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Original question:

THS3491: Amplifier power down not always behaving

THS3491: Amplifier power down - follow up

Jordan Smith
Prodigy 205 points
Part Number: THS3491

Hi

Follow up from this post: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1063699/ths3491-amplifier-power-down-not-always-behaving

"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."

We are still seeing this issue occurring on our production units, still several % showing the issue in each production run we do.

We have confirmed that when the THS3491 is failing to power down, there is either 0.2V present on the +IN and -IN pins, or 0V present on the input pins (depending where in the circuit the problematic part is). But it doesn't seem to matter what this voltage is. The PD pin is 0V when showing this issue, and the rails are present and stable at +/-14V.

Any ideas would be appreciated.

Thanks

Jordan

 

  • 0
    TI__Mastermind 25985 points

    Hello Jordan,

      Thank you for the update on our previous suggestions! From the original thread, it seems like on some boards, one amplifier out of the multiple for each board is having issues. Is this still the case, and for each board is this for the same channel? 

      Another suggestion would be to take a new THS3491 unit, test it with low power-down and note the quiescent current. Then replace the tested unit with the failing unit on the board. If the problem still occurs, would you be able to share the schematic? If so, feel free to share the schematic either privately via e2e's friend request or publicly through this thread. 

    Thank you,
    Sima 

  • 0 in reply to Sima Jalaleddine
    Prodigy 205 points

    Hi Sima

    It is random which channel and which amplifier is the problem.

    We have tried replacing the THS3491 and the problem is fixed.

    Thanks

    Jordan

  • 0 in reply to Jordan Smith
    TI__Mastermind 25985 points

    Hello Jordan,

       When you place the non-functional THS3491 back on the board does the problem reoccur, or does it go away? This is considered an A-B-A swap to help understand if the issue follows the device. 

    Thank you,

    Sima

  • 0 in reply to Sima Jalaleddine
    Prodigy 205 points

    Hi Sima

    We will try your suggestion and let you know what we find

    Thanks

    Jordan

  • 0 in reply to Jordan Smith
    Prodigy 205 points

    Hi Sima

    We have removed the problematic part from one of our boards, and fitted to a test PCB that we have manufactured. This board just breaks out the pins on the THS3491 to test points so we can connect up to a power supply. Both feedback resistors are 500 Ohm, and the input and output are terminated with 50R to ground.

    We also made up a test board with a 'working' THS3491. Please see below summary, we still see the fault on the problematic part where one rail does not shutdown (in fact, it draws more current).

    Please let me know what you think

  • 0 in reply to Jordan Smith
    Guru 140200 points

    Hi Jordan,

    this cannot be answered without a schematic. So please show a schematic.

    Kai

  • 0 in reply to Jordan Smith
    Guru 48395 points

    This could in fact indicate a test escape - seems time to get an RMA going so the defective unit can get run back through ATE. If it now fails, then something broke it enroute to your board. There is always the chance added screening needs to be added to the ATE test flow. 

  • 0 in reply to Michael Steffes
    TI__Genius 17671 points

    Hello Jordan,

    This has been brought to my attention; I have sent an email response but I will reiterate here: I am working with my team and I will let you know if/when to send faulty parts to TI by end of day tomorrow.  I appreciate your diligence and work so far.  

    Kai, Michael,

    Thank you for jumping in on this one.  It may well be a case where we need to examine the parts here at our TI site.  I do have a schematic from Jordan, but I do not think the intention was to share the circuit publicly.  I will leave that up to Jordan to decide who can view and comment on the schematic.  

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 17671 points

    Hello Forum,

    While a discussion around the specifics of this support request has been occuring over email, the relevant information for the forum is correct power down implementation.  Please refer to the datasheet when designing power down features into your circuit.

    Best,

    Alec