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TPS54320: Switch Node Rising and Falling Edge Behavior

Samuel Jones21
Prodigy 50 points
Part Number: TPS54320

While doing testing on a couple of different rails, all using a TPS54320 with 12V  supply voltage. Below is an image from one that exhibits behavior I would expect.

However, some of the nodes look like this. This is the worst one as it occurs on both the rising and the falling edge of the switch node. It seems like during the expected deadtime there is instead some ramping on the bus before the high-side or low-side MOSFET switches on.

I have been doing over-current/short circuit behavior testing. The ICs go into hiccup mode when shorted and so I wouldn't expect them to be damaged but am wondering if that is a possibility. 

I have also looked at the Gain/Phase plots and the systems have plenty of phase and gain margin (above 60 degrees and at or below -10dB in each system)

Any ideas on what would cause this behavior? Are my ICs damaged?

  • 0
    TI__Mastermind 35165 points

    Hi Samuel,

    What is the load current for the waveforms in the second plot? This shape of waveform can occur if the inductor current peak or valley is near 0 A when the FETs turn off. The near 0 A current is not enough to turn on the body diodes of the FETs.

    If the valley current is near 0A when the LS FET turns off, there is not enough positive current to pull down the PH node and force the LS FET body diode to turn on.

    If the peak current is near 0A when the HS FET turns off, there again is not enough positive current to quickly pull down the PH node and force the LS FET body diode to turn on.

    If there happens to be an RC snubber on the PH node this could also contribute to making this type of waveform more pronounced. The snubber adds capacitance to the PH node which slows down the rate the PH node falls.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 50 points

    Thanks Anthony

    This was definitely it. 

    While in both images my load box is off, the board on the second image has some other ICs removed that are pulling some quiescent current. When I set up my load box to pull the typical current the supply was designed for, this behavior went away.

    Sam