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ADS7812 high current when fault occurs.

Aaron McClure
Prodigy 30 points
Other Parts Discussed in Thread: ADS7812, ADS7809

We have a product that uses the ADS7812 to acquire data for display on an embedded controller.  On occasion the equipment will fault shutting down.  Most times this is not a problem but on occasion the ADS7812 starts to pull more current loading down the 5V 100ma regulator to about 3.1V.  We clipped the Vcc then measured about 175ma which explains why the regulator is limiting.  Vcc is also used to pull INT/EXT high.  When this drops the 3.1V the chips stops using the EXT clock to shift out data.

 

It appears that the 7812 may be locking up due to the fault.  It only happens in about 2% and only when a fault in the system occurs.  This is a high voltage power supply so a fault typically results in shutting the HV off.  The noise may be getting picked up by the ADC.  

 

We have the recommend bypass capacitance Vcc to ground and have followed the design guidelines for grounding and ground plane under the device. 

Has anyone seen this before? If so what was the fix?

 

-Aaron

  • TI__Expert 6925 points

    Hello Aaron,

    Welcome to the forum!

    This seems to be a common SCR latchup condition that many ICs experience when exposed to power transients. During a power transient, different voltage spikes can be generated across many traces in a circuit board. These spikes can exceed the absolute ratings of one or many pins of any ICs forcing a SCR latchup condition.

    Many times ICs are protected from power supply spikes by their internal ESD cells. However, ESD cells are not designed to protect against voltage spikes from power supplies. Thus, sooner or later, ICs show SCR latchup conditions and/or are damaged.

    Is it possible to see the schematic of your board and an oscilloscope screenshot of the main power supply when it is turned off?

    Thank you,

  • in reply to Rafael Ordonez
    Prodigy 30 points

    Hi Rafael,

    Sorry for the delay I am juggling a number of issues.  I believe I attached a schematic.  If not I will go and try it again.  The 7812 is in the upper right corner.  I don't have a scope screen shot of the supply voltage when the ADS is locked  up.  I do recall that the voltage was stable at 3.1V because when I checked it I suspected it might be oscillating which was not the case.

     

    Thanks

    -Aaron

  • in reply to Aaron McClure
    Prodigy 30 points

    Let me try the schematic again.  I don't see it attached.

     

    -Aaron

    ADS7812.pdf
  • in reply to Aaron McClure
    TI__Guru**** 174821 points

    Hi Aaron,

    Your post started off with the ADS7812, but the schematic you attached shows the ADS7809 - which part is it that you are having trouble with?  In general, I've not seen anyone having with issues using the ADS7812.  The ADS7809 can get into an SCR latch-up type of operating mode as Rafa mentioned if the DVdd rail is  brought up before the AVdd rail or if there is some sort of brown out condition where the digital supply stays up but the analog supply droops.