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LM2738: Vfb pin - pulled above the absolute max voltage of 3V

Bond - James Bond
Expert 4910 points
Part Number: LM2738

Dear Power Experts,

 

On the LM2738YMY switching regulator part the absolute maximum voltage listed for the feedback pin is 3V. 

 

  • Is this truly a voltage limit not to exceed on the pin or is it to keep the current flowing into the pin at a non-destructive limit? 

  • If the current is limited by the top resistor in the feedback network could the part survive if the current was limited below a certain level?

  • This technique can be used on some circuits since the ESD structure attached to the pin can bleed away a small current without being damaged.

     

  • 0
    TI__Mastermind 35115 points

    You are correct. Typically there is an ESD structure on the pin which starts to conduct current first when the voltage exceeds the abs max rating. Exceeding the limit MAY be ok if there is a resistor in series sufficiently large enough to limit the current into the pin to a safe level. The save level of current and other potential risks of doing so is not something we have documented so of course the general recommendation is to not exceed the abs max voltage rating.

    Do you have some more details on a scenario you are thinking of where the FB pin voltage may exceed the abs max?

  • 0 in reply to Anthony F
    TI__Expert 4910 points
    Hello,

    Case is the output voltage can under certain circumstances be shorted to an external voltage up to 36V

    it is a 51.1k ohm resistor in series with the feedback pin. The worst case intended voltage that could be shorted to it would be 36V. The lower leg resistor for the voltage divider is 9.09k. In that condition the feedback node would see ~5.4V.

    So current into FB pin would be approx. 600uA..
  • 0 in reply to Bond - James Bond
    TI__Mastermind 35115 points
    If you are using the LM2738 in the typical configuration, shorting the output voltage up to 36 V would actually be a bigger concern for the SW, BOOST and VIN pins. Shorting the output to 36 V will pull the SW node up to 36 V which is above the 24 V abs max rating. Pulling the SW node this high would also most likely cause an overvoltage on the VIN and BOOST pins.
  • 0 in reply to Anthony F
    TI__Expert 4910 points
    I'll send more info via email
  • 0 in reply to Bond - James Bond
    TI__Mastermind 35115 points
    The discussion has moved to e-mail so I'm going to mark this as resolved to close this post.