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LM2576-15 Slow Start Circuit

Jason Brinton
Prodigy 60 points
Other Parts Discussed in Thread: LM2576, LM2577

I am going to use the LM2576-15 in the buck-boost configuration and need to implement the delayed startup shown in fig 27 of the datasheet. My question is by using a larger value of C1 (The schematic shows .1uF) does it change the speed of startup? IE does larger C1 mean slower start and does that in turn lead to smaller peak startup current?

Thanks,

Jason

  • 0
    TI__Genius 16025 points
    Hi Jason,

    This part has no soft start. It has a finite delay with no voltage after which the part will turn on instantly. Increasing C1 only increases the delay before the part turns on and does not affect the startup current in any way.

    What are you trying to do? There might be a newer Simple Switcher part that can accomplish what you are looking for.

    Thanks,
    Anston
  • 0 in reply to Anston Lobo
    Prodigy 60 points
    Hi Anston,

    I am using batteries to power a board I am working on. On the board I need +/-5 and +/-15 volts. The batteries drain down to about 6V before they die. I am planning on using the LM2577 to get +15V and then I plan on using the LM2576 to get the -15V.

    My concern is the LM2577 (or 3 of them with their outputs tied together through small resistances) won't be able to provide the startup current needed for the LM2576.

    Thanks,

    Jason
  • 0 in reply to Jason Brinton
    TI__Genius 16025 points
    What type of batteries and how much current can they supply (alternatively what is their AHr) also what is your load requirement for the 4 output rails.

    I think you might need a SEPIC topology design. It will use one circuit to provide both positive and negative rails.

    Have you tried Webench? It's on TI home page.

    Thanks,
    Anston
  • 0 in reply to Anston Lobo
    Prodigy 60 points
    The batteries are NiMH, and I have 6 in series giving a min of 6V, each cell has 2300mAh. I don't have my load nailed down yet, but the +5V rail will have a 100mA (200mA on 50% duty cycle) load, and that will be by far the biggest of any loads. So my current estimate is 250mA max for the +5V rail and 100-150mA max for the other rails.

    I did a little reading about SEPIC from here: www.ti.com/.../slyt309.pdf

    is there a chip you recommend taking a look at?