• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 62 subscribers
  • Views 166 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.

LP8868V-Q1: Design Review

Malek Hattab
Prodigy 105 points
Part Number: LP8868V-Q1


Hello TI Support Team,

I am designing a backlight driver for a display that requires approximately 27 V @ 280 mA.

The desired input voltage range in my system was 5 V to 40 V. After reviewing several options, I found that the LP8868V-Q1 is suitable for my application. From the datasheet, I understand that the LED backlight is referenced to the input voltage, not the ground. Based on this, I designed a backlight driver to operate from 5 V to 36 V, with the OVP pin set to 63 V.

 

Design Parameters:

  • Input voltage: 5 V – 36 V
  • Typical output current (maximum brightness): 280 mA
  • Maximum designed output current: 500 mA
  • Switching frequency: 600 kHz
  • PWM dimming, with reserved components in case I decide to switch dimming methods after testing
  • Compensation network values follow the recommended design from the datasheet as a starting point; I amm currently deriving the transfer function and Bode plot for further verification.

 

The schematic is attached below.

Could you please review it and suggest any enhancements or improvements to help avoid potential issues before proceeding with manufacturing? If possible, I can also share the PCB layout privately for your review.

 

Thank you for your time and support.

Best regards,

Malek

Backlight Driver.pdf 

 

 

  • 0
    TI__Expert 5251 points

    Hi Malek,

    1. Regarding the maximum 63V for LED+ voltage and OVP voltage, it reaches the limitation of the recommended range, and we recommend leaving some margin. For the SW voltage, it may exceed 63V when the LED+ voltage reaches 63V. Additionally, please ensure that the maximum forward voltage of the LED string at 280mA is not higher than 27V.

    2. What's the meaning of "Maximum designed output current". Currently the maximum current is 280mA.

    Best regards,

    Suchen

  • 0 in reply to Suchen Dong
    Prodigy 105 points

    Hi Suchen,

    Thanks for your help!

    1. I was considering 65 V as the absolute maximum rating, but being more cautious is always better.

    I will set the OVP to 60 V, this leaves about 3 V below the maximum recommended voltage and 5 V below the absolute maximum rating.

    The 27 V @ 280 mA are typical values for maximum brightness. They mention that the backlight can reach about 28 V maximum, and these values are not confirmed yet as the backlight is newly customized.

    I initially thought of using an additional circuit (such as a voltage difference amplifier) to measure the voltage on the LED strip and feed it to the OVP input, but this would not protect the IC against high output voltage. correct?

    The main issue is that my board does not have a 5 V power supply, and space is limited to add one. However, it seems I’ll need to add it and use the LP8868-Q1 in boost topology to overcome all these issues. What do think?

    2. Yes, the maximum current is 280 mA, and the sense resistor is selected accordingly.

    However, I selected the inductor and diode to handle higher current(up to 500 mA)without overheating if I changed sensing resistor after testing.

    Thanks,
    Malek

  • 0 in reply to Malek Hattab
    TI__Expert 5251 points

    Hi Malek,

    Malek Hattab said:
    I initially thought of using an additional circuit (such as a voltage difference amplifier) to measure the voltage on the LED strip and feed it to the OVP input, but this would not protect the IC against high output voltage. correct?

    It's not recommended. Please use OVP pin as the LED+ protection of the device.

    Malek Hattab said:
    The main issue is that my board does not have a 5 V power supply, and space is limited to add one. However, it seems I’ll need to add it and use the LP8868-Q1 in boost topology to overcome all these issues. What do think?

    Sorry, I don't quite get your point. Where is the 5V power supply used for?

    Best regards,

    Suchen

  • 0 in reply to Suchen Dong
    Prodigy 105 points

    Hi Suchen,

    My system only requires a 3.3 V rail to operate, so there is no onboard 5 V power supply.

    I am considering adding a 6–40V to 5V, 3.5A buck converter onboard to power the LP8868-Q1 (boost version: LP8868UQDMTRQ1) from the 5 V rail, instead of directly from the input voltage.

    This approach would avoid limiting the input operating voltage of the device and provide flexibility in case I need to increase the voltage or current of the backlight. It would also allow the OVP to serve as protection for both the LED strip and the IC.

    Do you agree with this approach?

    Thanks,
    Malek

  • +1 in reply to Malek Hattab
    TI__Expert 5251 points

    Hi Malek,

    Understood. The LP8868X/U can handle a constant 5V input to produce a 27V output at 280mA.

    Best regards,

    Suchen

  • 0 in reply to Malek Hattab
    TI__Expert 5251 points

    You're welcome. I'll close this thread, but if you have any follow-up questions, feel free to ask them here.