MC34036 Step-down DC-DC

Discussion in 'General Electronics Chat' started by hazem91, Apr 15, 2014.

  1. hazem91

    Thread Starter New Member

    Apr 15, 2014
    i am trying to apply a 12 v battery based USB mobile phone charger using MC34063 (step-down DC-DC) :


    but i am having two problems:
    1-i get a constant output of 4.3v without the output capacitor, but when i place the output capacitor again the voltage drops to 1.25v . i tried multiple capacitors values and still the same result.
    2- even if i use a variable resistor in the position of R1 the voltage is constant doesn't change, but i get a different voltages by changing the same resistor.

    appreciate your help.
  2. ian field

    Distinguished Member

    Oct 27, 2012
    Somewhere on the web there's an online MC34063 calculator that also shows you the best circuit configuration, it would make it easy to double check that you've correctly scaled the inductor for load current, volt differential and switching frequency.

    You need to keep circuit trace inductance as low as possible - especially the ones carrying the SMPSU pulses - and even more especially on the earthy traces.

    Low ESR electrolytics are a must, if in doubt pad them with non polar types - bigger the better. These days you can get multilayer ceramic chip capacitors of tens of uF in tiny packages.
  3. IcedFruits


    Jan 15, 2014
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Could be inductor saturation, the 34063 likes quite a high value inductor. You didn't say what your load is?

    It's also possible your IC has been damaged during earlier tests, or you might just have it wired wrong. :)
  5. hazem91

    Thread Starter New Member

    Apr 15, 2014
    My load is a mobile phone (5v charger out of 12v battery).
    how could i configure whether its an inductor saturation? actually i am using a stock inductors from a broken TVs circuits so i cant tell the exact value of the inductor but i have tried many types with different shapes, cores and wires thin.
    but for the IC i dont think its damaged because i can get a 4.3v output when i don't place the capacitor.
  6. hazem91

    Thread Starter New Member

    Apr 15, 2014
    here are my calculation results:

    Vin = 12.50 ±10% V Vout = 5.00 V Iout = 500.00 A Vripple = 100.00 V [​IMG] Vin min = 11.25 V Ton / Toff = 0.94 Ton + Toff = 10.00 µs (micro seconds) Toff = 5.16 µs (micro seconds) Ton = 4.84 µs (micro seconds) Ct = 193.72 pF Ipk = 1,000.00 A Rsc = 0.00 Ω Lmin = 0.02 µH Co = 12.50 µF R1 = 10.00 R2 = ((Vout - 1.25) / 1.25) * R1 = 30.00
    i have tried 470pF Ct and 10nF.
    Rsc is 0.33 ohm ( 3 resistors is parallel)
    Co 220uF 35v / 220uF 10v
  7. KJ6EAD

    Senior Member

    Apr 30, 2011
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Remove the load and just test with no load, and the cap inserted because the cap should always be there.

    Your inductor is probably no good if you just pulled one from a TV without knowing what value it is.

    Inductor number codes are like resistors. An ideal inductor for this circuit is 470 uH which is marked 471 (47 with 1 zero).

    If you show a clear closeup photo of your inductors we can tell you if any might be suitable.