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TPS2121: Custom design review

Michael Gilge
Expert 1720 points
Part Number: TPS2121
Other Parts Discussed in Thread: TPS23751

Hello,

would you be so kind to review our schematics before we start the layout of the PCB?

The circuit is an ideal ORing between adapter power for 12V and PoE-Power using the TPS23751.

Thanks and best regards,

Michael

  • 0
    TI__Mastermind 32650 points

    Hi Michael,

    I noticed that you are not using any priority and that there are is not enough voltage difference between the two input to run in a purely VCOMP mode. What may happen is the deice might get stuck in a switching loop between the two supplies as there will be a slight voltage droop when current is pulled from a supply.

    Since you are not caring for the fast switchover methodology I would highly recommend using the PR1 pin to set priority to IN1 to make sure that IN2 will only be used is IN1 is unavailable.

  • 0 in reply to Shreyas Dmello
    Expert 1720 points

    Hi Shreyas,

    thanks for looking into our schematic. I get your point and have modified the schematic.

    Also, I have decided that overervoltage protection is not really required because D500 is there.

    Please give it another look.

    BR

    Michael

  • 0 in reply to Michael Gilge
    TI__Mastermind 32650 points

    Hi Michael,

    Looking at your schematic i have a few comments.

    1. I see that you are now using PR1 to set priority. Do note that the way this works is that when PR1>Vref after input settling time, the device will switch inputs. What I fear is that because of the resistor divider network used PR1>Vref earlier than wanted which could cause a larger voltage droop. I would therfore try and use a resistor divider of 45kOhm to 50kOhm in R574 and 5kOhm in R570.

    2. Your soft start slew rate will be quite small. This is a sensitive part and 1uF is quite large. This will slow down your transition time quite a bit as you are using the VCOMP mode with priority. Is this acceptable? If not, I would recommend using a 100nF or even a 10nF cap in C573.

    3. The ILIM resistor will limit the current to 3.82A according to Equation 1 in the datasheet. For 2.4A, I would recommend a limiting resistor closer to the 2.5A value listed in the datasheet (44.2 kOhm)

    4. The OV pins can be grounded if you are certain that D500 will account for all overvoltage transients. What about any overvoltage transient from IN2?

    Let me know if you have any further questions.

  • 0 in reply to Shreyas Dmello
    Expert 1720 points

    Thanks Shreyas,

    I have changed the values according to your suggestions and also added OV-protection for the PoE supply rail.

    Do you see anything else that needs my attention?

    Best regards,

    MIchael

  • 0 in reply to Michael Gilge
    TI__Mastermind 32650 points

    Hi Michael,

    As of now, I do not see anything that sticks out. I noticed you decreased your OV protection on OV2 from 63.4kOhm to 59kOhm but unless you expect higher voltage ringing, this divider is OK for your voltage values.

    Please feel free to reach out if you have any further questions. Thank you for using TI Power Switches in your designs.

  • 0 in reply to Shreyas Dmello
    Expert 1720 points

    Hi Shreyas,

    thanks for your help.

    BR,

    Michael