• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 65 subscribers
  • Views 587 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: Fast switchover time

Ivy Jin
Expert 4625 points
Part Number: TPS2121

Hi team,

My customer use TPS2121 in their new project.

According the datasheet, the fast switchover time is short and can reach 5us.

However, the customer actually measured the switchover time of 30-40ms, and the output voltage has a drop.

Below is the schematic, could you help review and give some suggestions? 

Thank you very much!

Regards,

Ivy

  • 0
    TI__Genius 13300 points

    Hello Ivy,

    In order to correctly diagnose the issue occurring here we will need more scope shots. If you can set the ST pin up it can help provide a better understanding and quickly resolve the issue. Can you please provide scope shots including PR1, CP2, Vout and the ST pin? The scope shot will also need to be in higher resolution of the switchover event. 

    Regards,

    Kalin Burnside

  • +1 in reply to Kalin Burnside
    TI__Expert 4625 points

    Hi Burnside,

    Thanks for your comments!

    The customer has found that the voltage drop occurs when a low voltage switch to a high voltage.
    It is also described in the datasheet that this situation increases the switching time.
    The customer attempted to adjust the current limit resistance to 18.7K, the current limit threshold was much higher than the load, and the CP2 was also connected directly to a 3.3V or 1.8V supply.
    The test found that switching from low voltage to high voltage is still more than 40ms, so is there any other way to reduce the switching time here?

    The following figure shows the VOUT drop waveform when IN2 is disconnected while both IN1 and IN2 are powered.
    The customer has fixed the CP2 to a high level of approximately 3.3V, and the CP2 has dropped simultaneously due to a VOUT drop.

    Could you please help give some suggestions?

    Thanks for your support!

    Regards,

    Ivy

  • +1 in reply to Ivy Jin
    TI__Genius 13300 points

    Hello Ivy,

    Can you please ask your customer to redo this scope shot focused in on the start of the switching event? Perhaps ~400uS/div.

    The reason I am asking for this scope shot is because I'm seeing odd behavior of your PR1, CP2 and Vout signals and since this switchover event should be taking place under 10uS a 20mS/div will not show the details clearly. 

    Also, how is the ST pin connected in this case?

    Regards,

    Kalin Burnside

  • 0 in reply to Kalin Burnside
    TI__Expert 4625 points

    Hi team,

    Happy holiday!

    The customer is currently testing as shown in the schematic below.
    The test waveform(400us/div)
    The test waveform(10ms/div)
    Thank you!
    Regards,
    Ivy
  • 0 in reply to Ivy Jin
    TI__Mastermind 32650 points

    Ivy,

    Looking at your setup you are trying to perform fast switchover between two 12V signals but you are using CP2 to toggle between the inputs. It also looks like the CP2 signal is dropping very slow when it is asserting low. There is no internal pulldown resistor and so I would recommend the use of an external pulldown. Finally I would recommend using hysteresis to avoid any bouncing between the inputs cause by this slow discharge of CP2.

    Please refer to the FAQ on fast switchover for more detail: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1023453/faq-tps2121-how-do-i-achieve-a-seamless-power-switchover-using-ti-s-power-mux-devices 

    Thanks,

    Shreyas