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TLV709: Negative GND and GND Input

Ken Kwack
Prodigy 300 points
Part Number: TLV709

Hello Team,

My customer has -25V line and want to make 3.3V as output. So I build below SCH but not sure it works or no. Could you review below SCH for me? Thank you!

  • 0
    TI__Mastermind 18466 points

    Hey Ken,

    This is not a configuration we recommend for our devices, as this can cause stability and reliability issues. 

    I am going to talk to a designer and see what input they may have for this schematic. 

    Best,

    Juliette

  • 0 in reply to Juliette
    TI__Prodigy 300 points

    Hello Juliette,

    please check with designer and if it can work or no. If not, Could you share the idea to make 3.3V from -25V? It is the display system so my customer has -25V line and need to make 3.3V. Thank you!

  • 0 in reply to Ken Kwack
    TI__Mastermind 32480 points

    Hi Ken,

    This second drawing you provided may work.  The -25V would be the RTN of the 3.3V rail.  It is very important that the system grounding diagram is reviewed and there are no issues with the system grounding when operating power supplies like this.  That is the only real risk you have here.  Otherwise the LDO will work just fine.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    TI__Mastermind 32480 points

    Hi Ken,

    Just to be clear, you cannot invert the power rail using a linear regulator.  You will need a switching converter to do that.  I assume you need to generate +3.3V relative to -25V.  Stated differently, you have -25V and GND coming into the LDO. You are then going to treat the -25V and GND as +25V and GND (by using the negative supply in a positive manner).  Then you will regulate that +25V to +3.3V.  This will have the net effect of regulating -25V down to -21.7V with respect to -25V.  But if you are using the -25V net as a new "GND" then the output of the LDO will look like +3.3V with respect to -25V.

    I hope this is clear. Let us know if you have any questions.

    Thanks,

    Stephen