• State TI Thinks Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 3 answers
  • Subscribers 63 subscribers
  • Views 1581 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS2121: TPS2121 reverse current and batteries

Petar Ivanov1
Prodigy 40 points
Part Number: TPS2121
Other Parts Discussed in Thread: LM66100, TPS2115A

Hello,

I’m trying to make hotswap battery scheme with TPS2121. Power consumption is around 20mA. My scheme is below. IN1 is general battery that will be used as main power supply (also will be charged). IN2 is backup battery that will be used only when the main battery is being replaced. That's why CP2 is grounded. BAT1 is Li-Ion 4.2V 4000mAh, BAT2 is Li-SOCl2 3.7V 1200mAh.

 

Power switching looks good and works perfect but there are two problems:

- When BAT2 voltage > BAT1 voltage there is charge current from BAT2 to BAT1. Also when BAT1 voltage > BAT2 voltage there is charge current from BAT1 to BAT2.

- From BAT2 I have constant discharge about 150-500uA. Is there a way for it to be reduced? Is Rilm affecting this current?

Thank you for your help!

  • 0
    TI__Mastermind 32650 points

    Hi Petar,

    Could you show us a scope shot of this reverse charging current? There should be no flow between the channels. Also can you please describe the situation in which you are seeing the 150-500uA discharge. Is this when VBAT2 is powering the output?

    By the way are you using the TPS2121 in VCOMP mode or are you setting priority. From the schematic you have set a PR1 reference but from your use case, you are using it as an OR-ing circuit?

    If this is a purely OR-ing circuit, I think a 2x LM66100 could be a better topology as your load currents and input voltages are quite low.

  • 0 in reply to Shreyas Dmello
    Prodigy 40 points

    Hello Shreyas, Thank you for the fast reply.

    I measure the charging current with multimeter. I will try to show you a scope. The datasheet says that Reverse Current Protection works from 0.2A. Is it right? This 150-500uA discharge are when VBAT1 is powering the output and VBAT1 < VBAT2.

    In the first iteration CP2 had voltage divider but it consuming from BAT2. Because of that I decide to put it down to GND. Can I set PR1 to VBAT1 without voltage divider?

    Thank you!

  • 0 in reply to Petar Ivanov1
    TI__Mastermind 32650 points

    Hi Petar,

    The reverse current blocking is specified from output to input. I do not think this is the case here. There is no situation in this device where both channels are ON.

    What I think you are seeing is the leakage current from IN to OUT. This can be anywhere from -500uA to 500uA over the operating temperature range on this device. This can occur whenever there is a voltage difference between INx and OUT. Please refer to the electrical characteristics table on the datasheet.

    What are your requirements for your MUX solution? Do you require priority or do you only care for diode OR-ing? 

  • 0 in reply to Shreyas Dmello
    Prodigy 40 points

    I want to set priority on IN1 and maximum battery save conditions for IN2.

  • 0 in reply to Petar Ivanov1
    TI__Mastermind 32650 points

    Hi Petar,

    Of the top of my head for your voltage, load and MUX requirements, I would suggest the TPS2115A.

    It is suited for the lower voltage range, it can set priority and has lower leakage than TPS2121. 

    Thank you for choosing TI Power Switches in your designs.

  • 0 in reply to Shreyas Dmello
    Prodigy 40 points

    Hi Shreyas,

    Many thanks due to your support we reach almost the full of our task. We manage to overcome the current leakage problem. The problem was in defective TPS2121. We made some changes in schematic but we found out a problem with out voltage drop under 2.4V. Our aim is to have maximum voltage drop to 2.8V due to minimum atmega128 VCC requirements.

    ldo_IN_OUT_wave_CSV.zip

  • 0 in reply to Petar Ivanov1
    TI__Mastermind 32650 points

    Hi Petar,

    This output voltage drop is during switchover correct? There are two ways you can get around this:

    1. Add more output capacitance which provides the energy for the downstream loads during switchover. This will increase the inrush current seen though.

    2. TPS2121 is able to perform fast switchover in XCOMP mode. This operation compares PR1 and CP2 voltage to trigger switchover. Please refer to Section 9.3.7 on the datasheet for more information on fast switching.

    Let me know if you have further questions.