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LP8860 VSYNC Usage

Hi team,

I have a question for LP8860 VSYNC pin. In datasheet, we can know it is used with VSYNC and HSYNC signal. I am confused when to use HSYNC and VSYNC. And how to choose the bandwidth of loop filter and why.

Regards

Michael

  • Hi,

    VSYNC pin input works as a reference clock for internal PLL. Internal PLL can be driven from internal oscillator or external clock(VSYNC).

    The reason there is VSYNC input is to synchronize internal timing with frame rate or line rate(HSYNC) of display to avoid waterfall effect which happens by interference between LED backlight and LCD display. In old days, LCD characteristics were not as good as current LCD, so the waterfall effect was bigger, but most of our LED driver have phase shift function which shift the phase of each LED string, so resulted LED freq becomes much higher. This improves waterfall effect as well as the improved LCD design itself.

    Please refer to this picture how external timing signal is used.

    And this is the table for filter component selection.

    Filter components only needed when VSYNC input is used. Otherwise, it can be open.

    Hope this helps,

  • It seems picture attachment didn't work well. I will try again.

    This is the first one above for PLL control diagram.

    And these are second and third for filter components.

  • Hi Yoon,

    I have a question for NTC sensing part.
    When a 24k resistor is used in Iset, I calculate the Iled according to the datasheet. But Iled I get is uA scale, rather than mA scale. I can see someone has the same question with me. e2e.ti.com/.../1927255
    So I wonder if anything wrong with datasheet or something I misunderstand.

    Could you please send me the calculation tool, too. Thanks!

    Regards
    Michael
  • Hi Michael,

    Do you want to set I_LED with external resistor or use external temperature sensor for temperature degrading?

    If it's for former, you can use this formula. 12 bit register setting for current, 3 bit scale, 4 bit correction factor can be ignored if you only use Riset for LED current setting. Just use 3000*Vbg/Riset

    For example, if 24kohm is used, 3000*1.2/24 = 150mA

    If you are talking about Iset at NTC description, it is pull down current to detect current setting, not I_LED itself, so the value is uA scale.

    Hope this helps,

  • Yoon,

    Thanks. I am talking about the Iset in NTC description. And I want to use RISET to limit the max current to 120mA now.

    I am a little confused. I though the IC detect the NTC voltage and get temperature information. Then control the current and lower down the current to cool down the screen. So I don't know the relationship between Iled and Iset in NTC description. Because the Itemp is very small, just uA scale,  it affects the Iled not very obviously...

    like others said,

    ///////

    Iset = 1uA,

    Itemp ≈ 40uA.

    and ILED = (1-40)uA + 150mA ≈150mA,

    I don't know how to make connection between this calculated result to the knee point.

    Even though the temp is 110deg, according to the formula, 

    Iset = 1uA,

    Itemp ≈ 51uA,

    and ILED = (1-51)uA + 150mA ≈150mA,

    This is not as the current de-rating curve.

    ///////

    Could you please kindly tell more detail about it?

    Regards

    Michael

  • Yoon,

    I almost got your point. Could you tell me more about pull down current ? And its relation ship with real I_LED?

    Regards
    Michael
  • Hi,

    Iset = Vbg/(50*Riset)*1000 : Riset in kohm, Iset in uA

    as shown in datasheet and max current will be

    max current setting from current scale * LED_CURRENT_CTRL[11:0]/4095

    If you don't have any preference of NTC function, I think PWM mode derating(shown as below) by NTC will be simpler and straightforward.

    Current mode derating using external resistors and NTC is more complicated and derating characteristics are changing with NTC parameters which is hard to support various NTC options.