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Varying the Vout on the LM3668

William Ham
Intellectual 260 points
Other Parts Discussed in Thread: LM3668

Is there a way of margining the output of the LM3668-4550?  I would like to vary +/-mVolts to +/- 10ths of a volt around 4.5V with Vin=3.3V+/-10%.  This would not be for normal operation - just for verification in my circuit.

One thought I had was to either vary Vfb or Vsel. If it can be margined - what is the maximum deviation around 4.5V that is achievable?  Again - I'm only looking to be able to margin <1Vdc.

Thanks in advance!

  • 0
    TI__Genius 14317 points

    Hi William,

    I looked over the Data Sheet and although its not real clear, it looks like Vfb can vary +/- 3% for either the high or low voltage selection (Vsel low or high logic level) modes. Its also likely that Vref to the error amp varies based on Vsel logic level (although not specified) and a resistor divider divides down Vfb voltage to the error amp as shown on the block diagram. So, it seems like a +/- 3% variation in Vfb should map to an allowed +/- 3% variation in Vout.

    I'll see if I can get confirmation from the LM3668 device engineers

  • 0 in reply to Jeffery Pfarr
    Intellectual 260 points

    Hi Jeffery,

    Thanks!  I look forward to their response.

    Best Regards,

    Bill

  • 0 in reply to William Ham
    TI__Prodigy 10 points

    Hi Bill,

     

    Jeff has it right.  The Vfb voltage is regulated to +/-3% accuracy, to the 4.5V can vary by +/-135mV. 

    If I understand correctly, you want to create these worst case condition via external stimulus.  You have access to the top of the resistor feedback network through the VFB pin.  By adding some resistance in series between the VOUT and VFB pins, you can increase the regulation voltage of the system.  I am not sure the values of the feedback resistors, so start small (5kohms) and slowly work your way up until you have the output voltage you desire.

    Note that adding the external resistor in series will degrade the performance of the regulator.  This change should only be used to "worst case" the regulation values to verify your circuit operation.

  • 0 in reply to Rich Dye
    Intellectual 260 points

    Rich,

    Thanks for the additional information!  Varying the external resistance looks to be a viable approach for the application.

    I think I have enough to go with for now.  If I have any further question I will post.

    Again, Thanks to ALL for your help and Happy Holidays!

    Best Regards,

    Bill