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TPS40400: Vout reading through GUI is lower than voltage measure on Vsns+ - Vsns-

Part Number: TPS40400
Other Parts Discussed in Thread: USB-TO-GPIO,

Hi Team, 

during test on EVM, we find Vout reading through GUI is lower than voltage measure on Vsns+ -  Vsns- . Please kindly see below. 

test point on EVM : TP6 and TP18.  TP6 and TP18 is connected directly to Vout and GND through JP1 and JP2. 

GUI config 

BRs

Given 

  •  

    It is strange that the GUI reading is starting 30mV low and rising to 41mV high at 10A load current, but it is also odd that the output voltage is rising 70mV from no load to full load.

    Can you check your setup to make sure you are creating ground loops, possible between an oscilloscope or bench supply earth ground and the computer's earth ground through the USB-to-GPIO adapter?

    Can you measure the FB to GND pin voltages to see if that is steady at 0.6V (VOUT_TRIM = 0) or 0.75V (VOUT_TRIM = 300mV) to see why VOUT is rising 70mV over a 10A load and why READ_VOUT is rising 140mV over the same 10A load current?

  • HI Peter, 

    Thanks for your reply. 

    please kindly check below 

    a. all GND is connected together.

    b.  FB-GND voltage as below in 0.6V.  It seems stable. 

    BRs

    Given 

  • if Feedback is stable at 0.601V, let's follow the voltage back to see where something is going wrong.

    Can you measure DIFFO (it is available on Pin Test point 13 and test point 14.  Check both.  There is only 50Ω between them, so there shouldn't be any voltage difference, but if there is, it would be evidence of current draw from DIFFO, which could be contributing to the issue.

    If the DIFFO voltage looks good, measure the voltage across the pads of C10 (should not be populated) and C11 (which should also not be populated)

    Also measure the voltage from TP5 to TP17 to see what that reads.

  • Hi Peter, 

    I have found the reason why the difference changes with Iout. 

    But there is still 30mV difference between GUI and DIFFO-GND.  Is it normal? Thanks. 

    BRs

    Given 

  •   

    Yes, +/- 30mV accuracy is expected for the READ_VOUT function on the TPS40400.

    For greater READ_VOUT accuracy, they might consider one of our newer PMBUS devices with VOUT telemetry that offer higher resolution and greater accuracy.