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LMZ12002 unexpected output voltage changes

Carlyn Loncaric
Prodigy 20 points
Other Parts Discussed in Thread: LMZ14202, LMZ12002

Hi,

I am using both the LMZ14202 and the LMZ12002 Simple Switchers to output 12V and 3V respectively. I'm currently powering the board using a wall adapter but I will eventually be powering from a 16.8V battery pack. The 3V regulator is powered from the 12V regulated output.

I have calculated RFBT and RFBB using the datasheet equations and double checked my results using TI's online calculator (WEBENCH). I'm a bit confused however since the equations in the datasheet do not take the input voltage into account and suggest resistors be chosen between 1 and 10k. But the WEBENCH suggests larger resistors whose values change with a change in input voltage. 

What I'm seeing is that as I change the wall adapter output the 12V regulated output changes as does the 3V regulated output. When I check the voltages on an oscilloscope however the voltages are steady (no noticeable ripple). My concern is that once I'm powering from a battery, since the battery's output will fluctuate, so will all of my voltage rails. The switchers are supposed to maintain a stead voltage are they not?? (within reason, input voltage varying by 1.5V). I'm using the suggested capacitor value and voltage ratings, what else can I do to ensure a steady voltage? How come the datasheet calculations are different from WEBENCH?

Thanks in advance for any suggestions!

  • 0
    TI__Genius 16025 points
    Hi Carlyn,

    I'm looking into your question and will have a detailed response in a few.

    Thanks,
    Anston
  • 0
    TI__Genius 16025 points
    Hi Carlyn,

    I tried this on the bench and couldn't see the issue you described. Would it be possible to see your schematic and/or layout?

    When the LMZ's are connected to the wall adapter what is the output of the wall adapter? If the wall adapter is 12V, then you might be running the first regulator in the dropout region which is why you are seeing these issues.

    What is your battery configuration and capacity?
    What will be the min and max values of the battery when discharged and fully charged respectively?
    Finally, how many output rails will you be powering up and what are the voltages and currents you desire?

    I can help you more effectively when I know these parameters.

    Thanks,
    Anston
  • 0 in reply to Anston Lobo
    TI__Genius 16025 points
    If your schematic is confidential you can send it directly to anston [*at*] ti [*dot*] com
  • 0 in reply to Anston Lobo
    Prodigy 20 points

    Hi Anston,

    I am SO sorry for the late reply!! The email notifications were going to my junk so I had no idea you had responded! I should have just checked the site.

    But yes, you were correct. My wall adapter was not performing as I expected it to, so the voltage being supplied to the input was not what it should have been. I've since switched adapters and things seem to be working as expected. Thank you so much for your feedback it helped me realize the problem!!

    Thanks!

    Carlyn