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TPS543C20: The function of the dummy resistor in the remote sensing pair

MINKYUNG KIM
Expert 1250 points
Part Number: TPS543C20


Hi team,

Customer is using TPS543C20 for 12Vin, 0.9Vout, Pout = 16W. For the remote sensing network, they directly sense the output voltage with the differential pair as below. One thing I want to ask is about the dummy resistor R74. I guess this resistor is not mandatory but used for keeping the level between RSP and RSN better. (Please correct me if any misunderstanding here.) Customer asked if it's okay with 150Ohm than 300Ohm. Could you see any difference on operation between 300Ohm and 150Ohm of dummy resistor here? Thank you for your kind explanation!

  • 0
    TI__Genius 17905 points

    Hi ,

    There should be no need for this resistor between RSP and RSN if they are using 0.9V reference and sensing output directly.

    Is there a reference circuit where they saw 300ohm between RSP and RSN being used?

    Regards,
    Kris

  • 0 in reply to Kris F
    Expert 1250 points

    Unfortunately, there's no reference circuit that I have. I was just requested to check if the dummy resistor 300Ohm can be replaced by 150Ohm on the customer's board. I understand that there is no need for this resistor. But do you see any potential issue if the customer uses this resistor?

  • 0 in reply to MINKYUNG KIM
    TI__Genius 17905 points

    I don't see a major issue with having the resistor there, but note that there will be 6mA of current flowing in the feedback path (0.9V / 150ohm). This will create a voltage drop based on the trace resistance in the remote sense lines and may reduce the output accuracy a little bit. For example, if the round trip trace resistance of the remote sense is 0.2ohm, then there will be an output error of 1.2mV (6mA * 0.2ohm).

    I am still curious why they believe they need this resistor there. Is the remote sense a differential trace on the same PCB, or is their remote sense going to a cable to sense a load off the board?

    Regards,
    Kris