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TPS2121: Schematic review

Part Number: TPS2121

Please review the schematic and resistor settings for TPS2121 and provide the feedback. The resistor calculation also attached below.

Both inputs are 20V, Input1 is DOCK station and Input2 is from VBUS (USBC). The default power path is DOCK if the dock voltage reaches less than 17.5V the output switch to Input2 which is VBUS from USBC. We are using automatic switchover mode. The current through the mux will be 2.25A continuous in both input cases (ours is 45W system). 

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Let me know if you need more info to review the design.

Thanks

Mani

  • Hi Mani, 

    Could you clarify your intention with the output to input resistors? Are they no connects?

    The nature of a muxing system is to reduce cross conduction between sources. In your proposed configuration, there will always be a conduction path between Vin and Vout. This increases the potential for damage given a floating Vin signal. 

    The following truth table illustrates what conditions are needed to operate in XCOMP mode. As your voltage dividers currently stand, the switchover where PR1 and CP2 intersect will occur when Vin1 drops to 17.5V (assuming Vin2 is held at 20V). This however cannot be assured in this current configuration.

    The device is always responding to its inputs. When the device switches from one supply to another, the unselected supply will slightly rise in voltage as it is unloaded and the loaded supply will slightly drop in voltage. The PR1 and CP2 comparator is quite sensitive and the bouncing of the VBUS line is quite extreme which changes the PR1 and CP2 comparison over and over again. This results in a situation where the device keeps switching between the inputs.

    This situation can be avoided by adding a hysteresis resistor from ST to PR1, further detailed in the following document: https://www.ti.com/lit/ug/tidue50/tidue50.pdf

    I think you may also need some input capacitance on the VBUS line to help maintain the voltage. This is because during fast switchover, there is no soft start or current clamping and your output capacitance is quite large. This means that an inrush will flow through the device which could cause the input voltage to dip noticeably.

    Best Regards, 

    Elizabeth

  • Hi Elizabeth,

    Thanks for the feedback.

    The output to input resistors are NC (not installed) and it will be for debugging purpose to bypass mux

    There will be a power path enabled between input to output all the time. Why the floating input has the chances of damage?

    If we are controlling the CP2 by MCU GPIO then the problem will be solved right?

    I have the resistor between ST and PR1 pins of recent schematic

    Thanks

    Mani

  • Hi Mani, 

    Yes that will work fine! 

    I don't personally see from the schematic where the ST and PR1 pins are tied but if they are tied with a resistor for hysteresis in a more recent schematic then that should work. 

    Best Regards, 

    Elizabeth