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TLV7031: Comparator input design issue

Chris Alzate
Prodigy 10 points
Part Number: TLV7031

Good morning, I am having some trouble with a comparator circuit I've designed on a IO breakout board that I'm hoping I can get some help with. The circuit in question is the front end to a fan circuit. The purpose of the comparator circuit, U199 is to detect whether or not there is a thermistor installed on J37. The reason for this is primarily thermal management. If there is no thermistor installed, the fan circuits for U195, U194, U196, and U197 will not be engaged, because there is no voltage on pin 1 of the FAN IC's. To solve this corner case, I decided to add a inverting comparator, U199 with a isolation switch U200 to prevent the feedback loop from following the output of the comparator. The primary issue I'm having with this circuit, is that when there is no thermistor installed, the -In pin is always about -10mv to -30mV above +in. This is interesting because without a thermistor installed I would expect -In to be 0V due to R1182 acting as a pulldown, and +In to be around 100mv. In this operational case, -In measured to be 125mV with +In measuring at 100mV. I expect the inputs to have an offset, but I did not expect the -In pin to have a DC bias voltage that tracks to the +in pin. Can you explain why this is happening? I've adjusted the resistor divider feeding the +in pin many times to voltages up to 1.5V and this behavior is consistent, which leads me to believe that I've made a design mistake. Can you explain what I have done wrong and how to possibly correct this, if you have any ideas? Schematic is attached for your reference.

IO Breakout board Fan Schematic .pdf

  • 0
    TI__Mastermind 24260 points

    Chris

    Thanks for your post.  Your assumptions on what to expect are correct and there is nothing in the comparator to make the inputs track or to have 100+mV of offset.  Are you sure that U200 isn't somehow contributing?  I would try unpopulating R1236 to disconnect the inverting input from everything.  Since a voltmeter is in the 10Mohm range, holding the probe on the pin should suffice in measuring something close to 0V.  Did you try another TLV7031 to make sure the comparator didn't somehow get damaged?  Sorry, I am just trying to walk through some troubleshooting to see what is causing the issue. Likewise, are you using leaded package option or the DPW leadless package.  Maybe there's some resistive short between the inputs that you cannot see?

    Hope one of these suggestions helps track down the problem.

    Chuck

  • 0 in reply to Chuck Sins
    Prodigy 10 points

    Yes, I have removed R1236 before. The behavior changes and the input acts as expected and drops to 0V. When its installed I think the pulldown, R1182 causes the comparator to act funny and increase the pins voltage to 115mV. 

    Secondly I have removed U200 all together and I still get the same results. I am very confident that U200 is not the cause.

    Lastly, yes I have tried several comparators on my boards. 4 in total. They all act the same. 

    Any ideas why grounding out the -in pin would cause the chip to backfeed voltage onto it?

  • 0 in reply to Chris Alzate
    TI__Mastermind 24260 points

    Sorry Chris.  There isn't anything inside the comparator to do that.  If you breadboard the circuit, I don't think you will get the same result.  Is it possible something is coupling in through J37?  And if you remove R1236, try soldering similar resistor to ground on the pad of R1236, even with a wire.  I don't expect you to see any problem if you do.  Have you tried a different board, or do you only have one prototype?  What impedance to ground do you measure?  You should only see 7.5k.  Our bias current is pA, so 7.5k can't generated that sort of voltage.

    Chuck

  • 0 in reply to Chris Alzate
    TI__Guru 52466 points

    Hi Chris,

    With the power both on and off, what resistance do you measure *between* the comparator inputs? And with and without R1236? If you reverse the DMM test leads, do you get the same resistance readings?

    Does any other nodes in the circuit use the node name vref or vin? I have seen this happen before in schematics and traces accidentally get tied together in layout. What is the node-name of J37 pin 2?

  • 0 in reply to Paul Grohe
    TI__Genius 11701 points

    Closing this post as we have not heard from you Chris. Feel free to reply back fi you have any updates.