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LP8866-Q1: Please review the schematic

Chase Jeong
Mastermind 7615 points
Part Number: LP8866-Q1

Tool/software:

Dear TI experts,

My customer made a schematic using LP8866-Q1. Could you review it?

LP8866_Q1_SCH.pdf

And they have some questions ;

- My customer wants to set switching frequency to 20MHz.

- Customer does dimming control using 200 KHz PWM frequency from deserializer. (customer will also test with 20 KHz PWM frequency)

- If customer sets direct PWM mode, 200 KHz PWM frequency for dimming affects to 12V main power supply. It is shown as noise on 12V power.

1) Is it normal operation? (noise on 12V power)

2) If my customer use phase shift mode, Can I avoid this noise problem?

Please check this issue. Thanks.

Best regards,

Chase

  • 0
    TI__Expert 3651 points

    Hi Chase, Our expert will reply to you later. Thanks for you patience.

  • 0
    TI__Mastermind 31695 points

    Hi, Chase

    Let me double confirm your requirement, so you want a 20MHz switching frequency and 200kHz PWM dimming frequency?

    20MHz is higher than our maximum switching frequency; 200kHz is also a little higher than our target.

  • 0 in reply to Zhiqian Wu
    Mastermind 7615 points

    Dear Zhiqian,

    Thank you for your support.

    There are some mistakes, Switching frequency is 2.0MHz or 2.2MHz, and PWM dimming frequency is set by external AP, but they can control it. (maybe 20KHz)

    And they decided that they use phase shift mode first because they do not see any noise on 12V power.

    So, I want you to review the schematic first, Is there any points to improve performance?

    And here are some questions;

    1. Customer does not want to use aluminium capacitor because it is big and end customer does not like it.

    Anyway, they want to replace aluminium capacitor to ceramic capacitor but the problem is rated voltage.

    Is it okay to use 25V rated voltage capacitor instead of 50V for VIN/VOUT lines?

    2. Could you check the values of resistors on LED_SET(P14) and ISET(P32) pins?

    Currently LED_SET(R223) is set to 124Kohm and ISET(R226) is set to 31.6Kohm.

    Backlight power supply current is typical 1000mA, supply voltage is typical 18V.

    Please check these issues. Thanks.

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Mastermind 30671 points

    Hi Chase,

    Thanks for your feedback! Our expert Zhiqian will give you reply soon. Thank you for your understanding!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Mastermind 7615 points

    Dear Lucia, Zhiquan,

    Please let me know if there are any updates about the issue above. Thanks.

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Mastermind 30671 points

    Dear Chase,

    Zhiqian will give you feedback once he has any update. Thank you!

    BRs

    Lucia

  • 0 in reply to Chase Jeong
    TI__Mastermind 31695 points

    Hi, Chase

    As you combine the all six channel together, and turn on at the same time. It will see a large ripple, and it leads to the audible noise. Depending on the waveform, it is 200Hz PWM frequency. Something you can do to reduce the noise, one is increase the switching frequency to 20kHz. 

    For your question:

    • The voltage rate is related with the maximum voltage on your input and output. 25V is not enough normally.
    • For the resistor value, it is OK.
  • 0 in reply to Zhiqian Wu
    Mastermind 7615 points

    Dear Zhiqian,

    Thank you for your support.

    Here is a question about capacitor from my customer.

    Last time I asked about rated voltage of capacitor, and you said that 25V is not enough.

    Then, Can I reduce total value of each capacitor circuit in red rectangle below?

    If I can, What is the recommended minimum value of each capacitor circuit?

    Please check this issue. Thanks.

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Mastermind 30671 points

    Hi Chase,

    Our expert Zhiqian will get back to you early next week. Thanks for your understanding!

    BRs

    Lucia

  • 0 in reply to Chase Jeong
    TI__Mastermind 31695 points

    Hi, Chase

    Yes, they can reduce some cap, we doesn't have a detailed value of minimum output cap. Depending on my experience, 33uF electrical cap + 2 * 10uF is fine from my side. for the input side, it is the pi filter, normally we will use it for better EMI performance.