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TPS544C25: Optimal feedback resistor position in the feedback path

Alexey Sivkov
Prodigy 30 points
Part Number: TPS544C25
Other Parts Discussed in Thread: TPS544B25EVM-681,

Could you please explain what is the best place for 50 Ohms resistors in the feedback path, close to TPS544C25 or close to the  point of load? and why?

  • 0
    TI__Guru 56925 points

     

    While is should not make much difference whether the loop injection point is close to the output or close to the IC, keeping the measurement points for monitoring the voltage at either side of the injection point will help keep the network analyzer results cleaner, so I would recommend placing the injection impedance matching resistor close to the output and using the output ground as the common ground point for the two AC coupled monitoring probes.  That should minimize the interference from the switching power-supply noise on the measurement.

  • 0 in reply to Peter James Miller
    Prodigy 30 points

    Hi Peter!

    Thank you for the answer, I think that I need to clarify my original question.

    In the document named “Using the TPS544B25EVM-681 and TPS544C25EVM-681” the recommended schematic for feedback is:

    I was asking about this 50 Ohm resistors. Why in the demo board they are placed closer to the load rather than DC/DC controller? And what is the best place for those resistors? if we have quite a long feedback path.

  • 0 in reply to Alexey Sivkov
    TI__Guru 56925 points

     

    My apologies for the misunderstanding.

    In the TPS544B25EVM-681 and TPS544C25-681 evaluation module, R13 and R15 are serving several purposes:

    1) They separate the VOUT and VOUTS+ nets and the GND and VOUTS- nets to ensure that the remote sense is kelvin connected to the load point and not inadvertently shorted to VOUT or GND at some other point along their return path

    2) They provide a controlled impedance to react with the differential filter capacitor, C5, to filter very high frequency noise on VOUTS+ to VOUTS- from getting into the remote sense and feedback signals.

    3) The limit the VOUT+ to VOUT impedance so that inductive loading on VOUT during a shut-down, such as an over-load condition with long wires leading to an electronic load during bench testing can not induce a low-impedance negative voltage on the VOUT+ pin, which could damage it.

    4) They provide a convenient matched impedance injection point for feedback and loop analysis using a network analyzer.

    To serve all 4 of those purposes, the TPS544B25EVM and TPS544C25EVMs both place R13 and R15 close to the load.  The only disadvantage to placing these  resistors close to the load is that it increases the net area of the VOUTS+ and VOUTS- nets that have higher impedance, which can make them slightly more susceptible to noise.

    In general, I would recommend customers follow this practice because the risk of the slightly increased net impedance is much lower than the risk that the kelvin sense lines are inadvertently shorted to VOUT and/or ground at some point other than the intended remote sense point is higher than the risk that noise injected into the feedback lines due to the higher impedance will interfere with operation.  However, if you are highly confident in your layout review process to ensure that the kelvin sense lines are not accidentally shorted to VOUT and or Ground along their return path to the IC, the resistors can be placed close to the IC.  Be especially vigilant when these remote sense lines change layers, as vias can easily connect to internal planes, especially ground, or are routed on internal layers that include ground pours.