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TPS61088: Shorting out of SW outputs at power-up

Franz Schellhase
Expert 3605 points
Part Number: TPS61088

Hi Team,

Can you have a look at the following customer inquiry, please? I can provide a schematic offline, if needed.

We are using a number of different TI boost converters in a new pump design. However, we are encountering a serious failure of the TPS61088 during our initial bring-up.

The failure appears to be shorting out (8 ohms to GND) ) of SW outputs of the device at power-up.  The max current we need is ~100ma @ Vin=5V , VOUT 9V.  We have the device set for PFM mode using  2.2uH uH 1A inductor.  

We did not take any oscilloscope measurements because the failure either was before enabling Vin (possible bad part) or immediately after applying Vin.

  1. While applying Vin there was no load on Vout.
  2. The output capacitors were disconnected by a jumper as well.

We have reviewed the TI knowledge base for similar issues and found some evidence of layout, but no mention of the specific reason. We have reviewed and components values, manufacturing issues along with the layout.

  1. https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/588165/tps61088-tps61088
  2. https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/588086/tps61088-ic-is-damaged-when-loaded-with-ptn78000a

At first we suspected some defective devices or poor soldering. However I’m not sure. My suspicion of a ‘possible’ issue was the reviewing of e2e reporting similar events.  

Once we replaced the device(s) from a different lot we did take some A/B comparisons (original design versus new design –layout only) of the inductor current. The failure appeared to be shorted (8 ohms) on all the SW pins. I’m thinking or speculating that our placement of the output caps may not be optimal or the inductor. So really I’d like to understand what the possible failure mechanism(s). Could having no output caps connected cause this failure? or some added inductance. We removed TP306 during initial debug (no load). 

We now have one board that is working with no changes or fixes implemented (just new device installed). We are few days away from receiving more boards and want to be better prepared if the issues arises again.

Thank you,

Franz

  • 0
    TI__Guru 65170 points

    Hallo Franz,

    Please clarify point 2 in above list: The output capacitors were disconnected by a jumper as well.

    Does this mean that there was not output capacitor at the converter at all?

    The converter needs to have output capacitance as soon as it starts up.

    Best regards,
    Brigitte

  • 0
    Prodigy 20 points

    There was a jumper placed before the output capacitors. When the Boost circuit was powered the jumper was not in place. This disconnected the output capacitors from the output of the converter. We were theorizing about this condition but did not know the ramifications with respect to the TPS61088 boost converter.

  • 0 in reply to willie Robertson
    TI__Genius 16225 points

    Hi Franz,

    When working without output capacitors, the output voltage will be extreme high when the switch is off, which causes the switch damage.

    Please try to add some capacitance before the jumper.

    Best Regards,

    Fergus