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Original question:

LM3488Q-Q1: LM3488Q-Q1 output 350V

LM3488Q-Q1: LM3488Q-Q1

user6334892
Prodigy 40 points
Part Number: LM3488Q-Q1
Other Parts Discussed in Thread: LM3488, LM348

Dear TI team,

I have a question about LM348 series. I have read on your website, that LM3488 can have output voltage to 500V, but when I tried to put my requirements to Your Webench software, it has told me that maximum output voltage is 300V.

My goal is to reach 450V. Output current will be very low (max in the order of few milliamps). Input voltage will be 9V.

If You are able to help me solve it, or recommend another device, I will be very delighted.

Thank You very much,

Štěpán Šípal

  • 0
    TI__Guru* 76745 points

    Hi User,

    The LM3488 is actually a controller, and the output voltage would not affect the IC. Theoretically the LM3488 doe not have the max Vout limit.  The Webench model is based on exiting evaluation  board or a reference design, and it has limited component library, so it may not support your 500V but the IC itself should.

    The only limitations regarding Vout has two factors:  the max duty cycle and the peak current limit.  9Vin to 500Vout requires a duty cycle of about 98.2% if you want to design the circuit in CCM, but this sounds tough.  Instead, you can design the circuit in DCM, namely select a smaller inductor, to operate with a smaller duty cycle (recommend up to 60% to avoid excessive peak current).

    For Boost, the required duty cycle in DCM can be calculated as:

    D = sqrt[ 2x Iout x Fsw x Lm x (Vout - VIn)] /Vin

    Where Iout is your max load current, Fsw the switching frequency, Lm the power inductor's indutance, Vout the output voltage, and Vin the input voltage.  Use this equation to choose Lm.

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 40 points

    Hi,

    thank You for equation, it helps a lot.

    I just would like one further question. If operating in DCM, is it necessary to edit some other equations, or just everything else can stay the same as in CCM. In datasheet is writen that example aplication and formulas are for CCM.

    Thank You for Your help,

    Štěpán Šípal

  • 0 in reply to user6334892
    TI__Guru* 76745 points

    Hi Štěpán,

    The majority of the equations should still apply.  Anyway, please refer to the following handbook which TI engineers put together to cover most commonly used topologies, including CCM and DCM boost. 

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 40 points

    Thank You very much for Your help Youhao!

    Wish You a great day,

    Štěpán