LTM4678 wrong or incorrectly-ranging current readings

When I program my LTM4678 outputs to +2.0V, I get a sensible small current from my open-circuit load.  When I then program it to >= +2.1V, suddenly it says it is drawing > 2.5A, which can't be correct for a small linear step and no load.  Changing bit 7 (and also tried with bit 1) of MFR_PWM_MODE  (Range of ILIMIT and HI/LO Voltage Range) makes no change/improvement.  Please if you an suggest what configuration I might have wrong.  Possibly related, I didn't understand what to do with the table of mV/IOUT_CAL_GAIN values for the IOUT_OC_FAULT_LIMIT: MFR_PWM_MODE[7] is not supposed to be changed while an output is enabled--am I supposed to predict what range I am going to be reading from my load before it is even turned ON?  Or, is the table just for when I am setting my current-limit and not related to the live readings of the current?  (if yes, then it makes sense I could program the range bit at the same time as setting my current limit register). 

In summary, it is implied there is some ranging/configuration needed for the live/run-time current draw, and I am not sure the configuration/procedure to follow to get the correct readings.  The LTM4675 and 76A modules on my board do not seem to have this behaviour.

Thanks!

  • 0
    •  Analog Employees 
    on Feb 10, 2019 7:41 PM

    Hello,

    At no load condition, the real output current would be zero. In LTPowerPlay GUI, the current reading can display slightly values.

    Is the LTM4678 used as dual output or single output dual phase? What is the date code of parts, numbers display in bottom of the right side?

    Can you share your schematic for us to review?

    MFR_PWM_MODE bit 1 and bit 7 enable your choice for high/low voltage range, high/low current range. They will not affect the output current and current read back.

    Table mV/IOUT_CAL _GAIN is used only for over current protection setting and it will not change fact of the output current values and the current read back as well. You do not have to calculate over current value as well since these value are internally calculated from LTPP GUI, under current tab, under output current, there are IOUT_OC_FAULT_LIMIT values in unit of ampers for selection.

    Thank you.

  • Thank you for clarifying that the range bits are only for controlling the current-limit trip-point accuracy.  Date code of my parts is: 1812MY.   I am attaching schematic fragments for your review.   (note in schematic, removing Q18 and 19 as an experiment, there still is the high current going from Vout 2.0 to 2.1V for open-circuit load). 

     Thanks!

  • 0
    •  Analog Employees 
    on Feb 20, 2019 12:06 AM in reply to sio2usa

    Hello,

    Can you remove the R147 and R148 to see if the current read back value decreases?Or anything connected to the output voltage except the output capacitors?

    For LTM4678, output current read back would have some offset at no load conditions.

    Do you know what is the IOUT_CAL_GAIN value in the LTPP shown below highlighted area? is it around 0.85mohm?

    Thank you.

  • 0
    •  Analog Employees 
    on Feb 20, 2019 12:31 AM in reply to sio2usa

    Also, can you let me know if it is at room temperature or other temp condition?

    Is the reading 2.5A or -2.5A at 2.1V?

    Thank you.