Step Down Switching Regulator

Discussion in 'General Electronics Chat' started by Ndjs, Sep 27, 2013.

  1. Ndjs

    Thread Starter New Member

    Sep 26, 2012
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    Hi Guys,

    I am trying to figure out from the maths of a Step-down Switching regulator. The attached image is from the data sheet

    1. How the output voltage is calculated?
    2. How would I know the peak current which this is designed.?
    I think from the data sheet Rsc=0.33/Ipk therefore Ipk=0.33/0.33 = 1 amp (i maybe way off hear)

    [​IMG]
     
  2. PackratKing

    Well-Known Member

    Jul 13, 2008
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    :confused: this information should be in the datasheet ?
     
  3. Ndjs

    Thread Starter New Member

    Sep 26, 2012
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    The data sheet only provides the details of the chip it self. If i wanted a Vout of say 20 Volts instead of the 10 what would I need to change for this to occur. ?
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
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    Output voltage is set by the voltage divider R1:R2 ratio, so that there is 1.25v across R2 when the output is at the right voltage.

    So you need to change R1 to change the 10v output.

    For 20v output you would have 1.25v on R2, and 20-1.25v (18.75v) on R1.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The 78S40 chip is so old, it's data sheet may have gone through some "cut downs" which is what they do with old products that they don't want to waste a lot of catalog pages on.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Current limit (from data sheet)

    http://www.ti.com/lit/ds/symlink/lm78s40.pdf

     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    That looks similar to the MC34063 or possibly the 78S something or other -there are various calculator apps you can download as EXE files or some you can enter values in boxes online.
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    IIRC - the MC34063 is a cut down version (I think, minus the op-amp bit) There is a fairly comprehensive appnote from Motorola/On-Semi and various calculator apps you can use online or download as EXE.
     
  9. Ndjs

    Thread Starter New Member

    Sep 26, 2012
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    If the output voltage is set by the R1:R2 and R2 = 1.25V that means 8.75 across R2.

    R1 = 12k (volt drop across is 1.25V)
    R2 = 85k This has to be 8.75 but is not the case

    R1 = 12k = 1.25Vdrop
    R2= 85k = (85/12)x1.25 = 8.85v

    Therfore output voltage is 1.25+8.85 = 10.1V.


    I think That is correct :)
     
  10. Ndjs

    Thread Starter New Member

    Sep 26, 2012
    18
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    The current limit is set by the data sheet but the design sets the Ipk with the following am I correct?.

    Rsc=0.33/Ipk

    Ipk = 0.33/0.33 = 1amp
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
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    Looks like you understand it correctly. :)

    Standard resistor values mean that it may not be a perfect 10v or 20v output, and there will probably be a small amount of voltage "sag" too making the real output a fraction less than the calculated value.
     
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