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Original question:

LP8866-Q1: I have a question regarding the circuit design using the LP8866-Q1.

LP8866-Q1: Additional inquiries regarding LP8866-Q1

SANGYUN KWAK
Prodigy 20 points
Part Number: LP8866-Q1

Tool/software:

About 4 months ago, I inquired regarding driving LEDs using two LP8866-Q1 ICs.

  • Connection method:

  1. LED configuration: Anode 1 and 2, Cathode 1, 2, 3, 4, and Anode 1 and 2 are shorted together.

  2. LP8866-Q1 #1 drives Anode 1, Cathode 1 and 2.

  3. LP8866-Q1 #2 drives Anode 2, Cathode 3 and 4.

This is how it is driven (although it is not the recommended method).

Both ICs share the same 24V input power supply, and the total current measured on the 24V input equipment is about 600mA.
However, about 10% of the time, instead of 24V, a voltage around 0.6V is measured, and the current spikes to 1000mA to 3000mA.

Could this phenomenon be caused by shorting the two ICs’ Anodes together?
I would like to understand the mechanism behind this.

For reference, the EN pins are connected together and used simultaneously.
If the EN pins are separated and enabled with a time delay, would that improve the boost circuit error?

  • 0
    Prodigy 20 points

    And could this cause only the 4th LED string to experience overcurrent, with all the current flowing through just the 4th string?
    Only the LEDs in the 4th string become excessively bright, and thermal damage has been confirmed.
    Could this be caused by using the two LED driver ICs with their anodes tied together?

  • 0 in reply to SANGYUN KWAK
    Prodigy 20 points

    And more than 1000mA is sometimes measured on the -24V line as well.

  • 0 in reply to SANGYUN KWAK
    TI__Mastermind 31445 points

    Hi, SANGYUN

    Sounds like you see a overcurrent in the input supply. Pls kindly check with the below item:

    • For two LP8866, we don't suggest to connect the output together, pls separated two boost output and try again. The boost control logic will conflict if combine two output together.
    • If the input power capability 
    • Check if the output is connected to the GND and which fault is reported.
  • 0 in reply to Zhiqian Wu
    Prodigy 20 points

    Hello,
    We have made a design change.
    That is, even though we temporarily modified the concept from driving LEDs with two LED driver ICs to driving with a single LED driver IC, the same issues of voltage drop and overcurrent still occur.

    In our measurement under normal operating conditions, the current and voltage were 550 mA and 24 V respectively.
    However, in 1% of cases, we observed in the log that the system drives at 0.6 V with more than 1500 mA.

    Currently, we are using 90 mA per string in 4 strings.

    How can we improve this phenomenon?
    We are sharing the circuit diagram of the LED driver IC block for your review.

  • 0 in reply to SANGYUN KWAK
    TI__Mastermind 31445 points

    Hi, SANGYUN

    Could you pls kindly share the waveform for below test point when the current and voltage changed?

    Boost output/Input voltage/Input current and FB voltage.

    You description is fuzzy, we need waveform to do detailed analysis.

  • 0 in reply to Zhiqian Wu
    TI__Mastermind 31445 points

    Hi, SANGYUN

    And pls kindly check if the TVS is conducted during the test and leads the inrush current for the system.

  • 0 in reply to Zhiqian Wu
    Prodigy 20 points

    This phenomenon occurs intermittently during mass production, making it difficult to capture and measure the moment.
    Such behavior has not been observed during testing.
    It occurs with a low probability in mass production.

  • 0 in reply to SANGYUN KWAK
    TI__Mastermind 31445 points

    Hi, SANGYUN

    For the 1500mA, is it the input current or output current? And what your mean about the 0.6V, input voltage?

    You need to describe your performance more clear and better to share the waveform. 

    If it is the output current and output voltage, it means boost is short to GND.

    As you mentioned, it doesn't reproduce in the test, pls take care any action in mass production may short it.

  • 0 in reply to Zhiqian Wu
    Prodigy 20 points

    Yes, that’s correct.
    1500mA is the output current, and 0.6V is the input voltage.

    • Normal output current: 550mA

    • Abnormal output current: 1500mA

    • Normal input voltage: 24V

    • Abnormal input voltage: 0.6V

    Could such a phenomenon occur if a momentary short happens?

  • 0 in reply to SANGYUN KWAK
    Prodigy 20 points

    Additionally, if a short occurs,
    is it possible that excessive overcurrent happens in a single LED string?
    We always observe that only one string of LEDs becomes excessively bright and shows signs of overheating.

  • 0 in reply to SANGYUN KWAK
    TI__Mastermind 34575 points

    Hi

    our expert will check and update you later...

  • 0 in reply to SANGYUN KWAK
    TI__Mastermind 31445 points

    Hi, SANGYUN

    Depending on your description, one string see the short and leads to the overcurrent, pls kindly check with the LED panel connection to see what happened.