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TPS7A33: LDO output stuck after unpredictable amount of time

Carlo Kleijnendorst89
Prodigy 30 points
Part Number: TPS7A33

On one of our designs, we have a few positive and negative analog 15V rails powering some opamp circuitry. For this, we use two 16V input supplies then use the TPS7A3301 and TPS7A4701 LDOs to get some nice clean voltage rails.

We currently have about 10 boards currently running without a problem (a few months).

Two boards ran for a while without a problem but now on one of them, two of the TPS7A3301 output rails is stuck at 0.8V. On the other board, one of the TPS7A3301's output rails is stuck at 0.8V. We think this happend after a power cycle.

Below is a screenshot of the negative LDO schematic:

We think the output transistor is damaged as we are still able to measure a voltage on the NR pin of the TPS7A3301. The rail is probably pulled positive through the opamp supply pins to the positive supply rail. We have a BAV99 diode on the output of every LDO, which is why it is clamped to only 0.8V.

We think the damage might be caused by a positive supply voltage on the output of the LDO during startup. A screenshot of the startup transients of one working set of 15V rails is shown below. 16V in in pink, 15V out in pink, -16V in in blue and -15V out in green. This is taken when powered by a dual bench power supply (one board failed under this condition). We see the negative rail going slightly positive (about 0.7V) at startup which is above the 0.3V absolute maximum rating of the TPS7A3301 output.

Is anyone else experiencing such an issue with this LDO?

Do you think this startup behaviour might be a cause of this issue?

If any other information is required, don't hesitate to ask.

  • 0
    TI__Guru 50190 points

    Hi Carlo,

    The startup could be damaging the device. Is it possible to measure the current into the device during this event? With the output clamped to ~0.8V, it suggests the device has been damaged. Can you contact your local sales to get them to help send the failed devices in for failure analysis?

    Regards,

  • 0
    Prodigy 30 points

    Hello John,

    Sadly, we do not have an easy way to determine the current flowing into the output pin directly as we have no way of inserting anything in line with the supply. I can try testing one of the broken LDOs in isolation tomorrow by lifting all of the loads.

    The output is clamped to ~0.8V due to the clamping circuit external to the device. This circuit is shown in the figure below. When we lift the pins of the BAV99 diode, we see the voltage rise a bit more to about 1.3V.

    I will try and figure out who our local sales representative for Ti is and try and come into contact.

    In the mean time, how would we normally overcome this issue? There is not really a guarantee that both supply rails will come up at exactly the same moment. 

  • 0 in reply to Carlo Kleijnendorst89
    TI__Guru 50190 points

    Hi Carlo,

    The best recommendation to mitigate this is to make sure both the positive and negative rails are up before enabling both of the LDOs'.

    And indirect means of doing this might be to use something like the TPS3840 to detect the positive input relay is up and set a reasonable delay to enable both regulators simultaneously. 

    A direct means to ensure both the positive and negative input rails are up before enabling the +/- regulators might be to use the TPS38600.

    Do you think either of these approaches would work for you? I will forward this to our sales team to see if they can reach out to you for FA analysis. 

  • 0 in reply to JCHK
    Prodigy 30 points

    Hello John,

    Thank you for the quick response. I look forward to hearing from your sales team for FA analysis.

    The approaches you suggest could work but would require changes to the layout or extensive patching. I will look into enabling the LDOs on a delay.

    In the meantime, we built a test setup with a TPS7A3301 eval module and a similar load situation. We are cycling the input supply every few seconds to see if we can reproduce the failure. So far, nothing has happened.

  • 0 in reply to Carlo Kleijnendorst89
    TI__Guru 50190 points

    Hi Carlo,

    Can you e-mail your contact details and I will get our local sales in touch with you.

    Regards,