Help with LM3488-something doesn't add up

Discussion in 'The Projects Forum' started by david1234, Jan 6, 2014.

  1. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    Hello ,
    I'm having problem with understanding this circuit
    from what I have read in the datasheet , Vo=1.26(1+R1/R2)
    http://www.ti.com/lit/ds/symlink/lm3488.pdf

    I have a working circuit with
    R1= 28K , R2=18K and I'm getting Vo=12V
    how could it be?
    what am I missing?

    this is the circuit -
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,766
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    C7 and C9 are higher values than shown in the datasheet. Could they be slugging the feedback too much and preventing the FET from turning off as quickly as it should?
     
  3. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    You think this is the problem?
    but in such a big difference? 7V~
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    I don't quite understand you circuit. You appear to have a flyback transformer connection but with a large capacitor from input to output. How is that supposed to work? :confused: The circuits in the data sheet use a simple inductor.
     
  5. BC107C

    New Member

    Apr 3, 2011
    14
    1
    Is the actual circuit switching? Maybe the feedback resistors are not having the values you want or maybe the inductor pins are crossed and you may get at the output the supply voltage not the generated one.
    Can you post any voltages around the U6 IC? FB, DR ?
     
  6. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    let me explain - someone gave my this circuit,and wanted to know why it's doesn't work. from what he have told\show me , in all the cards C6 has explode.

    this is what you have ask:
    Vin =18VDC
    V3(FB) - 1.26V
    V6(DR) - 2.3V

    and the Vo= 12.1V
    I just can't understand it

    Thanks,
     
    Last edited: Jan 12, 2014
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I don't see why C6 should fail so dramatically if it's been correctly specified. C5 is a more likely candidate since its ripple current is higher.
    If the dual-inductor is wrongly specified perhaps the inductor(s) is/are saturating and its inductance hence drops? A reduced inductance value would also increase the cap ripple current.
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    OP probably meant U6 failing not C6.
     
  9. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    what would be a good value of a new inductor?
    also how can I calculate it by my own?

    Thanks ,
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,766
    1,101
    I suggest you research DC-DC converters, specifically SEPIC converters. Semiconductor vendors (Linear Technology, T.I etc) probably have online calculators to aid inductor selection.
     
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