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LP8863-Q1: Calculation Sheet

Ting Ye
Genius 11800 points
Part Number: LP8863-Q1

Hello Team,

Could you help me check if the calculations in below attached excel file of LP8863-Q1 external resistors such as RFB1, RISET, and Rsense, and OVP threshold are correct? 

It would help simplify design circuit process.

Thank you!

LP8863-Q1_Cal_V1.xlsx

Regards,

Ting

  • 0
    TI__Prodigy 600 points
    This thread has been assigned; someone will reply shortly.
  • 0 in reply to Rodney Jones
    TI__Intellectual 2180 points
    Hello Ting,

    I reviewed the equations and everything looks ok, only few comments:

    - Use VBG = 1.21V instead of 1.2V for more accurate calculations
    - RFB1 equation is not exactly equal to LED Vf / I_RFB1 as boost output level will be higher than LED Vf voltage since LEDx pins will have some headroom voltage (~0.6V typical) to allow proper adaptive boost operation. Usually when choosing external components, VOUT_MAX is first calculated and recommended as ~20% higher than max LED forward voltage. Once VOUT_MAX target is set, then RFB1 can be calculated to yield that boost level.

    Hope this helps.
  • 0 in reply to Salome Ramirez Carrion
    TI__Genius 11800 points
    Hello Salome,

    Thank you for supporting!
    May I confirm if my understanding is correct?
    With LED forward voltage max of 3.7V, if I put them in 12S configuration, the VOUT_MAX should be set to ((12 x 3.7) + 1.21)*1.2 = 54.7V?
    And the VOUT_MAX exceeds LP8863-Q1 output max spec and means LP8863-Q1 is not applicable here.

    Thank you.
    Regards,
    Ting
  • 0 in reply to Ting Ye
    TI__Prodigy 670 points
    Hi Ting,

    Below is my calculation result.

    If 20% Vout margin is required, VOUT_MAX = (3.7x12+1.05)*1.2=54.54V.
    If 5% Vout margin is required, VOUT_MAX = 47.7225V.

    So basically i think LP8863 is little bit marginal to support 12S application, please advise customer to reduce the LED number to 10S or choose LED with VF_MAX=3.3V .

    Best regards,
    Xianghao
  • 0 in reply to Xianghao Meng
    TI__Genius 11800 points
    Hello Xianghao,

    Thank you for helping discuss together. I will check if LED number can be decreased.
    We also think that using 20% margin is for typical LED forward voltage calculation since previously some LEDs may only have typical VF. And it has low probability to have 12 LED all with maximum VF.

    Thank you!
    Regards,
    Ting