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LM5122: Softstart time and startup delay

Andreas Nestler
Intellectual 360 points
Part Number: LM5122

Tool/software:

Hello,

we have a booster design with the LM5122 (Vin=28V, Vout=66V) and would like to clarify some parameter.

Acc Eq. 11 in datasheet the softstart time is dependend from Input / output voltage ratio.

Q1: Which facts cause this dependency? Is this relationship caused by the fact that the output voltage basically starts with the input voltage due to booster topology?

We measured a start-up delay of about 6-7ms.Startup delay from my perspoective is the time reaching the UVLO threshold of 1.2V until switching acc. Figure 22:

Q2: Which component has impact to the start-up delay? Is it the soft-start capacitor?

Q3: Is there any dependency of start-up delay from in- to output voltage ratio similar to soft start time? If yes, can we predict this value (e.g. by means of an equation)?

We need to know the range of this start-up delay over the required input voltage range.

Thanks in advance!

Best Regards Andreas

  • +1
    TI__Genius 15615 points

    Hi Andreas,

    Thanks for reaching out.

    Q1: This is right. This comes from the fact, that the output voltage is not 0V at the start, but Vin.

    Q2: The startup delay acc. to figure 22 is the time from reaching UVLO threshold (SS ramp starts), until this SS-ramp starts affecting the output voltage (Which is Vin initially).

     At a given Css, this time depends on the level of the input voltage.

    The time from reaching UVLO threshold until softstart is finished will be constant with a constant value for Css: (Css*1.2V)/10uA

    With higher Vin, the delay will be longer, but tss (because of the 1-Vin/Vout term) will be shorter.

    Q3: Basically t_startup_delay=(Css*1.2V)/10uA  -  ((Css*1.2V)/10uA*(1-Vin/Vout))

    Best regards,

    Moritz

  • 0 in reply to Moritz
    Intellectual 360 points

    Hi Moritz,

    thanks for your reply and dedicated answers.

    Best Regards Andreas