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LM3880: power sequence fail

lei zhou1
Intellectual 260 points
Part Number: LM3880

Hi TI

        In our development, we use LM3880MFE-1AE/NOPB to control 66AK2G12ABYA60, the circuit is below:

 

    Recently we found a failure with LM3880, Failure phenomenon: When the PCAB is powered off at an interval of about 4s and then powered on again, the DSP cannot be started.

    LM3880 output correct power sequence enable is 3V3_EN, 1V8_EN, PORn_EN, the failure LM3880 1V8_EN is powered on simultaneously with VCC_5V, but 3V3_EN and PORn_EN output 0V, The waveform measured by the oscilloscope is as follows, so I have some question need your help:

1. What causes the abnormal output of LM3880?

2. Do you think this LM3880 failure is individual case?

3. Why does FLAG2 get up before FLAG1？

4. How to avoid and solve this problem？

5. Does this LM3880 is damaged?

  • 0
    TI__Expert 4175 points

    Hello,

    1. Abnormal or undefined behavior on the FLAG outputs predominately occurs when the EN voltage voltage is above the EN voltage threshold while VCC is still below its minimum operating voltage. 

    2/5. This could be a individual case, the device could be damaged. I would recommend replacing the LM3880 and testing again for this issue.

    3. Abnormal behavior on the FLAG outputs predominately depend on the EN/VCC behaviors.

    4. I need a little more information. See below.

    Are you needing power-up sequencing only, or is power down sequencing needed as well? You are using a voltage divider for the EN, which is recommended for power-up sequencing only. If you are needing power-up and power-down sequencing, the EN pin is recommended to be controlled via external signal.

  • 0 in reply to Sam
    Intellectual 260 points

    Hi SGradinaru

        Thank you very much.

    For point 1:

    In my circuit design, EN is obtained by dividing the voltage of VCC_5V. According to the requirements of the datasheet, Ven<VCC, Ven=(18/58.2)*VCC_5V+7uA*18K, when the threshold Ven=1.25V, VCC= 3.63V, when the minimum threshold Ven=1V, VCC=2.83V, which is within the working voltage range of LM3880 (2.7V to 5.5V), so the voltage of EN signal should not be greater than VCC in theory, the signal quality of VCC_5V is also very good，so how is this abnormality caused? 

    For point 4:

    Yes, we need power up sequence.

    Can you provide a reliability report for the LM3880MFE-1AE/NOPB?

  • 0 in reply to lei zhou1
    Intellectual 260 points

    Hi SGradinaru

        See picture below, when EN=1V, VCC_5V alredy 5V, it doesn't make sense：

  • 0 in reply to lei zhou1
    TI__Expert 4175 points

    How is the LM3880 powered off? Is this occurring on multiple units or is the just on a single unit so far? 

    SGradinaru

  • 0 in reply to Sam
    Intellectual 260 points

    The power off waveform  see below, we have replace the LM3880 and the fault has disappeared.

    This occurring just on a single unit so far, and we also hope it's an individual case.

  • 0 in reply to lei zhou1
    TI__Expert 4175 points

    Thank you for the information. Please let us know if this issue happens again on the new device.