Dimming a 3W led using mc34063

Discussion in 'General Electronics Chat' started by MCrowe, Oct 18, 2011.

  1. MCrowe

    Thread Starter Member

    May 29, 2011
    69
    0
    OK, so I have an LED driver (already built into a circuit) with the mc34063 chip on it.

    For reference, its set to output 750mA using a resistor to sense the feedback current, with 12V input.

    What I wanted to know is, has any one used this chip before, and is it possible to dim the LED using PWM? What I was thinking was that I PWM the feedback pin. I know this works on other constant current chips but they actually say in the data sheet that you can do it. Should this be possible? Or would you just end up putting too much current into the feedback pin and over driving the LED?

    The other chip I have used PT4115 is made so that you can set the upper limit for current with a resistor, and use PWM to vary the current from 0 to 100%. COuld you do the same thing with the mc34063?? OR is there another way you could dim the LED?

    Thanks..
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you alternately lifted the feedback pin (5) above ~1.3v and letting it float, yes - you could use PWM. However, if you pull the feedback pin low, then it will output its' maximum current possible, trying to get the feedback voltage to ~1.25v.

    You might accomplish something like that using a PNP transistor, pulling the feedback pin towards the positive rail. Might be better to use an NPN transistor as an emitter follower; the NPN as an emitter follower would source current but not sink it.
     
  3. MCrowe

    Thread Starter Member

    May 29, 2011
    69
    0
    Oh yeah, and maybe not as obviously as I though, im trying to do this with a PIC micro, the 16F977a to be exact.

    If I leave the circuit set up as is, but connect my PWM to the feedback pin, wouldn't that increase the current at the feedback, and thus dim the LED? Because the chip would think the current was higher than it actually was? Does that make sense?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    No, it doesn't make sense.

    You need to understand that the regulator IC wants to see 1.25v (nominally) at that feedback (pin 5). If the voltage is less than 1.25, it will increase the output current in an attempt to bring the voltage up to 1.25v.

    If you artificially pull that pin below 1.25v using your PWM output, the regulator will send as much current as it can from it's output to bring that feedback pin back up to 1.25v. Your LED won't like having that much current dumped through it.

    Now you CAN pull the pin higher than 1.25v. In that event, the regulator will decrease its' output current.
     
  5. MCrowe

    Thread Starter Member

    May 29, 2011
    69
    0
    Oh, yeah I guess that didn't make sense. Sooo, if I used PWM, and limit the minimum duty cycle I might be able to make that work.
    Thanks heaps. Annoyingly, I got home and tried my led driver out and my "1w led driver" is only putting out 120ma at 3v. So that kinda sucks.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I still think that you are not understanding what I am trying to tell you.

    If you put 0v on that feedback input, the regulator will immediately respond with maximum current output. You will effectively remove the current limiting ability of the regulator.

    You can INCREASE the voltage on the feedback input above ~1.3v, and the current output of the regulator circuit will decrease, which will make the LED more dim.

    If you try to DECREASE the feedback input below 1.25v, your LED will be subjected to excessive current, and likely burn up.

    You need to post a schematic or the specs for this 1W LED driver. I'll bet that it's a buck-type regulator and you do not have a sufficiently high voltage supply.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    SgtWookie, the 34063 has provision to add a current sense resistor between 2 of its pins (on the high side) and the current sense acts to switch the IC output off so it will regulate current.

    We don't really know without seeing his schematic, but if it uses the current sense option to regulate the load current that leaves the 1.25v Vreg system fully available for low freq user PWM.

    Alternatively if the circuit uses the 1.25v Vreg to control the LED current, the Isense input may also be possible to use for low freq PWM. It would be nice to see the schematic! :)
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yes, and the same (yet opposite) caveat applies; if you place Vcc on pin 7, you'll be telling the regulator that there is no output current - so (depending on the circuit) it may cause the regulator to output it's maximum possible current.

    Having access to the schematic would certainly eliminate a lot of questions!
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Agreed, but if you hold pin7 low it holds the osc in reset so there will be no output. :)
     
  10. MCrowe

    Thread Starter Member

    May 29, 2011
    69
    0
    Mmm. Not sure I can really help with the schematic. It's a stock Les driver circuit. 12 input, 3v output, 700mah. On the circuit there is an inductors (is that what that coil thing is called) a capacitor, a resistor, and some diodes for AC to DC. Its kind of hard to work out where everything is connects because it's quite a small board. It's a shame they didn't send menthe board with the PT4115 on it cos it's so easy to make Dimmable. It has an un used dim pin. Lol.
     
  11. ke5nnt

    Active Member

    Mar 1, 2009
    384
    15
    Maybe you could connect the Vout pin of the 34063 to the led, then LED to a NPN LL MOSFET, the Gate of the mosfet to your PWM device, and complete the connection through the MOS to ground. The PWM will use the MOS as a high-speed switch to complete the circuit from the +3V output of the 34063 through the LEDs to ground, then you can control LED brightness by varying the PWM rate.
     
Loading...