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Original question:

LM2903B: LTspice simulation shows different output compared to LM293

LM2903B: TL331I 2.6V Problem

Neslihan Yildirim
Prodigy 10 points
Part Number: LM2903B
Other Parts Discussed in Thread: TL331

Ich nutze den Komparator TL331I für die Simulation in PSpice. 

Für den Grenzentest beträgt die Versorgungsspannung 2.6V.

Die Spannung am - Pin(türkis) beträgt ca. 850mV.

Die Referenz am + Pin(gelb) soll beträgt etwa 470mV.

Da die Spannung am minus-Pin größer ist als die am plus-Pin, erwarte ich ein "low" auf Ausgang (grün).

Das ist aber nicht der Fall. Der Ausgang zeigt "high" bei 1.3V an. 

Im Datenblatt steht, dass der Komparator zwischen 2 bis 36V in Betrieb genommen werden kann. 

Wenn ich die Versorgungsspannung auf 2.9V stelle, dann funktioniert die Schaltung wie gewünscht. 

Woran liegt das Problem? Es handelt sich um eine wissenschaftliche Arbeit. Ich bitte um eine Antwort mit Begründung.

SchaltungSimulation

  • 0
    Guru 287370 points

    This circuit violates the valid input common-mode range of the LM2903B; you need to stay 2 V away from the positive supply.

    With the real device, the output is undefined in this case (either high or low, see the application note for details), but the model will make the output float to mid supply.

  • 0
    TI__Genius 11701 points

    Hi Neslihan,

    Thank you for your post. Sorry I do not know German but running your question through google translate, I understand that you are seeing issues in the output of your simulation.  As Clemens mentioned, the TL331's common mode range is 0 to VCC - 2V. Therefore, your voltage at the inverting input is violating the range (VCC - 2V = 2.6V - 2 = 0.6V). When the common range is violated, we have the model give an error condition where the output goes to mid supply (2.6/2 = 1.3V). This does NOT happen to the real device. This is why you do not see the error when you increase your VCC voltage. Please make sure to keep your inputs within the common mode range. 

  • 0
    Prodigy 10 points

    Hi Clemens, Hi Chi,
    thank you for the fast message.

    According to  the application note, 2.2 Input Voltage Range:

    I have calculated with VCC-1.5V.
    2.6 V - 1.5 V = 1.1V, so I am in the range)

    And because in data sheet (TL331 datasheet, 8.2.2.1 Input Voltage Range), a low is to be expected at the output.

    "2. When IN- is higher than common mode and IN+ is within common mode, the output is low and the output transistor is sinking current."

    Can i not rely on the statement from the data sheet?
    Do i necessarily have to keep to the 2V even though i have normal working temperature?

    Please help me further.

  • 0 in reply to Neslihan Yildirim
    Guru 287370 points

    The model is not identical with the real device.

    The real device will behave as described in the datasheet and the application note. But you should not exceed the recommended operating conditions. (And the temperature will change due to power dissipation.)

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    so my scientific work will fail on a positive result. thank you for the answer :) 

  • 0 in reply to Neslihan Yildirim
    TI__Guru 54076 points

    All,

    The new LM2903B fails high. The appnote has not been updated yet.

    As the appnote states, TI recommends staying within the specified input voltage range and not to rely on the "feature" as part of normal operation.