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LP8863-Q1: LP8863-Q1 Reduced current limit

Tomohiro Nagasawa58
Expert 1570 points
Part Number: LP8863-Q1

Dear support team

There is reduced current limit in boost start up state.( 36P of datasheet)

I have questions. Please teach about below.

(1)What is the value of the reduced current limit?

(2) Is the reduced current limit operation constant current behavior?

(3) How long is the period during which reduced current limit is applied?

(50ms:PL FET SOFT START STATE+50ms:BOOST START UP STATE=100ms?)

Regards

Tomohiro Nagasawa

  • 0
    TI__Expert 4565 points

    Hi,

    Max boost current during boost start up(after PL soft start) varies with SW freq. Max current at 300kHz is ~1.5A ~1.7A, and max 5A at 2.2MHz.

    But this limit applies only up to ~90% of boost initial voltage. Once boost voltage reaches up to 90% of initial voltage, this limit is removed and boost finish rest of 10% ramp up quickly. Not timer based.

    Hope this helps,

  • 0 in reply to Sung Ho Yoon
    Expert 1570 points

    Hello Sung-san

    Thank you for quick response.

    If reduced current limit is applied during PL SOFT START state operation due to inrush current,

    Is it operation to limit current to constant current? or hiccup operation?

    Regards

    Tomohiro Nagasawa

  • 0 in reply to Tomohiro Nagasawa58
    TI__Expert 4565 points

    Hi Nagasawa-san,

    There is no reduced current limit during PL soft start as boost is not switching. Reduced current limit is applied only when boost is ramping after PL soft start period.

    Reduced limit during boost ramp up is constant value varying with SW freq. I don't have data for all SW freq though.

    Hope this helps,

  • 0 in reply to Sung Ho Yoon
    Expert 1570 points
    Hello Sung-san

    I'm sorry for lack of explanation.
    One concern is that whether it will rise without problems with a large inrush current.
    Is there any current limit operation during PL SOFT START operation?

    Regards
    Tomohiro Nagasawa
  • 0 in reply to Tomohiro Nagasawa58
    TI__Expert 4565 points
    There is no active current limit during PL FET soft start as boost is not switching as I mentioned, but input OCP is monitored by sense resistor to cut off the circuit if current is too high. In order to avoid high inrush current, don't use too much Cin and Cout. Just use recommended values from datasheet or apps note. I saw some cases very high inrush with more than 200uF Cout.
    Thanks,