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LM3478: Voltage of FB pin beyond the limit of datasheet

chengdp
Prodigy 70 points
Part Number: LM3478

Hi e2e:

We have tested the voltage of FB pin. The peak value is -0.57V, less than -0.4V limited in the datsheet. And the over voltage  time lasted for 3ms. 

Could you please help to judge the risk?  Which part of the  device might be damaged? 

Besides,how could we revise our design to avoid the potential risk?

  • 0
    TI__Guru 55765 points

    Hello Chengdp,

    Did you measure the pin voltage referenced to the IC GND pin? This is essential, as the IC pin GND is the reference for this pin.

    If you really have such a negative voltage on the FB pin, I recommend adding a Schottky diode from GND to FB to limit the negative voltage.

    Where does this negative voltage come from? It is best to make sure it is not created before trying to clamp it.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 70 points

    Hello Brigitte:

    Did you measure the pin voltage referenced to the IC GND pin?

    Reply:Yes，you can see the the result from the below graph, the yellow one is just the voltage between PIN FB and GND. It accurred when the IC was power down. Could please telll us was there any risks?

    And if possible please help to check our design .Thanks!

  • 0 in reply to chengdp
    TI__Guru 55765 points

    Hello chengdp,

    What happens when you remove C6442 and C4057? Why do you need these components?

    In general if you are getting out of the abs max rating, degradation or damage to the device is possible.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 70 points

    Hi  Brigitte:

    C6442 works as a feedforward capacitor to reduce output voltage ripple at light loads.When we decreased the C6442 to 1nF, it seems become normal just like the below graph. So for your advice, shall we remove C6442 and C4057 ?  And could you please provide some reference circuit for us?  Thanks!

    Best regards,

    Chengdp

  • 0 in reply to chengdp
    TI__Guru 55765 points

    Hello Chengdp,

    If it works when reducing only C6442 and the ripple at low load is still within your requirements, I do not think that you need to change your external circuit further. Please just check stability with some load transients at different input voltages to make sure that the lower capacitor does not cause any stability issue.

    Best regards,
    Brigitte