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TL331B: TL331B Output jumping question

GaryC
Expert 3315 points
Part Number: TL331B
Other Parts Discussed in Thread: TL331, LM393

We test the TL331B has a problem below,

1. After remove the DC power, during the VCC(VZ) voltage drop to 4.25V, the output (PIN 4) has a high-frequency jump, which is not our expected phenomenon

2. The datasheet indicates the working voltage 3V-36V, the 4.25V should be considered as normal working conditions, and there should be no jitter

3. The same PCB, only replace new TL331, the output will be work normal.

Do you have any recommend? Thanks

BR,

Gary




  • 0
    Guru 317480 points

    The upper limit of the valid input voltage range (specified as VCM) is at least 1.5 V below Vz. When the difference is lower, correct operation is not guaranteed.

    The Application Design Guidelines for LM339, LM393, TL331 Family Comparators say that the comparator can work if only one input is in the valid range. Apparently, while Vz and the inputs are going down, the difference between pins 3 and 4 fluctuates just around the limit.

    Would it be possible to add a diode to Vz to force either C1 or C2 to discharge faster?

  • 0
    TI__Guru 61276 points

    Hello Gary,

    As Clemens said, you are violating the input range on the way down.

    Even though there is a valid 4.25V on Vz, the inputs are above 4.25-1.5=3V input limit.

    When both inputs are above the limit, the input stage is cut-off, and the otuput state largely relies on leakages to determine the output state. The output should be considered indeterminate (though the output usually goes low). Having the inputs and supplies moving at the same also adds to the chaos.

    The TL331 family has a unique pinout and we do not have a rail-to-rail equivalent, or one with a POR circuit in the same pinout.

    Since the inputs are being violated at the time, I do not think adding hysteresis will help.

    Is it possible to re-adjust the thresholds, by changing the resistors, so that the threshold is farther away from the positive rail? Removing C2 will reduce the lag on the negative input, keeping it lower than the positive...perhaps that will help a little.

  • 0 in reply to Paul Grohe
    TI__Expert 3315 points

    I would confirm TL331 VCC spec that is recommend 36V in section 6.5, but the datasheet page1 show the VCC max is 30V. May I know which one is correct? Thanks

    BR,

    Gary

  • 0 in reply to GaryC
    Guru 317480 points

    In section 6.5, the correct value would be 30 V.

    Revision F of the TL331 datasheet did not have a recommended operating conditions table. When that table was added in revision G, it used the wrong value (probably copied from the absolute maximum ratings). Revision H then added the "B" version, for which 36 V actually is correct.

    The TL331 is the single-comparator version of the LM393. In the LM393 datasheet, the family comparison table and the recommended operating conditions do match:


  • 0 in reply to Clemens Ladisch
    TI__Mastermind 26070 points

    Sorry for the conflicting information.  The TL331 is actually operable up to 36V however the EC table only warrants some of the specs to 30V.  In the next revision, we will update the front table to match the recommended operating range up to 36V.

    Our newer TL331B is the improved version and has specs warranted up to 36V.  So if that is important to the application, it is recommended to use the TL331B option.

    Chuck

  • 0 in reply to Clemens Ladisch
    TI__Expert 3315 points

    May I know you have plan to revise the datasheet?

    BR,

    Gary

  • 0 in reply to GaryC
    TI__Mastermind 26070 points

    Yes, the table on the front page  will be updated.

    Chuck