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TPS7A39: Multiple Devices with common NR/SS

Benjamin Bawkon
Expert 1100 points
Part Number: TPS7A39

Hello,

I'm working on a design that needs multiple +/- power rails (in my case, +/-2.5, +/-10 and +/-18V).  I intend to use TPS7A39 for each of the split rails, and I'd like to know if it's safe to connect all of the devices NR/SS pins together in order to accomplish ratiometric power-up sequencing across all rails.

A) Will this work as expected?

B) What impact might this have on noise performance?  Would the best practice be to connect all of the pins common, but still place a NR/SS capacitor at the pin of each device?

I should probably mention that I also intend to use 47nF feed-forward capacitors, in case that is of any interest.

Thank you!
Ben

  • 0
    TI__Guru 50090 points

    Hi Ben,

    To answer your questions:

    A) Will this work as expected? We have not tested this specific configuration but it should work as expected. The key is making sure the input rails are all up prior to enabling the TP7A39. A device like the TPS3847 might help in making sure the positive rails are up with a sufficient delay to have confidence the negative rails are within regulation as well. 

    B) What impact might this have on noise performance?  Would the best practice be to connect all of the pins common, but still place a NR/SS capacitor at the pin of each device? The NR/SS pin is the filtered Bandgap reference. this is a ~350K resistor from the bandgap reference to the NR pin which forms an RC filter. So whether you use one capacitor and three capacitors may be more a function of your layout. I would likely simply put each capacitor right at the NR/SS pin. 

     

    The use of a CFF capacitor will certainly help to improve both Noise and PSRR.

     

    I hope this helps to answer your question.