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LMZ31506 remote sense over long cable

Michael Schwager
Prodigy 40 points
Other Parts Discussed in Thread: LMZ31506, LMZ31506H

I'm using LMZ31506 to power a load at the end of a 25' (20AWG) cable, using remote sense to ensure the load is at the proper 5V (minus ground rise).

  • Are there any issues about stability over this length?  It's just a 25-pin bundled serial cable, so it's about 10mOhm/ft, 30pF/ft, 1nH/ft.
  • If the cable is unplugged (no SENSE+ feedback), what happens?  Should I add a local high-value resistor (like 10k) between VOUT and SENSE+ to ensure there is always something there?
  • The block diagram indicates this is a buck regulator, and all the numbers only show high voltage going to low voltage.  So just to confirm, this module cannot boost voltage, right?
  • Datasheet recommends to use open-drain to control INH pin, however I need this to be powered off by default, so I'd like to just put a pull down resistor, then use other 5V logic to turn this on.  Is there any reason this wouldn't work?

thanks

  • 0
    TI__Mastermind 28020 points
    Hi Michael,
    * Including the connector and the cable within the feedback loop will affect the stabilty of the device. It is recommended to connect the SENSE+ before the connector. You will need to verify proper operation if sensing after the connector and cable.
    * If the SENSE+ line is open, the power supply will be open loop and the Vout could rise to VIN. Placing a 10 - 50Ω local resistor between VOUT and SENSE+ will ensure the voltage feedback stays closed.
    * Yes, this is a buck regulator. It can only step down the voltage. No Boost.
    * The way you describe to control INH will work.
    Contact me directly if needed, jarrigo@ti.com. Thanks, Jason
  • 0 in reply to J Arrigo
    Prodigy 40 points

    Thanks for your response.

    I believe the INH pin would float as high as Vin, but only with a few uA of pullup current, correct?

    I wound up using an open-drain FET anyway, as the 5V logic I'm using is on a different supply from Vin, and it's possible that when Vin>5V, the INH pin would drive current back through the protection diodes on the logic gate, thus back-driving the supply, which ultimately goes back to an expensive National Instruments DAQ, so I'd prefer to zero out any chance of back-driving the supply.

    thanks

  • 0 in reply to Michael Schwager
    TI__Mastermind 28020 points
    The INH wil not rise to VIN. The voltage on INH will be 2.8V typical.
  • 0 in reply to J Arrigo
    Prodigy 40 points
    Hi, I'm looking at the "lmz31506h.pdf" datasheet SVN996 - JULY 2013 and it states on page 20, "Figure 30 shows the typical application of the inhibit function. The Inhibit control has its own internal pull-up to VIN potential. " Are you saying this is not accurate? It would save me a few components if you are correct.

    thank you
  • 0 in reply to Michael Schwager
    TI__Mastermind 28020 points
    The has an internal 2.8V (typ) pull-up when a valid VIN is present. It is not pulled up to VIN potential.
  • 0 in reply to J Arrigo
    Prodigy 40 points
    Great, thank you very much.