• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 47 subscribers
  • Views 196 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2902: The output waveform is incorrect

Tommy
Intellectual 400 points
Part Number: LM2902

Tool/software:

Hi E2E,

We found that there were more than 10 cases LM2902DR of abnormal output waveform during the use of products.

The main problem is PIN14 continuous output a high level(almost 12V).It cannot be eliminated by restart the product. The failure is recurring.

But when we did the ABA test, all the faults disappeared and could not be repeated.

the SCH and TEST report as follow:

LM2902waveform+impedance.xlsx

We have also applied for FA analysis and got the 8D report after TI CQE analysis. The 8D report show no problems with the IC. The IC is intact.
We checked the welding problem again and found no clear doubts.
May I ask what other reasons may lead to this situation? What aspects should we examine?

BR

  • 0
    TI__Guru**** 151256 points

    Hello Tommy,

    LM2902 doesn't accept voltage below ground. On the NG pin 13, the voltage is below ground. Nevertheless, the pin 14 output is full high as it should be. CH4 is OK.

    Pin 13 is too low because pin 1 voltage is incorrect. Pin 3 voltage is clearly higher than pin 2 voltage. So pin 1 should be high. Be sure to measure voltage directly on the LM2902 pins, to make sure the path to pin is good.

    What was the QEM number for the FA report?

  • 0 in reply to Ron Michallick
    Intellectual 400 points

    Hi,

    Thanks for the reply!

    The tracking number for this FA report is:CPR241107921

    The report#. is:QEM-CCR-2409-01037

    Yes, we agree with your analysis. According to the test waveform, both CH1 and CH4 of the IC is right.

    However, under the same external conditions, I could not figure out why the input signal of CH2 was different, so that CH1 lost the FAN carrier signal and became a near constant low level.

    BR

  • 0 in reply to Tommy
    TI__Guru**** 151256 points

    Channel 2 has a lot of direct capacitive load. That could easily lead to instability. Does pin 7 have small or large signal oscillation or excessive ringing?

    Based on the waveforms I thought pin 1 was incorrect. That's why I suggested looking a signal directly on device legs.

  • 0 in reply to Tommy
    Intellectual 400 points

    Sorry, there's a mistake in my description.
    It shoud be:

    “However, under the same external conditions, I could not figure out why the input signal of pin2 was different, so that pin1 lost the FAN carrier signal and became a near constant low level.”

    BR

  • 0 in reply to Ron Michallick
    Intellectual 400 points

    We did look the signal on the device legs directly.

  • 0 in reply to Tommy
    TI__Guru**** 151256 points

    Tommy,

    If you have a board with the voltages shown in the excel file, then keep it intact until this problem is found.

    Use a DMM , not scope, to get more accurate DC values. At minimum get the DC voltages for the 5 green nodes. 

    With enough data points , finding the reason becomes easy.