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TLC2272: Output Configuration understanding

MIT SHAH
Intellectual 715 points
Part Number: TLC2272
Other Parts Discussed in Thread: , LM7705

Hi Sir/Mam,

Greetings!!!

We are facing some difficulties in understanding of the U7A output configuration.The output of U7A is depends on D5 and R49.

We tested some cases as mentioned below :

TP29 TP21 U7A Pin #2 U7B Pin #7
10mV 149mV 47.4mV 198mV
20mV 155mV 50.2mV 204mV
30mV 161mV 53.6mV 212mV
40mV 168mV 57.6mV

223mV

We already gone through the datasheet of TLC2272,But we can't find the input output relation.

Please guide us in this.

Kindly let me know if you require more information.

  • 0
    TI__Guru* 93105 points

    Hello Mit,

    You are driving the TLC2272A, U7A input with an input common-mode voltage (TP29) very close to 0 V. The input common-mode voltage range goes down to about -0.3 V on the low end so that is okay. However, the issue keeping the output voltage (TP21) near 150 mV is likely the op amp output swing to the negative supply rail limit. Take a look at the VOL limits listed in the TLC2272A Electrical Characteristics table:

    It is hard to tell from inspection what exactly the output current is for U7A, but if the output is being required to source 500 uA or more, the VOL could be around 150 mV. That is as close as the output can swing to 0 V, and changing the input voltage won't move it off that lower limit until the input voltage becomes large enough to move the output off the rail. The limitation comes about from the ON resistance of the output MOSFET that is providing the current.

    Every op amp has its output swing limitations. And if the output needs to move very close to 0 V, then either an op having an output swing closer to the rail or a negative supply bias generator could be added to power Vs-. Finding an op amp that can get down to 10 mV of the rail is going to be tough to find.

    The LM7705 is a low noise, negative bias generator intended for powering Vs- at about -0.23 V so that most modern rail-to-rail output op amps can actually swing to 0 V. If you are interested in investigating that option you can find the datasheet here:

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 715 points
    Dear Thomas,

    Thanks for the valuable answer.
    Can you further more explain that how the output voltage of U7A increases as per the input voltage increases like 10mV,20mVetc..

    Regards,
    Mit
  • 0 in reply to MIT SHAH
    TI__Guru* 93105 points
    Hello Mit,

    When an op amp's output is pushed up against the output rail the output stage is in saturation, and the op amp is longer operating in its linear region. Nearly all of its dc, ac and noise performances become severely degraded and its is questionable if the op amp is still operating as a true operational amplifier.

    In the cases where the voltage level applied to the U7A non-inverting input was in 10 mV increments and increased in steps, the output was in saturation. Increasing the input voltage 10 mV at a time moves the output closer to coming out of saturation and closer to being in linear operation. However, since the output isn't quite in linear operation the small incremental input changes don't result in a perfectly linear, corresponding output response. Not until the output is back in its linear operating region would you expect to see the output change in accordance with the input voltage times the closed loop gain setting.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Intellectual 255 points

    Hello,

    What is the purpose of Diode D5 at the output of Op-amp U7A?

     

    Thanks & Regards

    Suraj

  • 0 in reply to Suraj Prajapati
    Guru 140200 points
    Hi Suraj,

    the circuit is a peak detector. D5 prevents the hold cap C11 from discharging too quickly.

    Kai
  • 0 in reply to kai klaas69
    Intellectual 255 points
    Hello Kai,

    Thanks for your quick response.

    This is really helpful.

    Best Regards
    Suraj
  • 0 in reply to kai klaas69
    Intellectual 255 points

    Hello Kai,

    We tried this circuit practically and we are getting constant 5V at pin 1 of Op-amp U7A  for different input voltage, But when I measure at pin 3 of diode D5 (TP21) I am getting around -2.2V.

    Please review the schematic and let me know if any errors are there.

    Thanks & Regards,

    Suraj

  • 0 in reply to Suraj Prajapati
    TI__Guru* 93105 points

    Hello Suraj,

    Do you have all the information you need to move forward with your TLC2272 application? If so, please close this e2e inquiry.

    Regards, Thomas

    Precision Amplifiers Applications Engineering