Constant current LED driver with NCP3065

Discussion in 'The Projects Forum' started by joro.a, Aug 22, 2011.

  1. joro.a

    Thread Starter New Member

    Aug 22, 2011
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    I have problems with the NCP3065. Something with the circuit is wrong - probably the sizing of the elements. I used their design tool, but unfortunately the circuit doesn't work as expected.

    My scenario is the following:
    I use the controller in boost mode. The minimal Vin is 10V and the output voltage is 30V. I need around 250mA @ 30V as output parameters. I've attached the final calculations the Excel file made for me. The inductor I used is 330uH.

    What happens when I run the circuit - the current the IC draws is around 1A (limited by the Rs resistor) and gets very hot in a few seconds. I think that the IC pulls down the MOSFET so that the inductor gets charged, but doesn't return from that state so the inductor gets discharged by the LED connected to the output?

    Any suggestions and ideas are appreciated.
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
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    What is the source voltage and source type.
    Are those parts listed the parts you have in your circuit, except the 330uH inductor?
    The IC should certainly not be consuming 1A. Not sure how you measured the current.

    DataSheet
     
  3. #12

    Expert

    Nov 30, 2010
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    My first impression is that the chip is bad, but a new assembly can have many kinds of errors. Keep checking to see if you made a mistake in assembly.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If timing cap CT were shorted to ground, the output would stay ON, and that's exactly what is occurring.
    [eta]
    I determined this by looking in the datasheet:
    http://www.onsemi.com/pub_link/Collateral/NCP3065.PDF

    On the bottom of page 7 is Figure 14, Typical Operating Waveforms.
    You will note that the output switch turns ON when the voltage on the timing capacitor is at its' minimum value, and turns off when the voltage on the timing cap is at it's peak.

    RS should turn the output off if the max current is exceeded, unless you are using too low of a value for it.
     
    Last edited: Aug 22, 2011
  5. joro.a

    Thread Starter New Member

    Aug 22, 2011
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    The capacitor is good. My multimeter cannot measure capacitance, but it reads very high resistance value on the cap and then climbs up. Pin 3 is not shorted to the ground. The problem is somewhere else.

    The source voltage I test with is 16V DC 1A power supply.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Are you really expecting 30v @ 250mA out? That's quite a bit for that IC.
     
  7. joro.a

    Thread Starter New Member

    Aug 22, 2011
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    According to the documentation and calculations it is possible, isn't it?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Well, yes - but did you try the spreadsheet with 16V for the input?

    When you're using a higher voltage, you need to increase the inductor size. [eta] It should still work, but your ripple percent will increase.

    If CT is good, and the correct value (2.2nF, or 0.0022uF, or 2200pF) then try another regulator IC.
     
    Last edited: Aug 22, 2011
  9. joro.a

    Thread Starter New Member

    Aug 22, 2011
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    Yes, thank you for the advice. I forgot to mention that I actually tried different voltage inputs from 9V to 16V but the effect still the same.

    Maybe I have burnt the MOSFET of the IC as I touch it as bare hands to solder it.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    It doesn't appear that the internal switch is a MOSFET; it is a transistor in a Darlington configuration.

    Overheating it during soldering will kill it.
     
  11. joro.a

    Thread Starter New Member

    Aug 22, 2011
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    Thank you once more. I'll solder another circuit with fresh parts and see if there is any difference.
     
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