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LMV7275 open-drain output short circuit protection?

David F.
Prodigy 195 points
Other Parts Discussed in Thread: LMV7275

I'm using an LMV7275MF/NOPB open-drain output comparator powered by +5V.  The output is connected to the input of an FPGA using a 1k pull-up to +3.3V.

In one instance, we had the input ball on the BGA FPGA shorted to +3.3V.  Therefore the output of the LMV7275 was shorted to +3.3V.  It probably operated this way for many hours.  After troubleshooting to find the shorted ball, then the comparator output still didn't work.  I measured about 170 ohms from output to gnd, with the part removed from the PCB.

Is the open-drain output of this part supposed to be protected from a short like this?  Most op-amps with push-pull output would be, but with an open-drain comparator, I don't know.  The maximum short-circuit current isn't specified on the datasheet, but even nominal shows 34mA at 5V.  With the output tied to 3.3V, this would be around 3.3 * 0.034 = 112mW dissipated. 

Any known answers?

Thanks.

  • 0
    TI__Mastermind 49966 points

    Hello David,

    Yes. They do have very basic fold-back current limiting. This is specified as "Output Short Circuit Current" in the electrical table, which is specified as 18mA minimum, 34mA typ. So we guarantee that you will get at least 18mA at 5V supply.

    With the output shorted to 3.3V, the temp rise is about 37°C. That is not enough to destroy it...unless the ambient was over 113°C!

    Which part was removed from the board? The 7275? Or the ASIC?

    "Ohming" the output with the power off may give erroneous results due to the ESD diodes clamping to the "dead" supply. Better to use the diode-check function and check both-ways (reverse the leads). You should see a solid diode (0.6 to 0.8V) in one direction, and a "soft" diode (diode + floating supply, 0.6 and climbing) in the other.

    If the output is truly resistively shorted, you should see the same low reading in both directions.

    I doubt if you 'destroyed' it. The currents and temps are just not high enough. I would not shoot it into space, but it should still be perfectly functional.

    If the output is not "moving", make sure the pull-up resistor, or any other devices that share the bus are still functional.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 195 points

    Paul,

    Thanks for replying.

    To provide some additional clarification and results, the FPGA is located on a separate board, so we swapped out that board to fix the shorted BGA ball.  Nothing ever saw any temperature extremes, just around 25C.

    I measured some additional voltages and resistances on the LMV7275 (uninstalled) as you suggested:

         measurement (+ to -)      voltage in diode mode          resistance

         output to gnd                             0.167V                            165 ohm

         gnd to output                             0.156V                            154 ohm

         output to vcc                              0.785V                             48.0 kohm

         vcc to output                              open                                2.2 Mohm

         vcc to gnd                                  open                                2.2 Mohm

         gnd to vcc                                  0.622V                            47.8 kohm

    From these results, it appears the output really is shorted with a fairly low resistance (around 160 ohm with both polarities).

    Before I replaced it in the circuit, it did provide a small voltage level output.  With my 1k pullup to 3.3V, it was outputting a square wave with around a 50 mV pk-pk amplitude (this is a very rough number from memory - I didn't save a scope capture).  So the input circuitry of the comparator was working, it just couldn't drive the output.  Replacing only the comparator did fix the circuit.

    I think the 3.3V shorted output should be a rare event (manufacturing defect).  So I'm trying to decide if any additional resistance should be added in the trace between the comparator and the FPGA.  Since the part is short-circuit protected, normally I'd say that's good enough.  But how do we explain this failure that I saw?  The comparator is located on a board that is very difficult and expensive to replace if there is a failure.

    Any more thoughts or advice?

    Thanks,

    David

  • 0 in reply to David F.
    TI__Mastermind 49966 points

    Hi David,

    I spoke to the original designer. We cannot see a way that this should have damaged the device if it occurred as you said.The LMV727x family has been out for 12 years on a stable high-volume process and we are not aware of any other issues with the LMV727x outputs.

    The current is limited by an internal series resistor in the drain of the output transistor. The output "transistor" is actually comprised of several smaller devices in parallel - each with it's own series resistor (not just one big resistor). The total output drive is the parallel combination of all the devices. The current is pretty spread out.  CMOS devices do not go into thermal runaway as they loose output drive as they heat up.

    How long was it "shorted"? A few minutes? An hour? A day?

    Since the output was trying to "wiggle", it could also be that the lower ESD diode blew - though they tend to blow in the low ohms, but can go resistive if partially damaged. A curve trace would tell the real story.

    However, we are making an assumption that applying 3.3V to the output was the only thing that happened. We all know many things can happen in a large system. You mentioned that they are on separate boards. Any chance it could have been hit by ESD? Are the boards separated by a long cable?  Are the supplies sequenced? Does that output trace go by something that may induce a large (negative) transient? Was the board hot-swapped?

    You should not have to add external resistance to protect the output - unless you are expecting the output to be pulled outside of the comparators supplies.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 195 points

    Paul,

    Again, thanks so much for investigating this.  Your questions are all right on point.  It is a complicated system.  We know we had an initial problem on a different board.  There was a fair amount of troubleshooting and handling going on.  While I think we manage ESD well, and also know better than to hot-swap, these things can still happen - either accidentally or by carelessness.  

    The short was likely applied for many hours to a couple days.  The supplies are sequenced, and if working correctly the 5V VCC comes up prior to the 3.3V output pullup.  We don't have a curve-tracer, and would be some work to manually do this.

    Your explanation makes me comfortable to just leave the circuit as is.  The part is protected by design from short circuits on its output.  This failure must have been caused by something beyond what we know happened (like ESD or an accidental scope-probe short to something else).

    Thanks again for your thoroughness, and willingness to help me out!

    Best Regards,

    David