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TLV3492: Long delay from power startup to valid outputs - many seconds

Richard Burnes1
Prodigy 30 points
Part Number: TLV3492

Some time ago, I measured the startup time of the TLV3492AIDCNT to be about 1 ms.  We now have a number of parts that take 3-25 seconds to generate a valid output after power up.  I've verified good power and inputs.  Some devices start with a low output and after some time begin drifting up slowly (analog output) toward our supply rail (3.3 V) over 3-4 seconds, then snap to ground, another few seconds pass, then the output is valid.  Once started it works fine.  Powering off and back on too quickly will result in a normal start.  Cold spray and a heat gun have no effect.  One of the inputs is a steady 2.2 V.  The other input is a Hall effect sensor that is mostly around 2 but increases to as much as 3.3V when a magnet passes by.

Parts that exhibit this problem constantly one day can test fine the next day.  This design that has been in production for several years and this is our first experience with this problem.

More detail:

The negative terminal is driven by a voltage divider 4.99k (top) and 10k (bottom) along with a 0.1uF cap.  The top of the voltage divider is tied to the same 3.3V rail powering the TLV3492.

The positive terminal is driven by a 5V Hall effect sensor after it's output has been scaled down with a 4.99k / 10k voltage divider with a 100 pF cap.  This prevents more than 3.3V from reaching the TLV3492's inputs.

The output goes to an Altera CPLD i/o pin configured as input only.  There's nothing else on the net.

We substituted parts from another reel which appeared to cure the problem, only to see a failure again in subsequent testing.  Not sure if the date codes were the same.

Thanks for your time and any advice you can offer.

  • 0
    TI__Mastermind 23395 points

    Richard

    Thanks for reaching out.  Sorry to hear about your adventure with our part.

    Really hard to say what is causing the variation from day to day or from one lot to another since startup delay is not something we have characterized on the TLV3492.  Would it be possible to capture some of the odd startup behavior with a multi channel oscilloscope?  We would want to see both inputs, the output and the supply pin if possible.  Just want to make sure what voltage all the pins are at when you experience the odd output behavior.  Sounds like the negative input is not really stable since it is coming on at the same time as the power to the comparator.  What about the signal from the Hall Effect sensor?  is it present before or after the supply voltage of the comparator?  Pictures would tell the story.

    I realize that your circuit has been in production for a while but I would suggest switching to TLV7032DDFR if that is at all an option.  This part has a known startup circuit (output starts low) and has the startup delay characterized around 200us.  This amount will vary from device to device but I would not expect parts slower than 1ms.

    Chuck

  • 0
    Guru 140200 points

    Hi Richard,

    the comparator could oscillate. Do you use a hysteresis? Can you show a schematic? What type is the Hall effect sensor?

    Kai 

  • 0 in reply to kai klaas69
    TI__Mastermind 23395 points

    Thanks for chiming in Kai 

    Richard

    Since we are working on this offline, I’m going to close the post. If we learn something worth sharing, we can always add the finding to the post. 
    Chuck