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LM27402: abnormal start up

Shawn Nie
Genius 10070 points
Part Number: LM27402

Hi Team,

Here is Shawn, now customer meets a start up issue, below is the schematic, below waveforms are tested with no load:

1. Why have the output voltage before EN enable?

2. The input capacitor C6889 of EN is changed into 1uF, and the resistance is changed into R6400=2.2k. There is no return groove on the waveform, but the input voltage will drop as below, then it will start up normally. Why the input voltage is pulled low?

3. The capacitance of SS pin is increased to 56nf, and the output voltage has a overshoot of 2V. After several times ,  it will start up normally. 

4. The waveform with Vout = 3.3V is as below.

5. Now the IC with screen printing VMO4AE and VM04AF have above issues, VM09AD has no this issue. and I have tested the EVM as customer configured, can not see this issue, the IC screen printing of EVM is VM24AD.

Could you help to give some comments on those issues, thanks very much.

Best Regards,

Shawn Nie

  • 0
    TI__Expert 5910 points
    Shawn,
    In the order of your questions.
    1.The EN voltage of 1.1V from electrical table is the guranteed turn on level. The actual voltage could be lower.
    2.What is the input voltage source used? Is it current limited? It could also be hitting current limit on start up due to the inrush current to output caps.
    3. and 4. Please try it by changing R6400 to 1.65k. C6889 can be removed.The Vin has a very slow rise time so its better to let EN cross threshold after Vin has reached a decent value. In this case I picked 7V for Vin for EN to cross 0.8V. This would ensure Vin is up before part is turned on and should solve some of the issues you are seeing.
    5. The different screen printing refers to different batch codes. We can explore this further once you checked the above.

    Regards,
    Mathew
  • 0 in reply to Mathew Jacob1
    TI__Genius 10070 points
    Hi Mathew,
    Thanks for you reply. I will push customer to test 3 and 4. But I still have two questions:

    1.The EN voltage of 1.1V from electrical table is the guranteed turn on level. The actual voltage could be lower.
    In principle, it will not turn on until EN exceeding 1.1V, and why does the output voltage drop low during startup?

    2.What is the input voltage source used? Is it current limited? It could also be hitting current limit on start up due to the inrush current to output caps.
    The current is not limited.

    5. The different screen printing refers to different batch codes. We can explore this further once you checked the above.
    What is the meaning of each of the Numbers of the screen printing?

    Best Regards,
    Shawn Nie
  • 0 in reply to Shawn Nie
    TI__Expert 5910 points
    Shawn,
    The numbers refers to the date code,manufacturing site code etc.

    Regards,
    Mathew
  • 0 in reply to Shawn Nie
    TI__Expert 5910 points
    Which part are you using LM27402MH or LM27402SQ (package difference)
  • 0 in reply to Mathew Jacob1
    TI__Genius 10070 points

    Hi Mathew,

    We use LM27402SQ, just the package is different? What's the difference in performance?

    Best Regards,

    Shawn