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LM3481-Q1: Resistor of Input RC filter burn down issue

Gary Jiang
Intellectual 980 points
Part Number: LM3481-Q1
Other Parts Discussed in Thread: LM3481

Hi team,

My customer find a failure on LM3481 board, and the resistance between VCC and GND is 1.9 ohm, and the 10 ohm resistor(R23) of RC filter in front of the Vin is burned down. Now they try to reproduce the failure, and they connect the VCC to GND. The 10 ohm resistor is burned down, and the resistor between the Vin to VCC is decreased from 3.4 Mohm to 5.87 kohm.

As I know, the VCC is derived from the internal LDO, does this LDO has a internal current limit, as the VCC is shorted to GND? If so, the resistor(R23) of the RC filter should not be burned down when VCC is shorted to GND, then why the R23 is burned down?

The second question is, If I connect 10V to VCC to simulate the EOS of VCC when the board is normally working with 0.9A load, will this cause the 10 ohm resistor burned down?

Similarly, if I connect a negative voltage beyond the absolute maximum rating, will this cause the resistance between VCC and GND decrease to a very low resistance?

Your answer will help me a lot, thank you!

Gary

  • 0
    TI__Guru 68495 points

    Hi Taoran,

    Thank you for posting. Yes the VCC regulator has a current limit.  However, if you ground the VCC, then all input voltage can appear across the LDO and the losses can be excessive to cause IC damage first, then R23 damage. 

    Regarding the 2nd question: you should not connect 10V or any external voltage rail to VCC pin.  It is an output pin not an input pin, so it does not accept external voltage or the IC can be damaged. 

    If you connect a negative voltage beyond the abs rating at any pin, your can cause the IC to be damaged.  This is why the abs ratings should not be exceeded.

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    TI__Guru 68495 points

    May I assume the issue is resolved and I can close this thread?  You are welcome to re-open this thread by adding a new post or creating a new thread.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Intellectual 980 points

    Hi Youhao,

    Thank you for your effort. Now customer is waiting for FA test, then they will decide to do further test.

    You can close this case now.

    Thanks again

    Gary

  • 0 in reply to Gary Jiang
    TI__Guru 68495 points

    Thank you Gary.  We can discuss again if needed.

    Again, thank you for promoting our product.

    Regards,

    Youhao