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LP8860-Q1 Questions for large LCD panel

Martin Hintermann
Prodigy 30 points
Other Parts Discussed in Thread: LP8860-Q1

Hello,


I am looking for a backlight LED driver for a large LCD panel. On the one hand we need a very wide dimming range (where LP8860-Q1 seems to be perfect) and on the other hand the LED-strings have a forward voltage of about 90V.

LP8860-Q1 datasheet describes a "High Output Voltage Application" in chapter 8.2.3 but figure 61 says "Up to 60V". I don't see any reason for the restriction to max. 60V output voltage - if the MOSFETs, resistors, diode and capacitors have adequate ratings.

Did I miss something causing this limitation?

Our LCD panel has 8 LED strings. Are there any recommendations for "cascading" two LP8860-Q1 for optimized matching of currents of the 8 LED strings, synchronizing boost-converters, etc.?
Or is there a similar backlight controller providing 8 channels which I didn't find yet?

Best Regards,
 Martin

  • 0
    TI__Prodigy 460 points

    Hello Martin,

    you are right, here is no limitation. Please note - maximum boost output/input voltage ratio is 10:1. 60V application has been tested, we didn't test LP8860 with 90V output voltage. One possible issue - because of many LEDs in string, string voltage mismatching can be high.

    Best regards,

    Vladimir

  • 0 in reply to V.Trondin
    Prodigy 30 points

    Hello Vladimir,


    first tests with 90V went pretty good.

    Now we would like to build a sample batch of 50 boards (with 2 LP8860 each) but the ICs are out of stock everywhere...

    Best regards,

     Martin

  • 0 in reply to Martin Hintermann
    TI__Prodigy 460 points

    Hello Martin,

    can you send me yours contact information? Please use my address vladimir.trondin_at_ti.com (please modify address)

    I will try to help you. Do you want free samples or prefer to buy bigger amount right now?

    Best regards,

    Vladimir

  • 0 in reply to V.Trondin
    TI__Intellectual 1830 points

    Hi, Vladimir,

    I search LP8860 and find this topic. I have one similar question. Now my customer have one LCD panel with backlight as 43V, and the input is from 12V car battery directly. 

    I have some questions about LP8860 that need your help：

    1. As OUT1-4 pins are all 48V, 52V transient, must them use FIGURE 61 to get more tolerance, which add protection MOSFET? Then any any special requirements for these FET, like Vgs?

    2. Also, should we add voltage divider for FB pin to get 43V, or just connect directly? Similarly, if connect directly, should we use one zener like 48V for protection?

    3. Customer would use direct PWM dimming with PWM pin, and no MCU communication with LP8860. Will there be any problem? And do you have more application note that could share with me about this part?

    Thanks for your help.

  • 0 in reply to Rocson He
    TI__Prodigy 460 points

    Hi He

    Rocson He said:
    1. As OUT1-4 pins are all 48V, 52V transient, must them use FIGURE 61 to get more tolerance, which add protection MOSFET? Then any any special requirements for these FET, like Vgs?

    When LED voltage is 43V, no needs to have extra nFET for protection. Protection should be used only when voltage is above 48V.

    Rocson He said:
    2. Also, should we add voltage divider for FB pin to get 43V, or just connect directly? Similarly, if connect directly, should we use one zener like 48V for protection?

    Same here, when voltage is below 48V - just connect directly.

    Rocson He said:
    3. Customer would use direct PWM dimming with PWM pin, and no MCU communication with LP8860. Will there be any problem? And do you have more application note that could share with me about this part?

    I recommend to use indirect dimming, when output PWM frequency is  higher - for example, input frequency can be 100 Hz and output PWM frequency 4.8 kHz. With 4 channel and phase shift equivalent frequency for boost load will  4x4.8kHz=19.2kHz -> ceramic capacitor noise will be outside audible range.

    Many valuable features are available for indirect dimming only.

    And customer can use this mode without any serial interface like i2c or SPI - just EEPROM should pre-programmed properly (can be done in factory). All internal compexity can be hidden from customer (see Figure 66 DS)

    We have internal LP8860 sharepoint site where can be found extra materials, if you want to have access - just send me a request.


    Best regards,

    Vladimir

  • 0 in reply to V.Trondin
    TI__Intellectual 1830 points

    Hi, Vladimir,

    Thanks for your quick response.

    I will send the request to you in Email and might discuss more details about this part.

  • 0 in reply to V.Trondin
    Prodigy 150 points
    Hi, Vladimir,I have some questions to consultation.
    LP8860-Q1 is uesd to support the backlight, which has 4 strings and every string has 11LEDs. (90mA/channel, VLED=34V,typ.)
    But ,when the Input Voltage=8.5V, the backlight starts flashing.
    the EEPROM setting is:60 0061 4F62 DF63 D164 DF65 1D66 7067 7768 7769 716A 3B
    6B 006C 0A6D 206E B06F 8470 C571 F372 E573 E374 3575 0676 DF77 FF78 3E
    when the flashing happens, read the register is:Read SPI 00 - 00
    Read SPI 01 - 00 Read SPI 02 - 00 Read SPI 03 - 4F Read SPI 04 - 00
    Read SPI 05 - 00 Read SPI 06 - 00 Read SPI 07 - 00 Read SPI 08 - 00
    Read SPI 09 - 00 Read SPI 0A - 00 Read SPI 0B - 00 Read SPI 0C - 00
    Read SPI 0D - 5F Read SPI 0E - 00 Read SPI 0F - 08 Read SPI 10 - 00
    Read SPI 11 - 00 Read SPI 12 - 10 Read SPI 13 - 04 Read SPI 14 - 6D
    Read SPI 15 - 04 Read SPI 16 - 7F Read SPI 17 - FF Read SPI 18 - FF
    Read SPI 19 - 80 Read SPI 1A - 00
    The above results show that BOOST_OCP fault happen，but I do not know how does it happen.
    Could you help to explain it ?Thank you very much.