• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 458 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: Slower than expected switching time

Randy Moore
Prodigy 20 points
Part Number: TPS2121

Dear TI support forum,

We are currently designing a power management system using the TPS2121 IC. The goal is to design a board that can control whether or not power is being drawn from ground systems (an external power source) or an on board battery pack. When ground systems power is disconnected the board should be able to seamlessly switch to the battery pack.

Our first test involved setting up the IC as shown in page 25 of the datasheet, Automatic Switchover with Priority (XCOMP).

Here is a summary of our test results:

  • Using the input voltage of 12V and a simulated load of 2A, we achieved a switching time of 50 microseconds. Resistor values were not exact, but achieved a switching voltage very close to the desired value. We recorded switching at 7.75 V, while the expected value was 7.6V

  • When one input is set to 12 V, and is then one is then suddenly disconnected, we observe a voltage drop to zero and then a ramp up, with a switching time of 207ms. This is done without any capacitor to smooth the output voltage. The ramp indicates that the current limiter is working. 

  • Overvoltage protection works well: When the input is increased past the designated value, the output switches to 5.0V

  • Had trouble reducing switching time: modified the SS (pin 11) by reducing capacitance to 100pF. To further minimize the switching time, we increase the slew rate by reducing the resistance of ILM output (Pin10) 17.6 kOhm. This produces a switching time of 624 microseconds.

Our current problem is that the switching time is far too slow and the output has a significant dip while switching. We have tried buffering the output with a 220uF capacitor to ground but it only seems to slow the switching time.

  • 0
    TI__Genius 13760 points

    Hi Randy,

    Welcome to E2E!

    In the first test it seems that the total time to do the switchover from 12V to the battery voltage, which I assume to be 7.5V, took 50us. The 5us switchover time described in the datasheet occurs once PR1 drops down lower than CP2, so that timing would depend on the values of the resistor divider used.

    Are you using the EVM of the TPS2121 to  run these tests? If so, could you share the jumper configuration you are using? If not, would it be possible to share a schematic?

    In the second test it seems like the device is restarting. In both test 1 and test 2 you state that the input voltage is 12V. Its a unclear what the difference is between the first test where the circuit worked, and the second test where the TPS2121 seems to restart. I have a some question to help understand the difference better.

    1. In the second test you state the input was suddenly disconnected. Is this a physical disconnection of the cables? How was this test different from the first one? (i.e. in the first test did you use a power supply that you simply turned off?)

    2. What is the voltage of the battery?

    Best regards,

    Andy Robles