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LM2776: The LM2776DBVT fails to be powered on. Procedure

ian wu
Intellectual 745 points
Part Number: LM2776
Other Parts Discussed in Thread: OPA2830,

Tool/software:

hi  Expert

We have encountered some problems in the circuit of TI's chip design for our production models, so I would like to ask you about it.

The details are as follows:

Use the LM2776DBVT to generate -5V to supply power to the negative terminal of the OPamp OPA2830 power supply. When the machine is powered on at a temperature above 50 ° C, the output voltage of the LM2776DBVT is abnormally 0.687V, resulting in no signal output of the opamp.

After the OPA2830 is removed, the equivalent LM2776DBVT output is not loaded, and the -5V output is normal when the machine is powered on at -40 to 70℃. The input signal is frequency 10K, pulse width 100ns and amplitude 500mV, and the output signal amplitude is 800mV.

Could you please help us analyze the problem according to the above information and the attached circuit diagram? If you need other test information, please tell me to cooperate with the test

  • 0
    TI__Expert 6510 points

    Hi Ian,

    Thanks for using E2E.

    Could you please provide the part names of the input, output and flying capacitor which you are using in your application.

    What is the load current at power-up and in steady state for our LM2776 because without load the device is starting correctly?

    What do you mean by "The input signal is frequency 10K, pulse width 100ns and amplitude 500mV, and the output signal amplitude is 800mV." Is this related to the OPA2830?

    Do you see this fail behavior on every board means every device or on one board only? Have you assembled a new/different OPA for the start-up testing already?

    Best regards,
    Sepp

  • 0 in reply to Josef Marsmann
    Intellectual 745 points

    Hi Josef

    1.input capacitor ：2.2uF/50V，output capacitor ：2.2uF/50V，flying capacitor：1uF/50V

    2.The current in steady state is 10mA, and the current in at power-up is also 10mA. There is no obvious overshoot in the test with oscilloscope + current clamp (the noise of current clamp is relatively large).

    3、“The input signal is frequency 10K, pulse width 100ns and amplitude 500mv, andthe output signal amplitude is 800mV”. this is related to the OPA2830.

    4, this failure is every device will appear, we have tested at least 20 circuit boards, all will fail; We did not use other types of operational amps for testing; Can you recommend a replacement model? Let's test that out;

  • 0 in reply to ian wu
    TI__Expert 6510 points

    Hi Ian,

    Thanks a lot for the new data.

    We have to find out which part of the schematic creates the fails above 50°C.

    I would recommend to remove the OPA an use a resistor as a load on the VOUT of the LM2776:

    • Load = 500Ω (10mA)
    • Load = 50Ω (100mA)

    Measure the power-up behavior at 20°C and 85° to see if the LM2776 is working correctly.

    If the results are ok, measure the load current at power-up with the OPA at 25°C and a higher temperature at the edge before failing (~48°C).

    Best regards,
    Sepp

  • 0 in reply to Josef Marsmann
    Intellectual 745 points

    hi  

    1.The LM2776 outputs are connected to the following loads

    • Load = 500Ω (10mA)

    • Load = 50Ω (100mA)

    As shown below:

    At 50℃, 55℃, 60℃, 65℃ and 70℃ respectively, the output voltage is normal, and all are -4.8V.

    2.At 48°C, test the LM2776 power supply current to the opAMP OPA2830 at 10mA. The current at the moment of power-on and startup is tested using the current clamp, as shown in the following figure:

  • 0 in reply to ian wu
    TI__Expert 5055 points

    Hi Ian,

    Due to holiday season, Sepp will get back to you by Jan/2.

    Thanks for your understanding!

    Yann

  • 0 in reply to Yann Ducommun
    TI__Expert 6510 points

    Hi Ian,

    If I understand it correctly, it works with a resistance load over the temperature range, correct?

    So please create a plot with the VIN and VOUT in addition to the output current and zoom in to the power-on transition in more detail with max resolution and bandwidth. I would like to see the behavior at the max temperature were it passes and the fail case on the slightly higher temperature.

    I hope this helps to identify the root course for this issue.

    Best regards,
    Sepp