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TPS54061: Mode Change within the 4mA to 20mA operating Range of the system

Part Number: TPS54061

We are attempting to use the TPS54061 in a power supply for a 4/20mA loop device.  We are finding that around 4.8mA the part changes in operating efficiency.  The change has proven difficult to model on the digital side of the system.   Does anyone have any thoughts as to why TI would recommend a part for a 4/20mA application and not provide a stable operating profile through the 4-20mA range of the power supply.  Is this a known issue with a fix?


  • What is the operating frequency and inductance value in your design? This part operates in diode emulation mode to improve light load efficiency. Would that be the reason? Please provide more details of the input voltage, output voltage, frequency, L, C to help us figure out the reason. Please share your schematic for review.
  • Attached is a schematic showing the usage case for the power supply.  The distortion in the system occurs around 4.7mA on the primary side (Loop) of the system.  I believe the issue is associated with the mode change under light load. Problem is the mode change is happening above 4mA. If the change occurred at less than 4mA it would not be a problem.


    Dean McDaniel

    Aquadyne Computer Corp.

    858 663-1400


    2248.Schematic Prints.pdf

  • Since the mode change is at 4.7mA and it is pretty close to the <4mA preferred by this application. I would suggest to change the inductance of the L101 to shift the mode change current level.

    I am not sure this part the frequency change threshold is Tonmin or Ipeakmin. It could be either a smaller inductance will shift the threshold down or a larger inductance, depending on the control strategy.

    I would suggest you try a 120uH and a 82uH inductor, to see which one can move the mode change threshold below 4mA.