• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 64 subscribers
  • Views 158 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

TPS92641: LED Shunt Dimming Issue

Sean Hung
Prodigy 90 points
Part Number: TPS92641
Other Parts Discussed in Thread: LM3409HV, , , LM3409

Tool/software:

Dear respected TI experts,

I have implemented a LED driver with shunt dimming feature by using LM3409HV. It works well with high dimming performance.

Now I would like to upgrade the LED driver for higher current application. However, because of the issues of the component heating and efficiency, I have to use  Synchronous Buck topology. Therefore, TPS92641 is my first candidate.

To evaluate its performance, I tested the TPS92641EVM board. Based on my test results, the dimming curve is roughly linear, but it becomes unstable and not smooth in the middle range (5%–65%).

According to my experience with LM3409HV, This issue happened before, but it can be resolve by using the below circuit from its datasheet (LM3409HM page 17):


I understand that the values of ROFF1, OFF2 and COO are very critical, if these values are calculated and selected well, then this dimming issue can be resolved.

Now my question is: Can this circuit be applied to the TPS92641? Since the TPS92641 does not have a COFF pin for OFF-time programming, I would like to confirm whether a similar solution can be implemented.

Thank you for your time! Any opinions or suggestions are welcome!

Sean

  • 0
    TI__Mastermind 27805 points

    Hi

    Our expert is out of office due to Chinese New Year, he will reply you on 2.5

  • 0
    TI__Mastermind 22830 points

    Hi Sean,

    I am currently out of office for Chinese New Year Holiday. I will reply to you on Feb.8 when I am back to office. Thanks for your understanding.

    Best Regards,

    Steven

  • +1
    TI__Mastermind 22830 points

    Hi Sean,

    Sean Hung said:
    Now my question is: Can this circuit be applied to the TPS92641? Since the TPS92641 does not have a COFF pin for OFF-time programming, I would like to confirm whether a similar solution can be implemented.

    You cannot, and also do not need to, implement a similar solution in TPS92641.

    TPS92641 implements controlled on-time architecture, whereas LM3409 implements controlled off-time architecture.

    The architecture of LM3409 makes it necessary to add above additional circuitry if you want to use shunt dimming with it. If there is no such circuitry, when all LEDs are short, the C_OFF will never be charged up to the threshold that can end the off cycle until t_OFF-MAX (Maximum OFF-time, check Section 7.5 Electrical Characteristics from LM3409’s datasheet) is triggered. At this time, the output current is out of regulation in the switching-off period (In the switching-on period, the output current is still under the peak current control). You need the above additional circuitry to force the OFF timer to react earlier before t_OFF-MAX is trigged, which will make the output current still under regulation.

    With TPS92641, when all LEDs are short, the ON timer still works as expected thanks to the feedback (R_VOUT1, R_VOUT2, VOUT pin) from the output node. In the switching-off period, the output current is still under the valley current control. So TPS92641 does not have the problem of running out of control when all LEDs are short. Therefore, no additional circuitry like LM3409 is needed.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 90 points

    Hi Steven,

    Thank you very much for your explanation. It is very clear. 

    I truly appreciate you help!

    Sean

  • 0 in reply to Sean Hung
    TI__Mastermind 22830 points

    Hi Sean,

    You are welcome. I am going to close this thread. Please feel free to contact me again if you have any further question. Thanks.

    Best Regards,

    Steven