ask one of driver experts to look at this issue. It is likely that they will need more details

    Regards

    Peter
  • 0
    TI__Intellectual 2230 points
    Hi Francesc,

    Could you put the input(1V/div) and VDD(2V/div) signals of UCC27324 together with the output(5V/div), and zoom the issue waveforms into one switching cycle, also the schematic will help us analysis, thanks!

    Best Regard,
    Oliver
  • 0 in reply to Oliver Pan91
    Intellectual 375 points

    Enclosed are the oscillograms showing a right pulse and the following missing. There are two oscillograms as my scope has only 2 channels. I enclose also the schematic of the power part.

    In the waveforms the positive glitch is clearly visible and I suppose that this is what causes de driver to latch up. A schottky diode to VDD removes the spike and solves the problem, as well as a resistor in series.

    The length of OUTA and OUTB tracks are 30 mm and the OUTC and OUTD 20 mm. Both tracks run close and over a ground plane. The driver associated to OUTsCD runs right in several boards tested, so I suppose that inductance in the cause of the spike.