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TPS2121: Please review my circuit

user5834448
Prodigy 140 points
Part Number: TPS2121

Hi team,

I would like to achieve the following condition.

* IN1 = 20V / 3A(max)  ,  IN2 = 19.5V / 3A(max)

* If both inputs are valid, IN2 is selected.

* If IN2 is valid after IN1 is valid, output will switch from IN1 to IN2 at around 17.55V with fast switching mode.

* Vout-min >= 16.7V   (I'm worried about this condition especially.)

Please review my circuit whether it satisfied  the above conditions or not.

Also, please give me some advice if you have some concerns.

Best regards,

Kensho

  • 0
    TI__Mastermind 32650 points

    Hi Kensho,

    Why do you have 19.5V on IN2 and 20V on IN1 but then prioritize IN2? 

    Do note that fast switchover is only in the XCOMP modes of operation so by grounding PR1, fast switchover will not take place.

    I am looking through your setup as I am troubleshooting your previous issue: https://e2e.ti.com/support/power-management/f/196/t/827475

    Apologies for the delay. 

  • 0 in reply to Shreyas Dmello
    TI__Mastermind 32650 points

    Hi Kensho,

    I recommend revisiting this design where the inputs are exchanged and to use PR1 to set priority rather than using CP2. I would also recommend using a smaller SS cap. Please look at this post for my reasoning.

    https://e2e.ti.com/support/power-management/f/196/p/825271/3061663#3061663

    Thank you for using TI Power Switches in your designs. 

  • 0 in reply to Shreyas Dmello
    Prodigy 140 points

    As you review my circuit on https://e2e.ti.com/support/power-management/f/196/p/825271/3062175#3062175, I redesigned my circuit. Please check my circuit again. I would like to achieve the following condition.

    * IN1 = 19.5V / 3A(max)  ,  IN2 = 20V / 3A(max)

    * If both inputs are valid, IN1 is selected.

    * If IN1 is valid after IN2 is valid, output will switch from IN2 to IN1 at around 17.55V with fast switching mode.

    * Vout-min >= 16.7V   (I'm worried about this condition especially.)

     


    Best regards,

    Kensho

     

  • 0 in reply to user5834448
    TI__Mastermind 32650 points

    Hi Kensho,

    I would like to reiterate that you need to reduce the SS capacitance. A longer SS will increase your switchover time. Fast switchover is only achieved with PR1 and CP2 larger than Vref (XREF operation mode). I would recommend 10nF or lower. 

    So you cannot ground CP2 and have fast switchover. This schematic would result in prioritizing IN1 as soon as its connected irrespective of the voltage on IN2. Without a voltage on CP2, you also cannot set a switchover voltage for the device. CP2 being grounded will result in IN1 being selected as long as it is valid which means all voltages between UV and OV.

    The overvoltage protection is set to trigger between 21.39V to 23.76V taking into account Vref variance. Is this acceptable?

    Finally I want to ask you to reiterate what you mean by

    * If both inputs are valid, IN1 is selected.

    * If IN1 is valid after IN2 is valid, output will switch from IN2 to IN1 at around 17.55V with fast switching mode.

    These points are contradictory. This is two different behaviors expected at the same state (both inputs valid). Could you please clarify?

  • 0 in reply to Shreyas Dmello
    Prodigy 140 points

    Hi Shreyas,

    Thank you for your kind support.

    Sorry for my misunderstanding. I exchanged to 10nF as SS capacitance.

    I can accept the current setting of the overvoltage protection. Because the range of the input is 18.5V<IN1<20V and 19V<IN2<21V respectively.

    I would like to achieve "If both inputs are valid, IN1 is selected". (It means that IN1 has priority.)

    Also, I'm worried about Vdip ( = Tsw*(Iout/Cout) ) during switching output because I can allow the output from 16.5V to 21V in my system. That's why I'm nervous about switching time (Tsw) too.

    Please let me confirm the following case.

    IN2 is valid and selected at first. At the time, CP2 is higher than Vref (CP2 is not grounded). And then, IN1 is valid and PR1 is higher than Vref and CP2. In this case,output will switch from IN2 to IN1 with fast switching mode when PR1 is higher than CP2. Is my understanding right?

