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LM3431 type 2 faults occur <urgent>

Lucifer
Expert 2600 points
Other Parts Discussed in Thread: LM3431

Dear Sir,

we uses LM3431 for 3ch LED driver (9/9/8+znear diode). and we found the znear diode vf will increase to 4.2V in worst case or high temperature. it causes  LM3431 enter type 2 faults occur (auto restart ) and the LEDs blinking . after checking the CFB pin the voltage is 1.38V  (the typical shoule be 1.7V) . May I have your advise which protected Mechanism is?  is it current unbalance caused the problem ? Thanks,

  • 0
    TI__Prodigy 130 points

    Hi,

    LM3431 faults can be triggered in different ways. This could also be an OVP condition if the values for the AFB pin is not selected properly. Could you either send me the detail schematic or monitor SC and AFB pin voltages when fault is being triggered.

    Thanks

    Vahid

  • 0 in reply to Vahid Yousefzadeh
    Expert 2600 points

    Dear

    Please let me explain our experiment.

    We try to replace the Zener  by a resistor then the problem is sort out. It doesn’t have any problem like using Zener.

    We thought it was OVP protection and increased OVP threshold but it was not work at all.

    In addition,  I have checked the CFB pin the voltage is 1.38V  (the typical should be 1.7V) .  So we think CFB should be the root cause of this issue, could you kindly give us your opinion, Thanks.

    please have a look at the attached file waveform.

    e.pptx

  • 0 in reply to Lucifer
    TI__Prodigy 130 points

    Hello,

    Thanks for sending the waveforms. Let me see if I understand your results correctly. The waveforms you sent both for the case of LED operating in normal or in fault condition, shows switching of the LED strings. Is that because of the dimming you are applying?

    I still think that the cause is probably the SC or something else other than CFB. The reason that CFB does not regulate at 1.7V is most likely because of the fault that occurred in the system. For me to be able to help you I need: a- the detail of your circuit and also more waveforms, b- I would like to see the waveforms at the last cathode of the LED string (after the diode connections to SC and CFB pins), and the DLY pin. Also, it would be good if you wouldn't apply any dimming and capture the moment that fault occurs (DLY pin voltage reaches the threshold 2.8V) and the switch node waveform is disrupted.

    Best regards

    Vahid

  • 0 in reply to Lucifer
    TI__Prodigy 130 points

    Hello,

    I just wanted to follow up and see if you still need our help on resolving this. If you are concerned about putting sensitive information on e2e. We can connect directly and then you could send me detail of your circuits. For that you could send me your email address. Please let me know how I can be of further help.

    Regards

    Vahid