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LP8861-Q1: About "Headroom Adaptation"

Junya Kato
Expert 1635 points
Part Number: LP8861-Q1

Hello Team,

Is it possible to calculate the time of "Headroom Adaptation"? Because it changes with load, is it difficult to calculate?
The set value of Vboost is 24V and it stabilizes at 19V. The current at 19V is 335mA.

Best regards,
Kato

  • 0
    TI__Prodigy 670 points
    Hi Kato,

    I am not quite sure your question? Could you please clarify the target LED current value for output current through ISET pin (pin 10)? And what the meaning for time of "headroom adatation"?

    Best regards,
    Xianghao
  • 0 in reply to Xianghao Meng
    Expert 1635 points

    Hello Xianghao-san,

    I'm sorry I didn't explain it enough.
    "Headroom Adaptation" is the part surrounded by the following red circle. I am checking whether time to stabilize can be calculated.

    In addition, Riset=24kΩ is used, and it is configured to connect 4 outputs together and output 400mA.

    Best regards,
    Kato

  • 0 in reply to Junya Kato
    TI__Prodigy 670 points

    Hi Kato,

    There are several factor for this time period.

    1. VOUT voltage initial value, you could calculate the Vout initial based on equation in datasheet page 14.

    2. Target Vout voltage is depending on the VF voltage of LED string, but it normally depends on the temperature.

    3. VOUT voltage divider resistor value on FB pin.

    It will be a complicated equation. I think overall it's quite short time period around tens of ms. Could you please advise why you want to calcuate this time period? Thank you.

    Best regards,

    Xianghao

  • 0 in reply to Xianghao Meng
    Expert 1635 points
    Hello Xianghao-san,

    It takes about 14 seconds until the voltage stabilizes at "Headroom Adaptation" now.
    My customers are monitoring the voltage of the LED and asking questions as we want to estimate the timing to start the monitor.

    Best regards,
    Kato
  • 0 in reply to Xianghao Meng
    Expert 1635 points
    Hello Xianghao-san,

    Can it be said from a design perspective that it will stabilize in tens of ms?
    Is it possible to take 14 seconds?
    If yes, please tell me the mechanism that may take time.

    Best regards,
    Kato
  • 0 in reply to Junya Kato
    TI__Prodigy 670 points

    Hi Kato-san,

    I understand your questions and below is my answer.

    Because of the adaptive headroom loop control, the voltage on the out pin voltage couldn't be always stable value during the operation. I think it takes 14 seconds to be relevant stable because the LED forward is depending on tempeature. In early few seconds after startup, the LED was heated up due to the self heating, so the VF of LED is dropping until thermal balance. So the heardroom votlage will be stable for a while.

    In the other side, if the LED is in PWM dimming conditions, the OUT pin voltage will be square waveform. So the voltage can't be always stable. I expect I answer your questions.

    Best regards,

    Xianghao

  • 0 in reply to Xianghao Meng
    Expert 1635 points
    Hello Xianghao-san,

    Thank you for your logical answer. It was very helpful.

    Best regards,
    Kato