    Datasheet (excerpt): When CP2 is pulled high, this enables fast switchover and is compared to PR1. If PR1 > CP2 then IN1 is used, and if PR1 < CP2 then IN2 is used.

    Best regards,

    Kensho

  • 0 in reply to user5834448
    TI__Mastermind 32650 points

    Hi Kensho,

    If you need to use both PR1 and CP2 to run the TPS2121 then CP2 cannot be grounded as in your schematic above. (grounded through R106)

    You are correct in worrying about the output voltage dip and adding capacitors to maintain the voltage but remember that too much inrush current can cause the device to hit  thermal shutdown. 

    You also may need to change the resistor divider on the PR1 pin to ensure that the voltage on PR1 is larger than CP2 when PR1 is 19.5V and CP2 is 20V. In the case that both PR1 and CP2 is used, care has to be taken when deciding these voltages as not only to they set priority but also are compared to one another to make the switch. 

    I would recommend three major things to keep in mind.

    1. Both PR1 and CP2 should be sufficiently higher than Vref.

    2. The voltage on PR1 rises above the voltage at CP2 at the correct time for switchover. For example: you do not want a resistor divider network that causes PR1>CP2 when IN1 is only 16V.

    3. Even though you want the fast switchover time, this may not happen due to the fact that there is an input settling time when Inputs are coming online. (Refer to Section 9.3.1 in the datasheet)

    Now that I see your application involves priority and autoswitching with two similar voltages I recommend taking a look at http://www.ti.com/lit/ug/tidue50/tidue50.pdf. This is a TI reference design that perform seamless switchover for two similar voltage sources and a priority to IN1. This document can form a better basis and understanding of seamless switchover operation in your application.

    Thank you for choosing TI power switches in your design. 

  • 0 in reply to Shreyas Dmello
    Prodigy 140 points

    Hi Shreyas,

    Sorry for the late reply. I had a national holiday.

    I redesigned my circuit around TSP2121 (Fig.1). Please check my circuit again. Each parameter is decided based on Tbl.1. Tbl.2 shows the functional behavior in my system.

    Even though I wanted to add an external precision voltage reference IC like reference circuit, I almost fix my circuit pattern design and can't change pattern anymore. However, I already added a voltage detector beforehand. I think it makes me help to achieve my requirement.

    If you have any concerns or advice, please tell me.

    Fig.1:

    * IC100 is a voltage detector with output delay. The IC can detect 4.5V (=-Vdet). When the supply voltage is higher than released voltage (=+Vdet), the output is Hi-Z. If not, the output is grounded.

    Tbl.1

    Tbl.2

    Best regards,

    Kensho

  • 0 in reply to user5834448
    TI__Mastermind 32650 points

    Hi Kensho,

    I hope your holiday went well!

    I have not seen a voltage detector IC used in a similar situation so please take into account any delays(propagation or intentional) into consideration. By your description of the IC, I understand that is holds the PR1 node to 0V until the input voltage is around 17.5V to 17.9V. At this moment, the PR1 node is not grounded and the voltage is 2.77V. The voltage on CP2 is held at 1.81V which will cause the mux to switch over from IN2 to IN1 when IN1 is around 17.5V to 17.9V.

    The voltages on OV1 & OV2 look good. The ST pin is pulled up correctly and ILIM is set at 22kOhm which is compliant with your current requirements.The only addition I would make is to increase the input capacitance on IN1 and add input capacitance on IN2. 3A is a larger load current so during switchover (and with 100uF output capacitance), you might see a supply droop but this is not necessary and is dependent on your supplies.

    Overall, this operation should be fine for the TPS2121.

    Thank you for choosing TI Power Switches in your designs!

  • 0 in reply to Shreyas Dmello
    Prodigy 140 points

    Derar Shreyas,

    Thank you for checking my circuit.You have concerns about input capacitance.

    I already use 10uF on the side of IN2. IN2 is supplied from USB and is allowed up to 10uF for input capacitance according to the USB specification. On the other hand, I added a 10uF capacitance on the side of IN1 too.

    I hope my circuit will go well. Thank you for your kind support in this time.

    Best regards,

    Kensho