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TPS7A1633 latch up from CM event?

Bryce Billing
Intellectual 260 points

Hi There,

I am having an issue with a TPS7A1633DGNR part (henceforth LDO)

I am using it as a local supply for some SOC monitoring circuitry supplied by a multi-cell pack in a multi-pack power supply.

When I turn off an AC connected power supply that is far removed from this, the LDO appears to latch up and outputs a relatively hi-Z ~150mV to 100mV instead of 3.3V. I expect turning on/off the AC power supply will generate some common mode noise which may get injected into the system, however I have no problems with any other devices.

This condition does not clear until either input power is removed, or the enable pin is brought low/high again which I can do with the circuit/opto shown.

This problem existed before the remote disable circuit was implemented, so I doubt that is related to the cause of the problem.

I have scope shots showing input power is steady, and output drops.

Current draw is quite low < 5mA.

Probing the enable pin, I can see it sits at the same voltage as the input pin, so it does not appear to be a problem with the enable pin.

Using a thermal camera, I can see the part is not hot, so it does not appear to be in a current limit mode (?).

I have tried increasing the output capacitance, but that does not help the problem.

Here is the schematic;

Here is a scope capture somewhat showing the problem, capturing the glitch occurring is somewhat challenging however it can be seen happening on two regulators that are supplied by different batteries at the exact same instant.

Ch1/yellow: regulator #1 input, 5V/div

Ch2/blue: 3v3 regulator #1 output, 1V/div

Ch3/purple: regulator #2 input, 5V/div

Ch4/green: 3v3 regulator #2 output, 2V/div

Any help that can be provided would be appreciated.

Thoughts?

Thanks,

Bryce

  • 0
    TI__Guru 55940 points
    Hi Bryce,

    It looks like you did a lot of debugging already. I appreciate the information of what you have tried already.

    On your schematic, I see that you have labeled all of your unused pins with net names. Could confirm that these are all floating or were they used for routing?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Intellectual 260 points

    Hi Ryan,

    Thanks for the quick response.

    Yes the pins that are not connected through are indeed floating.

    Labelling them that way is a work around for a bug in the layout tool we use.

    Kind Regards,

    Bryce

  • 0
    Intellectual 260 points

    I have some done some more investigation into this.

    Adding a small capacitor (~1nF) from the enable line to gnd has mitigated this issue enough that I cannot see it occurring anymore.

    I would be interested to know if there is a specific recommendation for an ESD diode to be added to this pin or not.

    Regards,

    Bryce

  • 0 in reply to Bryce Billing
    TI__Guru 55940 points
    Hi Bryce,

    I am glad that you seem to have found a work around for your application. In order to help us find the root cause, could you provide a zoomed in scope shot of Vin, Vout and Ven? I know you mentioned it is difficult to catch the issue but this will would go a long way for root causing.

    Very Respectfully,
    Ryan