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Original question:

LP8860-Q1: EEPROM Configuration Support

LP8860-Q1: when input voltage decrease, the backlight blink

Qianqian
Genius 5010 points
Part Number: LP8860-Q1

Hello TI Experts,

I'm designing an LCD Backlight driver using the LP8860A

3 strings of LEDs, their are 9 LED in each string~ output voltage is 27V

As described in below chart, Fsw=300KHz，VDD=5V,  I have set LED_STRING_CONF[2:0] to 001~

I meet with a problem really really needs your suggestion:

When the input voltage is 13V,the backlight is normal~when input voltage decrease to 11V,the backlight will blink,when I increase the input voltage higher thn 11V,it will not blink

The 22uH inductor's Isat is 3A;Rsense is 25mΩ, I see fault register(0x0F) is 0x08, which means BOOST_OCP

I change Rsense from 25mΩ to 20mΩ，when I decrease input voltage,it'll not blink until 10.2V(when not change this,it will blink when input voltage is 11V)

I measure the output current of each string is 69mA.

Can you give me some advice on this problem?Thank you very much~

And I have done this test:

I use LP8860C,and connect I2C to GND,when powered on, I can measure VDD is 5V,EN is 3V, but the backlight is not working,I cannot figure out why~

  • 0
    TI__Prodigy 670 points

    For you flickering issue, I think boost OCP is triggered when you decrease the input voltage. There are couple of suggestions for you.

    1. Set LED_STRING_CONF[2:0] to 001. Please make sure you unlock the EEPROM before writing and burn the EEPROM after writing. You can refer to the apps note trough below link for EEPROM programming. You need to make sure the EEPROM is writen correctly first. 

    LP8860-Q1 End-of-Line Programming guide of EEPROM


    2. Please measure the VIN close to the inductor as much as possible. The real input voltage to the boost convert might be lower than the power supply output because of the voltage drop on the wire between power supply and boost converter.

    3. Check the inductor current by using the current probe to make sure the inductor is not saturated. I calcuated the peak inductor current, it shouldn't be saturated if the real VIN at inductor is over 7V. But please also measure the peak curernt of inductor to make sure it's not saturated.

     

    Best regards,

    Xianghao 

     

  • 0 in reply to Xianghao Meng
    Genius 5010 points
    Hello Xiaohao,
    Thanks for your reply~
    1. Yes, I have checked the register.I read the register,it‘s what I have written.
    2. Yes,the real input voltage is 10V, after 20mΩ resistance and diode their is a voltage drop to 9.5V
    3. When the backlight isn't blink,the inductance current is 1.2A, when it's blink,the inductance current is changing, the max will be around 2.5A
  • 0 in reply to Qianqian
    TI__Prodigy 670 points
    Hi,

    Based on current information, it's hard for me to have further suggestion. Could you please read out the EEPROM value for 0x70 address? we want to confirm if the current setup is correct.

    Best regards,
    Xianghao
  • 0 in reply to Xianghao Meng
    Genius 5010 points
    Hello Xiaohao,
    Thanks for very much~
    0x70 address value is 0xCE
  • 0 in reply to Qianqian
    TI__Prodigy 670 points
    Hi,

    Even the BOOST current limit threshold is 9A. I don' think it's triggered. but I think the Vout voltage is too low comparing to target value caused the BOOST_OCP triggered. But I don't know why output voltage can't reach target value. What kind of MOSFET you choosed for switching FET? And please share the schematic you designed for LP8860. Thank you.

    Best regards,
    Xianghao
  • 0 in reply to Xianghao Meng
    Genius 5010 points

    Hello Xiaohao,

    The mosfet I choose is FDD86581-F085

  • 0 in reply to Qianqian
    TI__Prodigy 670 points
    Several issues:

    1. LP8860A has to ultilize I2C for brightness control. But both SDA and SCL are short to GND. So how to control the brightness and read the EEPROm value?
    2. Please mount 10uF C124, even charge pump is not used, a 10uF capacitor should be connected to Cpump pin for gate driver.

    BEst regards,
    Xianghao
  • 0 in reply to Xianghao Meng
    Genius 5010 points
    Thank you for your prompt respond~
    1.I connet SDA and SCL ,pull up to 3.3V
    2.Yes，when I test , I have mount the C124
    Sorry for my not clear expression~
  • 0 in reply to Xianghao Meng
    Genius 5010 points
    Hello Xiaohao,
    The problem has solved~
    I change the boost mosfet then it works all right~

    From the datasheet, the requirment of boost mosfet is 60V 15A,
    I used to use FDD86581-F085,Now I change to BUK9Y53-100B, then it works well~
    I'd like to know besides 60V 15A,what should I take into account when choosing the mosfet?