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THS3491: Amplifier power down not always behaving

Jordan Smith
Prodigy 205 points
Part Number: THS3491

Hi

We are using the THS3491 (RGT) package in our design with bipolar +/-14V rails. We use a single power down signal for a bank of THS3491 amplifiers to power-down all of them at once, with the power down either at 5V or 0V.

We have observed on some boards that have been built one amplifier will appear to only partially shut-down, and appears to still draw the full quiescent current from one of the rails (i.e. the 14V OR -14V, but not both). The power-down line is measured to be 0V and the other amplifiers are fully shut-down, but the problematic part is only half shut-down and will generate a several volt offset voltage at the output.

It happens on a few % of boards, so we suspect faulty parts at this point but have yet to try to replace the amplifier to confirm. Is this an issue that has been observed before?

  • 0
    TI__Genius 14166 points

    Hi Jordan,

    I've not seen the PD pin not work for the THS3491 yet, but I have helped someone power down multiple amplifiers at once before. They just needed to buffer the power down signal with another amplifier, since the logic was not strong enough on its own. For the THS3491, each PD pin will source 25uA into the 0V, and the logic signal needs to sink all that current otherwise the voltage will start to rise and maybe some of the amplifiers start to turn back on. 

    How is the PD pin trace being driven? In parallel with the 0V logic, can you short the PD pins to ground and see if the last amplifiers turn fully off? 

    Best regards,

    Sean

  • 0 in reply to Sean Cashin
    Prodigy 205 points

    Thanks for the ideas Sean. The PD pin pulled up to 5V through 10k and is driven low by a spare low-side MOSFET driver output, and the VLOW that I measure on the PD pin is ~0V. I have tried shorting the PD to ground but no difference.

  • 0 in reply to Jordan Smith
    TI__Genius 14166 points

    Hi Jordan, Can you also measure the voltage directly on the ground reference for that device's PD pin? An check the solder joints?

    Is there a >1V input signal to the IN+ pin of the device? I just learned that this might cause the device to draw some Iq when powered down.

    the next step is probably to replace the erroneously "ON" amplifiers and confirm that the amplifiers were not just damaged somehow.

  • 0 in reply to Sean Cashin
    Prodigy 205 points

    I'll measure those pins and let you know what I find (will have to wait until the new year when I have access to the boards again). From a visual inspection the solder looked OK

    Thanks for the suggestion, I had not checked whether there is an input signal applied so I will have a look at that as well.