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TPS54360: Short circuit condition between Vin, SW & GND

Part Number: TPS54360

Hello,

I've been using this IC to drop a 42V rail down to 12V to power an array of high power LEDs. The load current is no greater than an amp, although this is pretty choppy (switching approx 300-500mA at a time). Switching frequency is approx 640kHz.

Over the past months, I've seen almost 10% of these components have their SW and Vin pins short to GND. Replacing the IC clears the issue, but I worry that it's only a matter of time until it happens again. 

There is a 100nF capacitor between the Boot and SW pins (as per the datasheet), but I notice that the voltage across this sometimes violates the absolute maximum value in the datasheet (sometimes spiking to almost 16V for <10ns). What could cause this? Also, where does the 8V abs max value come from? Is this a silicone restriction (FET Vgs max etc), or something else?

I'm *not* seeing a great deal of overshoot on Vin on power-up and, besides this Boot-SW voltage spike, the part seems to be working reasonably well until chip failure.

I notice that there are a few threads of this nature surrounding this part, and wonder if anyone's been able to resolve this issue?


Thanks in advance,

Nick

  • Hello

    Please share your schematic and views of your PCB layout.

    Also, can you share 'scope waveforms of the SW node and BOOT node.

    Thanks

  • Good Morning Frank.

    I've attached a schematic and scope trace.

    On the scope, trace 1 (yellow) relates to the BOOT pin, and trace 2 (green) relates to the SW pin. Probes were attached directly to the pins of the IC.

    I look forward to hearing back!

    Thank you,

    Nick

  • Hi Nick,

    Could you confirm which TPS54360 you are using (There is a B version)?  The reason I'm asking is that the regular version is basically in NRND status, and the B version is a drop in replacement.

    https://www.ti.com/lit/an/slvaea1/slvaea1.pdf?ts=1646696266426&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPS54360

    A couple comments on your design:

    • Decoupling capacitor return path to ground is not great (it's going to ground through a via).

    • If you really want to limit the voltage spike on the Bootstrap capacitor, then you could add a zener diode in parallel (this will clamp the voltage down to the maximum rating of 8V).
    • This portion of the wave form does not look typical:

    • What is the purple waveform?

    Thanks,

    Andrew

  • Hi Andrew,

    We are using the original TPS54360 (ie; not the "B"), but we will be using the B-model for our next units, provided we can stop these failures. 

    The grounded side of C718 (decoupling capacitor) is connected directly to the ground plane - albeit through thermal relief spokes. I don't quite follow your annotation - my apologies, could you please elaborate?

    Do you think this violation of the maximum voltage across these pins could be contributing to the failures I'm seeing? I will add a zener here to prevent this in the future... although I'm curious as to where this overshoot is coming from.

    This abnormal feature of the waveform makes an appearance once the load is removed - I've attached traces to support;

    - trace 27 contains the BOOT and SW waveforms with a load of 500mA.

    - trace 28 is a zoomed copy of this 

    - trace 29 contains the BOOT and SW waveforms with no load.

    - trace 30 is a zoomed copy of this. 

    **EDIT: Traces uploaded in order: 29, 30, 27, 28.

    I'll add a zener between BOOT and SW tomorrow.

    Thank you,

    Nick

  • Hi Nick,

    I apologize for the unclear notation. One of the best ways to get rid of ringing in a switching power supply is to place the input capacitor as close as possible to the IC.

    It's good that you placed the bypass capacitor as close as possible to Vin. However, to get the full effect, the grounded side needs to be as close as possible to ground in the IC. This picture should explain what I mean.

    Let me know how the Zener diode works.

    Thanks

    Andrew