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LM3480: Input to ground short at 9V for 5V version. Up to 2.5A.

Joe Carabetta
Prodigy 40 points
Part Number: LM3480

Hi Everyone, Im new to this forum so sorry if this is a repeated or obviously question.

I have designed a circuit with this regulator intended to take input from a 9V battery and regulate it to 5V for an attiny microcontroller.  I first tried testing just the LM3480 with the battery but it pulled >2.5A when connecting the battery with no load attached to the output.  The output voltage swings up and down and sometimes negative.  

I have attached two 0.1uF capacitors to the input and output, respectively, as shown in the data sheet.  I checked all of my connections for shorts and everything seems to be connected correctly. 

Is there something obvious I did wrong or didn't understand from the datasheet about this regulator? I've attached my circuit for the regulator below.

Thank you in advance!

  • 0
    TI__Mastermind 22610 points

    Hello Joe,

    I'm sorry to hear that you are having problems.

    Unfortunately, I'm not seeing anything out of the ordinary here. The PCB layout looks OK.

    Best that I can offer is to suggest that you remove the LM3480 from the PCB and try reconnecting the battery to see if the problem persists.

    Then remount the LM3480 with the 'OUT' pin isolated from the PCB. Only the 'IN' and 'GND pins connected, to see if the problem persists.

    Then, all the usual re-checks.

  • 0 in reply to Donald Jones
    Prodigy 40 points
    Hey Donald,

    It worked! i desoldered and resoldered the chip and it outputs 5V now. Weird. I'll check again to make sure.

    Quick follow up, I program the Attiny with a regulated 5V that is then applied to the output of the LM3480 while no power is connected to the LM3480. Is a quick fix to protect any reverse current through the output to ground is to put a diode across the output to ground? and does the characteristic of this diode have to be specific or can I use any old diode (zener or otherwise).

    Thanks again for the suggestion!
  • 0 in reply to Joe Carabetta
    TI__Mastermind 22610 points

    " ... to protect any reverse current through the output to ground is to put a diode across the output to ground?"

    I don't have any information on hand to answer this question, but I don't see how a diode from OUT to GND could add any protection without interfering with normal operation.

    I can order some LM3480 sample devices and bench test to determine what the OUT pin behavior might be in this condition. That will take at least one week based on the shipping times that I am seeing now.

  • 0 in reply to Donald Jones
    Prodigy 40 points
    I just saw somewhere else on a TI forum that this might be able to protect the LM2480 when 5V is applied to the OUT pin with no voltage on the IN. But I don't think that is necessary, I can probably test it myself. I have some extra. Thank you so much for the help though!