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LM2577: Is this a typo on the Data sheet

John Scoles
Prodigy 40 points
Part Number: LM2577

Is this a typo on the DataSheet

Typeon

or is that referring  to the >=47 uf cap from this paragraph

and the designer picked 100uf vs 47uf

And one more quick question.  

Am I correct in assuming this 100uf cap would be an electrolytic?

Cheers

  • +1
    TI__Guru 61695 points

    Hello John,

    The capacitor on the input shall be above 47uF, so we used a 100uF cap there. This will reduce the input voltage ripple. The input capacitor needs to have a certain size for the converter to work properly, but it might need to be increased if the input ripple is still to high for the system. Normally increasing the input capacitor only has positive effects on the system as well as on the converter operation.

    You can use an electrolytic capacitor, but I would recommend to add a ceramic one in parallel, e.g. 1uF or 10uF to have a low impedance on the input and get lower ripple. For the converter an electrolytic one is fine.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 40 points

    Ok thanks for this.  It clears things up for me.  Now if I can only figure out what ∑ILoad(max) is trying to sum my day would be perfect

  • 0 in reply to John Scoles
    TI__Guru 61695 points

    Hello John,

    I expect that you have been able to find out the sum of your max load? Can I help somehow?

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 40 points

    Sure you can help. 

    On that datasheet  formula #15

    Can confirm that the substitution for the term "∑ILoad(max)"  would be (.225+.225).  

    This it for Rc in circuit #44. 

    I get something well over 2k.

    ie .  (750*(.225+.225)*(15+5*1)^2)/5^2

    Cheers

    John

  • +1 in reply to John Scoles
    TI__Guru 61695 points

    Hello John,

    Yes, the sum of Iload is in this case the current in the positive and negative output. So if your load current is 225mA on both outputs, the value is 450mA.

    Even if the value that you calculate is above 3 kohm, please do not use a higher value as 3 kohm.

    Best regards,
    Brigitte

  • +1 in reply to Brigitte
    Prodigy 40 points

    Ok thanks allot.

    It  been like 30+ years since my Electric Circuits 101 course,  so it has been a while since I had to read and interpret a data-sheet ;)

    Thanks again