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LM25149-Q1EVM-2100: Modified evaluation board failed during testing

Chris Poulsen
Prodigy 30 points
Part Number: LM25149-Q1EVM-2100
Other Parts Discussed in Thread: LM25149

Tool/software:

I am trying to figure out what I did to kill my LM25149-Q1EVM-2100.

I want to use the LM25149 as a battery charger for a 12 Volt SLA battery.  For that I need an output voltage of up to 14.7 Volts.

I used the Quickstart calculator to work up the design but wanted to verify that with the evaluation module.

I first changed the output capacitors from the existing 10 Volt parts to 22uF 25 Volt parts.  I then verified that the module still worked as original.

I calculated that a 8.25 K resistor in parallel with the existing 19.1K (R22) should give me the desired voltage.

Rather than put that value in directly, I chose to raise the voltage slowly with a 50K pot in series with a 6.8 K resistor.  This pair was connected to pins 1 and 4 of the J2 header.

I connected the board to my bench supply that was set to 24 Volts with a current limit of 1 Amp.  The output was connected to an electronic load set at 1 Amp.  

On power up, the output came up to a level a little higher than 5.0.  I then turned the pot and the voltage rose as expected.  But, before I got even close to my target, the output voltage fell to zero and the bench supply went into current limit.  The board would not work even after power cycling the board without the external resistor.

I’ve studied the schematic and BOM for the board and couldn’t find anything else that should be sensitive to the higher output voltage.

What am I missing?

 

 

 

 

 

  • 0
    TI__Guru 50120 points

    Hello,

    Chris Poulsen said:
    I first changed the output capacitors from the existing 10 Volt parts to 22uF 25 Volt parts.  I then verified that the module still worked as original.

    So during this point the EVM is okay.

    Chris Poulsen said:
    Rather than put that value in directly, I chose to raise the voltage slowly with a 50K pot in series with a 6.8 K resistor.  This pair was connected to pins 1 and 4 of the J2 header.

    Was the device regulating and EVM okay at this point as well? 

    Chris Poulsen said:
    On power up, the output came up to a level a little higher than 5.0.  I then turned the pot and the voltage rose as expected.  But, before I got even close to my target, the output voltage fell to zero and the bench supply went into current limit.  The board would not work even after power cycling the board without the external resistor.

    So you were changing the pot resistance live and then experienced a failure here.

    The moving part here is the pot resistor and manually tuning of the bottom resistance. There may have been a EOS event during this live testing that could damage the FB pin or circuit given the nature of live tuning. 

    Since you know your desired Vout regulation point, you can change the bottom resistor to that fixed value and check regulation.

    This removes potential user EOS and error from the setup.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 30 points

    Thank you,

    I thought that it was probably a transient from adjusting the pot live, but thought I'd ask while waiting for repair parts about any other possibilities.

    I expect that I will have more questions as I adapt this to my needs.  Should I add them to this thread, or start a new one?

  • 0 in reply to Chris Poulsen
    TI__Guru 50120 points

    Hello,

    I suggest starting a new thread to address the other concerns here.

    Regards,

    Jimmy