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CH1 and CH2 Asynchronous opening

Lin_Charles
Expert 4260 points
Part Number: TPS92518
Other Parts Discussed in Thread: LM5122,

Hi all,

    One of my customer use TPS92518 to control 8 series (CH1) and 12 series (CH2) LED. Each LED has 3 V voltage drop. They use LM5122 to boost 12V to 50V. And then using 50V as input voltage of TPS92518. But these two LED series have asynchronous opening time, the CH1 will be on first and then CH2 will be on later. Not clear about the time interval, but people can feel this change. Can you help look into this issue? Thank a lot!

  • 0
    TI__Mastermind 25500 points

    Hello Charles,

    I am not sure I understand what you mean by "asynchronous opening time". Do you mean channel 1 lights up before channel 2? Are both channels running the same LED current? Do you have a schematic for this?

    Thanks,

    Clint

  • 0 in reply to Clinton Jensen
    TI__Expert 4260 points
    Yes, you are right. The CH 1 will light up first. They have the same current. The voltage is not the same. I do not have the schematic. Do you have some idea about this?
  • 0 in reply to Lin_Charles
    TI__Mastermind 25500 points

    Hello Charles,

    If they have the same amount of output capacitance and the same LED current it will take channel 2 longer to charge the output capacitor high enough for the LEDs to start conducting. If that is a correct assumption then you could use less output capacitance for channel 2 so that it charges faster. If no output capacitance is being used then something else must be going on that I am not sure about. Let me know.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    TI__Expert 4260 points

    Hi Clint,

       Thank you fro your quick response.  I do have the schematic see picture below. They have the same output capacitor (20 uF). I am curious about why the output capacitor will cause this issue. For the charging time is very short, will people notice such change?

  • 0 in reply to Lin_Charles
    TI__Mastermind 25500 points

    Hi Charles,

    The difference in charging time would be a couple hundred microseconds and I am not sure if that would be noticeable or not. Do you have information on what the time difference is between the two lighting up? The original post didn't specify. If they are enabled at the same time there should not be a noticeable delay between the two. If it is more than the delay caused by the output capacitor charging then it must be a delay in the enable commands between the two. You should be able to enable both with a single command though.

    The only other thought I have is that the 10k from MISO is supposed to be terminated at the uC VCC power supply, not the 518 VCC. Perhaps it is pulling down on VCC2 enough that is causes a delay? You could try and reconnect that resistor to the uC supply to see if that makes a difference.

    Regards,

    Clint

  • 0 in reply to Lin_Charles
    TI__Mastermind 25500 points

    On second thought MISO should for sure not be connected to the 518 VCC as the voltage would exceed the MISO pin abs max. I'm not sure if that might have anything to do with anything or not.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    TI__Expert 4260 points
    Thanks. I will check the detail with customer and see how this things goes.
  • 0 in reply to Lin_Charles
    TI__Expert 4260 points
    Thanks, the customer change the output capacitor and solved this problem.