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TL720M05-Q1: Startup failure when output pulled negative

Mike Twieg
Intellectual 960 points
Part Number: TL720M05-Q1

Hello all, I have a project with positive and negative supply rails. The TL720M05-Q1 regulates the positive input supply from +7V to +5V, and a negative regulator regulates another input supply from -12V to -10V. There are some biasing resistors connecting the +5V rail to the -10V rail. I've noticed that if I apply the -12V input whithout the +7V input, the -10V rail starts fine, and pulls the +5V down to around -0.47V through some of those biasing resistors. If I then apply the +7V input, the TL720M05-Q1 will not start, and its +5V output rail will remain at -0.47.

If I apply the +7V input before the -12V input, then both LDOs start fine, but I need to be able to deal with both possibilities. I presume it's something with the TL720M05-Q1's internal reverse polarity protection, but the datasheet does not any details.

Please advise.

Mike Twieg

  • 0
    TI__Guru 55940 points

    Hi Mike,

    Some LDOs have difficulty starting from a negative bias.  There are some application workarounds that may help while satisfying your application requirements.  Normally I would suggest sequencing the negative rail after the positive rail; however, you already stated that your application does not allow for that.  Here are two other possible options:

    Very Respectfully,

    Ryan