• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 166 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM51231-Q1: High Cout from Designcalc

Isabelle Olars
Prodigy 20 points
Part Number: LM51231-Q1
Other Parts Discussed in Thread: LM5122

Tool/software:

Hello,

I have used the Excel designcalc for the LM5123 but the minimum output capacitance displayed is 3454,50 uF which sounds like a lot. I have done the same calculations for the LM5152 which gives the exact same values for everything but the Cout (246,75uF for LM5152). I can't seem to find an equation to figure out what is causing the high capacitance. These are my inputs to the calculations:

Vsupply(min)=4V
Vsupply(typ)=12
Vsupply(max)=36V
Vload=9V (note: the intent is for essentially constant bypass mode)
Iload=20A
Fsw=600kHz
Lm=0,47uH
Rdcr=0,67mOhm
Rs=0,75mOhm
deltavout=150mV

Is the output capacitance correct, and is my only other option to allow for larger ripple? (or a smaller inductor, but that's not really suitable for the system)

Also, is "constant" bypass mode suitable for the LM5123? Is there any way to calculate theoretical losses during bypass?

I'll add my intended circuit below if it helps.

Thanks for your time! Best regards,
Isabelle

  • 0
    Prodigy 20 points

    Oops, sorry! Iload=28A (not 20 as I accidentally wrote)

  • +1
    TI__Mastermind 19595 points

    Hello Olars,

    thanks for reaching out to us via e2e.

    Regarding the output capacitance:
    If you are choosing an inductor where the inductance is much higher than calculated, it will not have enough time to charge and store the required energy.
    Increasing the switching frequency makes it even worse.

    But in general, these requirements are very extreme.
    75A .. 100A input current is a lot. Even if you use dual FETs.

    The thing that worries me a lot is the following:
    The LM5123 has a range between 100% and 108% on the nominal output voltage where it is neither boosting nor going into bypass.
    Both FETs will just be off in that range and the whole load current will go across the body diode of the high-side FET.

    If you need 28A on the output side and if you use dual FETs, this will be more than 14A per FET (because the distribution is not exactly 50%).
    If the voltage drop of the body diode is 0.7V this would be more than 10W loss per FET.

    The LM5122 would do a better Job because it does not have that gap.

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Prodigy 20 points

    I understand, I will swap to the LM5122 instead. Thanks for your help!