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LP8863-Q1: Loop Compensation ＠SEPIC

T Kishi
Genius 4425 points
Part Number: LP8863-Q1

Hi

My Customer has question.Please cooperate me.

LP 8863-Q1 is a device recognition that can be used in SEPIC configuration.
I confirmed using "Power Stage Deigner" whether SEPIC configuration is possible according to customer's specifications.
Please check attached file.

LP8863-Q1.pdf
When VIN = 4.5 V, "RHPZ" becomes 5.81 kHz.
I think that it is necessary to make the crossover frequency about 1 kHz from this result,can customer use LP 8863-Q1?
Is there a way to set crossover frequency etc?

Best Regard

T Kishi

  • 0
    TI__Expert 4565 points

    Hi Kishi-san,

    If Vin = 4.5V, is there any chance for Vout to be lower than Vin level? SEPIC mode will work for the case Vin > Vout, and max Vout will be limited to 24V.

    I think boost mode should be used for this apps condition.

    Usually, crossover freq is recommended to be <<0.3*RHPZ, and RHPZ can be changed by conversion ratio, Iout, and inductor values easily, so it's hard to calculate fc or RHPZ accurately.

    Another one is conversion ratio is too high. Even if boost mode is used, required conversion ratio will be 36/4.5 = 8. At this high load current condition, boost conversion ratio will be lowered do ~5. I guess 4.5V is just min voltage during cold crank condition, right? In that case, boost can trip OCP fault, and come back to normal operation after cold crank period with higher normal Vin level.

    This is the efficiency curve for Vin = 5V, and Vout =  18.5V(5 LEDs), 22V(6 LEDs), 25.6V(7 LEDs). Efficiency becomes worse at higher Vout which means high power loss turned to thermal issue on boost components such as SW FET, inductor, and diode.

    You can send me an email for more questions due to limitation of editing capability of E2E.

    Hope this helps,

  • 0 in reply to Sung Ho Yoon
    TI__Expert 4565 points


    Somehow, image file was not attached correctly.

    This is the efficiency curve image.

  • 0 in reply to Sung Ho Yoon
    Genius 4425 points

    Hi Sung

    Thank you for reply.

    I understand that it is quite a severe condition.
    The specification of Vf of the LED used by customers is 2.6 V to 4.0 V.
    We intend to use this LED in 9sires 6parallel.(BoostVout=23.4V~36V)
    Therefore, it can not be realized without SEPIC configuration.
    For SEPIC, maxVout = 24 V,
    In SEPIC maxVout = 24 V, can TI's solution be able to satisfy the following specifications?
    Also, why is it limited to MaxVout = 24 V with SEPIC configuration with LP8863-Q1?

    The following is described in Fetures. Is this a mistake?
    "Up to 47-V VOUT Boost or SEPIC DC-DC Controller"

    VIN:4.5V～35V
    LED Vf:2.6V～4.0V　(9sires 4parallel)
    ILED:130mA (6×130m=780mA)
    SWf:300kHz~500kHz or 1.8MHz~2.7MHz
    Dimming PWM:200Hz(0.01%～100%@0.01%step)

    If possible, could you tell me your e-mail address?

    Best Regard
    T Kishi

  • 0 in reply to T Kishi
    TI__Expert 4565 points

    Kishi-san,

    Max 24V is based on simulation and validation results during development stage. So any Vout level higher than 24V is not guaranteed.

    If DC level of Vin is 4.5V to 35V, using a voltage regulator will be recommended. Vin level is too wide to meet.

    My email address is sungho.yoon@ti.com if you have further questions.

    Thanks,